Personalized solutions for ENT implants: The role of 3D/4D printing

Abstract

3D printing, a newer manufacturing technology, is gaining prominence in the pharmaceutical and healthcare sectors, particularly ENT implants. It enables the production of customized biological tissue scaffolds, portable models, and surgical training aids. The emergence of 4D printing offers the potential for enhancing ENT therapy safety and efficacy. The manuscript explores the potential of 3D printing to revolutionize pharmaceutical and clinical practice, enabling the development of personalized drug formulations, patient-centric implants, and anatomical models. This review delves into the emerging concept of “smart” biomaterials used in 4D printing, which are capable of mimicking natural tissues and responding to external stimuli. This paves the way for significant advancements in ENT tissue engineering with the potential to increase treatment safety and efficacy. This highlights the importance of healthcare staff in translating 3D printing innovations into clinical practice for successful adoption. The manuscript highlights the transformative impact of 3D printing in the pharmaceutical and healthcare industries. 3D printing and bioprinting technologies are revolutionizing ENT therapy, offering novel avenues for improved patient care and fostering advancements in the healthcare field.

Keywords:
ENT implant; 3D printing; 4D printing; Additive manufacturing; Bioprinting; Biomaterials; Smart polymers; Tissue engineering

Graphical Abstract

INTRODUCTION

3D printing, also known as rapid prototyping or additive manufacturing, is a fast and efficient fabrication method that uses computer-aided digital (CAD) 3D modeling to create a robust three-dimensional product (Goyanes et al., 2016). It has become popular in the pharmaceutical and healthcare sectors for the development of medical devices, individualized therapy, tissue reconstruction, and oral dosage forms. 3D printers are often used in the medical field to develop 3D-printed medical implants and scaffolds for tissue regeneration.

Current developments in bioengineering have revolutionized surgical therapies in ENT, allowing surgeons to replace, repair, or regenerate organs and tissues damaged or destroyed by injury or illness. Bioengineering with CAD and 3DP promotes the reconstruction of new tissues, which can be used in ENT implants to regenerate cartilage, bone, or skin (McGivern et al., 2021). Significant improvements in 3D printing technology have been made in materials, equipment, and processes, revolutionizing every aspect of life. 4D printing involves the utilization of smart biomaterials that enable the printed entity to undergo structural modifications in response to environmental stimuli, including light, pH, magnetic fields, humidity, and temperature (Gualtieri et al., 2019). Common materials such as shape-memory polymers, dielectric elastomers, stimuli-responsive hydrogels, smart nanocomposites, and shape-memory metal alloys are used in various areas (Zhao et al., 2019).

The application of 4D printing technology has shown promise in overcoming various obstacles linked to surgical implants, including the requirement for donor tissue, inadequate tissue compatibility, and the risk of transplant rejection. The significant progress in 3D/4D printing has dramatically expanded the possibilities for creating accurate tissue reconstructions for various implants. This technology offers precise solutions to address challenges such as the complex structure of tissues, the need for vascularisation and innervation, selecting appropriate materials, ensuring long-term stability, and compliance with regulatory requirements. This review offers a comprehensive examination of smart biomaterials utilized in creating patient-centric 3D/4D printed objects for ENT implants.

Bioinks are biocompatible materials that can act as scaffolds for cells to grow and differentiate, resulting in functioning tissues. With the addition of growth factors and cells, bioinks can be designed to imitate the characteristics of actual tissues in the reconstruction of tissue. With the development of newer smart bioinks, 4D printing has led to the establishment of promising research in tissue engineering and medical devices.

BIOINKS FOR 3DP

4D printing involves the production of materials that change their shape, characteristics, or function when subjected to environmental triggers. This transformative process leverages “smart” materials that respond dynamically to environmental changes, which can be further classified as physical, chemical, or biological stimuli-responsive materials based on the stimuli used to trigger the 4DP process. The selection of appropriate biomaterials is crucial for the success and functionality of 4D printed structures, especially in biomedical applications. The bioinks used in 3D and 4D bioprinting must have a range of biological and physicochemical properties to ensure successful printing, followed by cell viability, proliferation, and differentiation.

A bioink must have biological characteristics such as cell adhesion, proliferation, migration, water and oxygen permeability, nontoxicity, and biocompatibility to enable tissue creation and breakdown while preserving cell activities. (Roseti et al., 2017). Bioink, a biomaterial that mimics the endothelial cell matrix (ECM) in real tissue, aids in cell growth and regeneration by promoting cell proliferation, adhesion, migration, and differentiation. It promotes cell attachment by activating integrin receptors and releasing growth-promoting components. Biomaterials with immune-inert characteristics have been designed to prevent immune responses and modulate the immune response by suppressing T and B lymphocyte-mediated resistance and inhibiting the function of natural killer cells (Roseti et al., 2017). Recent studies have revealed that the hydrophilicity and hydrophobicity of bioink surfaces have a substantial impact on biological responses, with hydrophilic surfaces stimulating cell development, whereas the size of pores and the level of porosity have a significant effect on the movement of cells, their ability to penetrate tissues, the availability of nutrients and oxygen, and the exchange of waste products (Sultana, 2018).

Biodegradability is critical for replacing implanted constructs with the body’s endothelial cells (ECMs), which allow cells to proliferate and eventually replace the construct. The bioink should be nontoxic and biodegradable without interacting with other tissues or organs. Tandem degradation necessitates the controlled injection of cells with modified inflammatory reactions. Immunology is a major scientific field, and scaffold fabrication must be sterilized to avoid infection (Lyons et al., 2010).

The structural functions and durability of bioink are crucial, and it must meet specific architectural framework and functionality requirements, including mechanical properties such as tensile strength, elastic modulus, and stiffness. For ENT tissues, the tensile strength needs to be sufficient to withstand everyday stresses without failure. For example, ear cartilage must be resilient enough to withstand mechanical manipulation (such as pulling or bending). Native cartilage tissues have tensile strengths ranging from 1-10 MPa. The elastic modulus, or modulus of elasticity, measures a material’s ability to deform elastically when a force is applied. Bioinks should have an elastic modulus that mimics that of native tissue for flexibility and resilience. Ear cartilage has a low elastic modulus, ranging from 0.1 to 2 MPa, which bioinks should match to ensure flexibility. Nasal cartilage has a 0.2-1 MPa elastic modulus, and bioinks should match proper function and structural support. The trachea requires a relatively high elastic modulus (Elsner, Zilberman, 2009). Bioinks are essential components in 4D printing, and these bioinks are composed of three main components: smart biomaterials, cell sources, and bioactive factors.

Smart biomaterials

Smart biomaterials in 4D printing technology are transforming the design and manufacture of ENT implants by allowing them to change in response to external stimuli such as temperature, pH, moisture, or magnetic fields. These materials may be designed to alter form, degrade, or release therapeutic compounds, providing new possibilities for customized therapy. Shape-memory polymer implants can fit tightly during insertion, lowering their degree of invasiveness, and improving patient outcomes. Furthermore, materials with responsive drug release characteristics can distribute drugs in a targeted, controlled manner, lowering the need for systemic therapies and limiting adverse effects. External stimuli may be classified as physical, chemical, or biological stimuli.

Physical stimuli-responsive materials

Physical stimuli-responsive smart biomaterials are a class of materials that undergo significant and reversible changes in their properties or structure in response to physical stimuli such as temperature, light, magnetic fields, and mechanical forces. These materials have unique shape-changing behaviors that may be adapted to physical changes in environmental conditions in biomedical applications.

Thermoresponsive materials

Temperature is a common external stimulus in 4D printing for shaping and geometric arrangement of responsive materials such as SMPs and sensitive polymer solutions. These materials can be permanently deformed or temporarily changed by heating and cooling. The original morphology can be recovered by reheating at an elevated temperature, which is useful for bone defect repair and recovery. This approach works only with polymers with a lower or equivalent Tg to body temperature. (Li et al., 2023). Polyglycerol dodecanoate acrylate (PGDA)-based 3D-printed SMPs for biomedical implantation have shown excellent adaptability, with a 100% shape-fixity ratio at 20°C and a recovery ratio of 98% at 37°C, making them suitable for 4D-printed constructs for biomedical implantation (Zhang et al., 2023).

Temperature is the most common physical stimulus used to alter the shape of printed objects, with materials such as poly (N-isopropyl acrylamide) (PNIPAM), PEG, poly(caprolactone) (PCL), gelatin, and chitosan (Zhao, Qi, Xie, 2015). Shape-memory materials can be made or deformed under specific conditions, such as mechanical stress, temperature, and cooling. These shape regeneration features offer an opportunity to develop self-fitting implants for minor bone defects. Poly (N-isopropyl acrylamide) (PNIPAAm) is widely used in the field of cell sheet engineering because of its temperature-responsive properties. Below the lower critical solution temperature (LCST) of approximately thirty-two°C, PNIPAAm is hydrophilic, supporting cell adhesion. When the temperature increases above the LCST, the material becomes hydrophobic, helping cell detachment without the need for enzymatic treatments. This makes it particularly useful in creating cell sheets for tissue engineering applications. For example, Zarek et al. (2017) developed a methacrylate-based PCL thermosensitive tracheal stent, which expands and fits comfortably after implantation and avoids potential harm during the implantation procedure. PEG-based hydrogels are used in controlled drug delivery systems owing to their ability to swell in response to environmental changes, enhancing therapeutic efficacy and reducing side effects (Wang et al., 2023). Chitosan hydrogels are also used in cartilage tissue engineering because of their ability to mimic the natural extracellular matrix, providing structural support for chondrocytes and enhancing cartilage regeneration (Pita-López et al., 2021).

Electroresponsive materials

These materials can be stretched, compressed, or folded depending on the direction and intensity of the electrical field to which they are exposed. Some polymers, such as thiophene, aniline, and pyrrole, have good biocompatibility and printing capabilities, so they can be used to create 4D structures that respond to stimuli. Conductive electrophilic hydrogels can be prepared via polypyrrole interfacial polymerization via 3D printing to design neuroprosthetic and bioelectronic hydrogels. Carbon-based nanomaterials, such as CNTs and graphene, have been used to regulate and study stem cell fate and biology. This could improve bone and brain regeneration. 3D/4D bioprinting has led to the creation of an efficient muscle tissue scaffold via the use of a cell-aligned bioink, helping advancements in muscle repair or replacement in the field of tissue engineering. (Yang, Kim, Kim, 2021). Zolfagharian et al. (2019). developed a Takagi-Sugeno fuzzy model and discrete rigid finite element model to predict the bending behavior of 3D printed porous polyelectrolyte hydrogel actuators, which were verified by printing and measuring actuator end-point deflection. Zolfagharian et al. (2020) developed a porous chitosan actuator design using stiffness-based topological optimization, resulting in greater end-point deflection compared to actuators with uniform lattices.

Photo-responsive materials

Photoresponsive materials can show mechanical responses upon exposure to near-infrared (NIR), infrared (IR), and ultraviolet (UV) light, hence facilitating the propagation of optical impulses. The prevalent responses observed in light-responsive bioprocesses are extensively employed to produce dynamic 4D shape-altering structures, primarily due to the photoisomerization of polymer chains and the photodegradation of polymers. Wei et al. (2017) developed a 4D printed tube-shaped shape-memory design using the photoresponsive polymer poly (lactic acid). Arakawa et al. (2017), employed the process of photodegradation to create three-dimensional multicellular endothelium vascular networks within hydrogels that were loaded with cells. This multi-layered approach with 4D printing can help quickly construct networks with microchannels of the same size as human blood vessels. Luo et al. (2019) developed 3D printed structures using the light-sensitive ink photothermal polydopamine along with alginate that can change shape under NIR stimulation and preserve their new morphology for at least 14 days photothermal method has been employed to create NIR-sensitive nanocomposites that can be reversibly shape-modified, using fused deposition modeling and graphene nanoplatelets in a temperature-responsive epoxy, enabling more controllable NIR light stimulation (Cui et al., 2019).

Humidity-responsive materials

The ability to respond to changes in humidity is crucial for improving the temporal form transition of structures developed through 4D printing. Hydrogels are three-dimensional networks of polymers that can absorb and keep significant quantities of water without dissolving in a solvent. Their ability to expand is a result of their chemical or physical cross-linked structure, which undergoes a reversible change in volume when immersed in an appropriate solution. Gelatin and collagen, which are hydrophilic natural polymers, are employed as humidity-responsive materials in 4D printing (Jamal et al., 2013). 4D bioinks are ideal for moisture-sensitive materials because they can change size and shape. Transition methods must control shrinking or swelling to support structural strength. Humidity-sensitive polymers have been used for 4D printing. Gladman et al. (2016) developed a bioink by embedding cellulose fibers in an acrylamide matrix, but localized anisotropic swelling occurred. Mulakkal et al. (2018) developed a reversible hydrogel construct using carboxymethyl cellulose and hydrocolloid. This hydrogel may be activated by water and has potential uses in tissue regeneration.

Magnetic-responsive materials

An applied magnetic field from a specific distance may also enable remote and safe control of the 4D-printed constructions. Magnetism-responsive materials are generated by incorporating ferromagnetic or paramagnetic additives into polymeric structures, such as metal alloys, oxides, and magnetic nanoparticles. These components may change form via direct magnetism, thermomagnetism, or electromagnetism, allowing 4D-printed materials to respond to a magnetic field trigger. (Álvarez et al., 2022;

Wu et al., 2020) a tubular structure was created via 3D printing by mixing poly (lactic acid) with magnetic iron oxide nanoparticles. These constructions can be easily controlled and recovered via magnetic forces. Iron oxide is heated within a magnetic field, transiently restoring its earlier shape. The use of this technology in tissue engineering enables the manipulation of scaffold morphology and geometry, facilitating its diverse applications. The rheological features of hydrogels that are replicated via a magnetic field present promising opportunities for fabricating biomaterials through printing techniques. Zhu et al. (2018) successfully fabricated bioproducts with magnetoresponsive properties via a 4D printing technique in their study. This was achieved by incorporating Fe-based nanoparticles into poly(dimethylsiloxane) (PDMS) material. The use of magnetized bioinks enables the fabrication of scaffolds showing anisotropic characteristics while simultaneously facilitating the control of nanoparticle synthesis during the bioprinting procedure. Zhao et al. (2019) created a personalized tracheal scaffold using 4D printing techniques and magnetic-responsive shape-memory composites. The scaffold can adapt to external magnetic fields, mimicking the behavior of the natural trachea.

Chemical stimuli-responsive materials

Chemical stimuli-responsive materials can change their characteristics in response to certain chemical triggers, such as pH and ionic concentration. This responsiveness enables the implants to dynamically adjust to the biological environment, making them right for delicate and varied situations in ENT applications.

pH-sensitive materials

According to recent studies, many tissue structures or scaffolds can be made from pH-sensitive materials with flexible shapes and mechanical properties, which are useful for biomedical applications. Fabricating tripolyphosphate/ chitosan scaffolds with varying cross-linking amounts of protein or primary amine loaded is a practical approach for bone regeneration therapy (Xu et al., 2018). The

release behavior was sensitive to pH and the degree of cross-linking in the polymer structure. Pyridine, sulfone, carboxyl, and phosphate-related chemical groups create pH-sensitive self-assembled structures that can change from spheres to spirals when the pH is adjusted to a specific range. In one study, poly (N-iso phosphoric acid) formed bubbles when the functional group of the polymer was neutralized.

Ion-sensitive materials

The use of ion-sensitive materials in the fabrication of hydrogels offers novel opportunities for developing cell-laden and morphing structures in tissue engineering, especially when combined with 4D bioprinting methodologies. Bioprinting has been proven to be a viable method for fabricating scaffolds with a durable cell-filled structure that can support clinical-scale applications. Bioprinting allows for the cross-linking of multivalent ions such as Zn²+, Ca²+, and other ions. Tabriz et al. (2015) proved that bidirectional dipole-dipole interaction can be induced by Zn²⁺, resulting in a reversible, shape-memory, ultrahigh-strength hydrogel with cell-laden properties. A novel shape memory hydrogel, which has a bidirectional memory function, can be fabricated by coupling ions such as zinc-imidazole. To preserve the flat sheet of the hydrogel with cells, the sheet can be compressed into a tubular shape and cultivated with Zn²⁺ ions. Chelating substances can eliminate zinc ions, allowing for the recovery of stable forms of the produced structures (Nan et al., 2013).

Hydrogels are a prime example of ion-sensitive biomaterials suitable for 3D printing in ENT. These crosslinked polymer networks incorporate hydrophilic groups that can interact with ions. By incorporating ion-responsive monomers, such as methacrylic or acrylic acid, hydrogels can exhibit swelling or shrinking behaviors in response to pH or ionic strength variations. This property can be leveraged to create pH-sensitive nasal implants that react to inflammatory conditions or drug delivery systems for otitis media. Conductive polymers are another class of ion-sensitive materials with potential ENT applications. Polymers such as polypyrrole and polyaniline exhibit conductivity that can be modulated by changes in ion concentration. This characteristic is particularly interesting for developing neural interfaces or hearing aids. For example, ion-sensitive cochlear implants can be fabricated using conductive polymers to adapt to varying conditions within the inner ear (Green, Abidian 2015).

Ionic liquids, which are salts in a liquid state at room temperature, offer another approach to ion-sensitive biomaterials. These materials can be engineered to respond to specific ions, enabling targeted drug delivery or biosensing applications. In the ENT context, ionic liquids could be employed to create ion-responsive carriers for delivering antibiotics in chronic sinusitis. (Moshikur et al., 2023).

Biological stimuli-responsive materials

Bioenzymes play a crucial role in the development of advanced biomaterials, particularly hydrogels that exhibit bioactivity and shape-memory properties. These hydrogels are often created through the incorporation of enzymes and substrates that facilitate ionic and hydrogen bond cross-linking. One notable approach involves the use of glucose and enzymes to form bioactive materials. Glucose can serve as a source of energy or a reactive component in the synthesis of hydrogels, whereas enzymes such as matrix metalloproteinases (MMPs) are essential for dynamic cross-linking and functionalization of the hydrogel network. Enzymes contribute significantly to various biological reactions by facilitating specific interactions between polymers, such as polypeptides and polynucleotides, which are crucial for the formation of robust hydrogel structures. For example, MMPs are a group of enzymes involved in the degradation of extracellular matrix components that can be harnessed to create hydrogels that respond to changes in their biological environment, such as tissue remodeling or disease progression. By incorporating enzyme substrates such as MMPs into hydrogels, researchers can develop materials that exhibit tailored mechanical properties and controlled degradation rates, increasing their suitability for tissue engineering applications.

Recent advancements in enzyme-sensitive hydrogels highlight their potential for applications requiring precise control over material behavior. For example, hydrogels sensitive to hyaluronic acid (HA) have shown promising results because of their excellent cell attachment capabilities and flexible swelling and breakdown properties. HA is a naturally occurring glycosaminoglycan involved in various biological processes, including cell proliferation and tissue repair. Hydrogels that include hyaluronic acid (HA) can imitate the natural extracellular matrix. This allows agents to create a favorable environment that supports the proliferation of cells and the regeneration of tissues (Sobczak, 2022)

Cell sources

With the advent of 3D printing and 4D printing, tissue regeneration has emerged as a promising area of research. To successfully regenerate tissue, it is important to have this type of cell. These cells should be strong, capable of fighting infections, and able to transform and multiply into the types of cells that are necessary for repairing damaged tissue. Moreover, achieving dependable outcomes in terms of tissue regeneration, differentiation, and proliferation is crucial. These characteristics play a role in ensuring tissue regeneration. Epithelial stem cells and chondrocyte osteoblasts are some examples of cell sources used in 3D/4D bioprinting to create ENT implants that can support tissue engineering efforts.

The synthesis and maintenance of cartilage tissue rely on chondrocytes, which are frequently utilized in the regeneration of nasal or auricular regions. In contrast, osteoblasts play a vital role in bone formation and mineralization, making them essential for repairing bone tissue within these areas. Stem cells have the distinct potential to transform into multiple types of cells in the human body, making them well-suited for regenerating tissues. The use of induced pluripotent stem cells (iPSCs), adult stem cells, and embryonic stem cells presents a diverse range of options for three-dimensional bioprinting applications. Scaffold materials such as hyaluronic acid, collagen, and synthetic polymers are commonly used in 3D printing techniques aimed at regenerating tissues; notably, these materials exhibit biodegradability properties (King et al., 2012). These materials are biodegradable.

Epithelial cells are essential for the health of all body tissues, including the ear, nose, and throat. They can be extracted and cultivated in laboratories from various sources, such as skin or mucosal tissues. According to Park et al. (2019), incorporating epithelial cells with a hydrogel-based biomaterial has been demonstrated for the fabrication of vocal fold tissue, nasal cartilage, and an alternative for tracheal tissue via 3D and 4D printing techniques. 3D/4D printing can create structures that replicate cartilage tissue via cell sources. Biodegradable materials are used as scaffolds for cartilage regeneration. This approach has the potential to enhance reconstructive treatments and minimize the dependence on artificial implants or donor tissue in ENT implants, potentially enhancing tissue regeneration results (Park et al. 2019).

Bioactive factors

Bioactive compounds are often used to promote the growth, restoration, and rejuvenation of tissues. The growth factors epidermal growth factor (EGF), namely, transforming growth factor-beta (TGF-alpha), platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF), are used to stimulate tissue repair and regeneration. The scaffolding function of extracellular matrix (ECM) components, such as hyaluronic acid and collagen, facilitates cell adhesion and motility, hence facilitating the regeneration of tissues and wound healing processes. The use of platelet-rich plasma (PRP) has become a widely accepted and established therapeutic approach for the management of chronic sinusitis. This therapy is known to effectively facilitate the process of wound healing and regeneration, as demonstrated in the study conducted by Stavrakas (2016). Stem cells can undergo differentiation into many types of connective tissues, rendering them valuable for tissue regeneration. Cytokines, such as tumor necrosis factor-alpha interleukin-1 and interleukin-6, play important roles in regulating inflammation and the immune response. The use of gene therapy includes the introduction of genes expressing growth hormones or extracellular matrix components to promote tissue regeneration (Hosseinkhani et al., 2023).

TECHNIQUES OF ENT IMPLANTS

Conventional techniques

Initially, designed for drug delivery, scaffolds have since been utilized in the ENT field. Conventional approaches are used for developing scaffolds with precise pore sizes and shapes, but their application is restricted in terms of internal structure and interaction.

Solvent casting involves dissolving a polymer in a solvent, adding an insoluble salt, evaporating it to create a salt polymer composite, and then immersing it in water for a porous structure. This technique is cost-effective and user-friendly for cartilage and bone tissue engineering, producing scaffolds with high porosity and adjustable pore sizes, with porosities ranging from 50% to 90% (Li et al., 2020). Using the solvent casting method, Sin et al. (2010) and his team fabricated polyurethane scaffolds for cardiac tissue engineering, and Güney et al. (2020) and his team created a scaffold of gelatin/collagen hybrid polymers mixed with bioactive glass that mimics native bone, offering new treatments for bone tissue repair. This method led to the formation of simple, organized frameworks like flat tubes and sheets, which contain residual solvents that might harm cells and tissues owing to their toxic nature.

Lyophilization is a freeze-drying technique that involves dissolving a polymer in a solvent, placing it in a mold, and chilling it below the freezing temperature. This solidifies the solvent, which evaporates by sublimation to form a structure with interconnecting pores. The temperature ranged between -20°C and -80°C. This technique allows for variable pore diameters while keeping excessive temperatures from compromising the scaffold’s function or biological factors (Roseti et al., 2017). Valencia et al. (2018) developed a technique for creating chitosan and graphene oxide scaffolds for tissue engineering, which has demonstrated favorable compliance for the restoration of tissue architecture. The technique was also used to create a sodium alginate base containing magnetite nanoparticles for bone cancer treatment (Valencia et al., 2018).

Thermal-induced phase separation (TIPS) is a method that involves separating a uniform polymer solution into distinct phases by decreasing its solubility via temperature reduction. This technique consists of two distinct phases: a phase with a low concentration of polymer and a phase with a high concentration of polymer. These phases are then transformed into a solid state to create a fibrous network at the nanoscale level, which has a high level of porosity. This fibrous network is utilized as a construct/scaffold (Li et al., 2020). This technique produces a thermoplastic crystalline polymer scaffold suitable for bioactive molecule incorporation because of its low temperature. The use of TIPS followed by the freeze-drying process enables the precise regulation of high porosity, resulting in the fabrication of scaffolds with porosities exceeding 95% (Kim et al., 2014). Thermoplastic polyurethane has been used to create multi-walled carbon nanotube (CNT) foams with thermoplastic polyurethane scaffolds, increasing their stiffness and strength without cytotoxic effects. This composite scaffold significantly influences cellular behavior but has limitations such as limited fabrication material availability and resolution inadequacy, and its use in bone tissue engineering is restricted owing to its incompatibility with scaffold pore size (Conoscenti, 2017).

The gas foaming technique employs an inert gas to generate pressure within biodegradable premolded polymers dissolved in solvents such as water or fluoroform at high temperatures. The pressure is applied until the polymer reaches its saturation point, causing the formation of gas bubbles (Eltom, Zhong, Muhammad, 2019). Gas foaming and electrospinning techniques are used to develop 3D nanofiber sponges containing nerve guidance channels that promote nerve regeneration, infiltration, and cell proliferation, confirming good peripheral nerve function recovery. Similarly, gelatin foaming and freeze-drying were used to create a macroporous bone scaffold made from an agarose/chitosan matrix and reinforced with nanohydroxyapatite (Rao et al., 2019). The main disadvantages of this technology include the excessive utilization of heat during compression molding and the creation of a closed, disconnected pore structure with a nonporous outer layer. Harris and colleagues (1998) developed a technique that combines GF (gelatin foam) with particle leaching to increase the porosity of scaffolds. This approach yielded PGLA (polyglycolic acid) scaffolds with an impressive overall porosity of 97% (Harris, Kim, Mooney, 1998).

The powder foaming process creates high-porosity glass and ceramic scaffolds by suspending ceramic particles in liquids such as water or ethanol. This slurry suspension is used to prepare green bodies, which are weakly bound clay materials that undergo sintering to form pores A powder foaming process was used to generate a 90% porosity silicate bioglass scaffold with pore diameters ranging from 510-720 μm. The scaffold has strong mechanical strength, bioactivity, and biodegradability but lacks protein binding ability (Chen, Thompson, Boccaccini, 2006).

The sol-gel method is a technique for polymerizing metal alkoxides, resulting in ceramic or glass-based constructs/scaffolds in various configurations, including thin film coatings, ceramic fibers, and ultrafine powders (Raucci, Guarino, Ambrosio, 2010). An inherent limitation of this method is the relatively weak mechanical strength demonstrated by the constructed scaffolds. Chen et al. (2010) devised an improved sol-gel technique to create bioglass ceramic scaffolds that have been modified with sodium oxide. This modification results in increased mechanical strength, biodegradability, and cytocompatibility of the scaffolds.

Electrospinning is a method that uses an intense electric field to create very thin fibers from polymer solutions that carry an electric charge. The shape and diameter of fibers are influenced by key characteristics such as fluid viscosity, charge density, molecular weight, and electric field intensity, making them versatile for various materials and diameters (Pham, Sharma, Mikos, 2006). The drawbacks of this method are the use of organic solvents and the challenge of creating complex scaffolds with homogenous pore distributions. Electrospinning is a method that uses an intense electric field to create very thin fibers from polymer solutions that carry an electric charge. The shape and diameter of the fibers are influenced by key

characteristics such as fluid viscosity and charge density. Research investigating the use of nanofiber scaffolds made from gelatin/chitosan has revealed that these scaffolds are biocompatible and can retain the shape of human dermal fibroblasts and provide optimal support for cell proliferation (Lu, Li , Chen, 2013).

3D/4D printing techniques

There are many techniques used to create three-dimensional objects by layering materials, such as fused deposition modeling, stereolithography, selective laser sintering, digital light processing, binder jetting, material jetting, selective laser melting, direct metal laser sintering, and extrusion. The commonly used techniques for 3D/4D bioprinting of ENT implants are light-based 3D bioprinting, inkjet printing, and microextrusion, as discussed below, and their important comparative points have been compiled as a table (Table I).

TABLE I
Comparison of 3D printing technologies

Technology	Materials Used	Advantages	Limitations	Potential Improvements	Ref
SLS	Metals, polymers, ceramics	High precision, strong properties, versatile materials	High cost, long production times, limited build size	Affordable lasers, multilaser systems, better process control, material advances	(Roseti et al., 2017)
SL	Photopolymer resins	High accuracy, smooth finish, good for complex geometries	Limited materials, post-curing needed, high resin cost	Durable resins, reduced costs, faster light sources, resin recycling	(Du et al., 2018)
Inkjet Printing	Polymers, ceramics, metals	High-resolution, multi-material/full-color printing	Postprocessing needed, limited mechanical properties, expensive inks	Stronger inks, better nozzles, integrated multi-material printing, cost reduction	(Eltom et al.,2019)
Micro-Extrusion	Thermoplastics, biopolymers, composites	Good for large objects, broad materials, low-cost, multi-material	Lower resolution, requires support, and postprocessing needed	Finer nozzles, improved extrusion control, composite material advances, faster printing	(Murphy et al.,2013)

The commonly used methods for 3D/4D bioprinting of ENT implants are light-based 3D bioprinting, inkjet printing, and microextrusion.

Light-based 3D bioprinting

Light-based 3D bioprinting includes two commonly used methods: selective laser sintering printing and stereolithography.

Selective Laser Sintering (SLS)

SLS is a popular 3D printing method in which lasers are used to solidify liquid resins and polymers to create a 3D structure. It has the key benefits of high surface quality and high printing resolution. This process involves laying resins and photopolymers to create solid structures, which are dried and cross-linked to form solid polymers. It is often used for complex structures and is being developed for various materials, including shape-memory polymers (SMPs), liquid crystal polymers (LCPs), and hydrogels. Shape-memory polymers can be designed to change shape based on specific stimuli and can be made from thermoplastics, thermosets, or elastomers. Hydrogels prepared using natural polymers (such as gelatin collagen and alginate) and synthetic polymers (such as PVA and PEG), which are similar to the extracellular matrix of tissues, can also be used for 4D printing scaffolds (Tamay et al., 2019). Mei et al. (2023) employed SLS 4D printing to fabricate shape-memory thermoplastic polyamide elastomers that are capable of undergoing shape transformation upon exposure to heat. These materials can experience substantial deformation and maintain a temporary form. Through the manipulation of printing settings and the regulation of heating‒cooling cycles, they created complicated 3D-printed objects with shape-memory behavior. This technique has the potential to be applied in the field of biomedicine, namely, for the fabrication of 4D-printed scaffolds that may dynamically alter shape and adhere to specific anatomical places. These scaffolds can be implanted in an inert form and then change into their desired shape when exposed to body heat or other external factors (Mei et al., 2023).

Stereolithography (SL)

SL is a 3D printing technique that uses laser technology or UV to solidify a resin made of photopolymer. The procedure involves the transformation of a powdered suspension into a monomer solution, analogous to the gel casting method. The properties of the monomer and photoinitiator impact how the photopolymerization process occurs. A base is submerged in a plastic monomer, which is repeated until an entirely constructed structure is achieved (Pham, Ji, 2000). Furthermore, this method may be employed in scaffold 4D printing to fabricate scaffolds and microarchitectures that closely show the extracellular matrix within biological tissues. Scaffolds are often printed using photopolymer resins, metallic powders, PCL, hydrogels, and ceramics. The advantages of SLS include high-resolution printing, being free of nozzles, requiring low thermal stress, and

eliminating layer connection problems. There are certain drawbacks, such as the ability of the initial materials to be photocurable, the limited number of polymers permitted for pharmaceutical utilization, and the need for post-curing steps. Zopf et al. (2015) fabricated nasal scaffolds with polycaprolactone and subsequently implanted them into a porcine model. This intervention yielded a satisfactory outcome and full integration of soft tissues.

Inkjet printing

The inkjet bioprinting technique involves fabricating three-dimensional structures by depositing bioink droplets onto a substrate. The substrate is composed of a hydrogel-based material incorporating essential nutrients, growth factors, and living cells (Öblom et al., 2020). Inkjet printing is the process of expelling a continuous tubular stream of ink from nozzles, which is then fragmented into small ink droplets by a stimulating jet. It is possible to regulate the dimensions and arrangement of these droplets. The printing information is generated by manipulating the electrical charges of the nozzles to produce ink droplets, either with or without charges. The spatial electric field modifies the course of the droplet, guiding it toward the collecting plate to record characters or graphics. (Jiang et al., 2024). Some commonly used polymers, such as gelatin, alginate, collagen, PEG, PVA, and hyaluronic acid, can be modified to achieve the desired porosity, stiffness, and degradation rate to achieve the required mechanical and biological properties of scaffolds. The incorporation of stimuli-responsive materials, such as shape memory polymers or hydrogels, into scaffold designs can enhance overall biological performance (Saska et al., 2021). The method offers high resolution, accuracy, adaptability, and scalability advantages. Nevertheless, it has limitations such as limited printable biomaterials and potentially decreased cell survival due to significant shear stresses during printing.

Microextrusion

Microextrusion-based bioprinting is a widely used 3D bioprinting technique that involves the precise dispensing of biological components and bioinks in the X‒Y plane (Mironov et al., 2003). This technique uses pneumatic, mechanical, or solenoid-driven extrusion to release bioink filaments sequentially through microscale nozzles, following instructions from CAD software. The dispensing quantity may be controlled by adjusting either the air pressure or the displacement of the pump. The bioink must undergo fast gelation by either physical or chemical polymeric crosslinking, preserving its structure and preventing its spread (Zhang et al., 2021). Extrusion-based bioprinting offers several key benefits, including the ability to place thick bioinks (6-30 × 10 7 mPa s−1) with a high concentration of cells (including cell spheroids), user-friendly operation, and a wide range of potential applications. One drawback of this technology is that the rate of cell survival is lower than that of other bioprinting technologies, often ranging from 40% to 80% in terms of cell viability (Potyondy et al., 2021). One solution to this problem is to increase the nozzle diameter. However, doing so might result in a decrease in resolution. Increased viscosity bioinks offer more structural support, whereas low viscosity bioinks create an environment that preserves cell viability and function. Therefore, achieving a balance between the two is crucial. One variant of the extrusion

technique is referred to as "embedded 3D printing" or "freeform reversible embedding" (FRESH method). In this approach, the extrusion process takes place in an ionic support solution, which helps to preserve the printed structure until the deposited ink solidifies. The FRESH technique uses a support hydrogel as a temporary, thermoreversible support, which may be removed by washing after printing, resulting in the remaining free-standing scaffold. The support hydrogel consists of gelatin microparticles that form a gelatin slurry. This gelatin slurry enables the printing of biomaterials in complex shapes that would otherwise lack the necessary structural strength to be printed independently, without the assistance of a support bath. (Hinton et al., 2015).

To create 3D custom-shaped structures via extrusion-based bioprinting, an automated robotic machine and fluid-dispensing equipment are used to extrude gel-like bioink. The commonly used materials are cylindrical filaments containing living cells and polycaprolactone (PCL). Other materials, such as shape-memory polymers (SMPs) or hydrogels, can be added for 4D printing. High-viscosity biomaterials are used for small tissues or scaffolds, cell spheroids, and complex tissue self-assembly. However, these techniques work only with highly viscous biomaterials, as low-viscosity biomaterials need high pressure, leading to cell death. Extrusion-based bioprinters are inexpensive and easy to assemble. A comparison of different techniques is shown in Table I.

The conventional techniques and 3D/4D printing techniques are compared in the table below (Table II).

TABLE II
Comparison of conventional methods with 3D and 4D printing

Aspect	Conventional Methods	3D Printing	4D Printing
Introduction	Conventional manufacturing and formulation processes for creating pharmaceutical products. (Chen et al., 2010)	3D Printing process creating 3D objects by layering materials (Khoo et al., 2015)	Extension of 3D printing where printed objects can change shape or function over time in response to external stimuli. (Tibbits, 2014)
Material	Active pharmaceutical ingredients (APIs), excipients, binders, coatings, and traditional materials. (Kim et al., 2014)	Pharmaceuticals, biocompatible polymers, hydrogels, resins, and other specialized materials for drug delivery. (Javaid et al., 2018)	Smart materials that respond to stimuli like temperature, light, humidity, etc. (Javaid et al., 2018)
Process	Conventional methods such as freeze drying, solvent casting, etc. (Chen et al., 2010).	Layer-by-layer deposition of material (Jeong et al., 2020).	Similar to 3D printing with the addition of materials that enable transformation (Jeong et al., 2020).
Complexity	Limited control over internal architecture, resulting in less customized structures (Chen et al., 2006).	High complexity in geometric shapes. (Khoo et al., 2015)	Adds complexity with the ability to change shape/function postproduction (Khoo et al., 2015).
Time	Fixed structure and properties once fabricated.	Fixed shape after printing.	Changes shape over time in response to stimuli.
Cost	Lower initial cost due to established fabrication techniques and materials (Javaid et al., 2018).	Moderate to higher cost, depending on the complexity and material used, especially for custom or patient-specific scaffolds (Javaid et al., 2018).	Higher due to advanced materials and technology (Javaid et al., 2018).
Design Flexibility	Limited by traditional fabrication techniques, leading to less customization.	High design flexibility, allowing for patient-specific scaffolds with complex internal architectures.	Very high flexibility, enabling designs that can change or adapt postproduction based on specific stimuli.
Durability	Dependent on material; designed for gradual degradation in the body.	Controlled degradation, with materials engineered for specific mechanical and biological properties.	Durability varies based on stimulus-response and the number of transformation cycles, with some materials designed to adapt over time.
Environmen-tal Impact	Potentially high due to waste from conventional manufacturing processes.	Lower waste due to precision manufacturing and material efficiency.	Potentially lower with the use of adaptive and self-healing materials, reducing the need for replacement or additional scaffolds.
Current State	Well-established with extensive clinical and commercial use.	Emerging technology with increasing use in personalized medicine and complex tissue engineering.	Experimental and in the early stages of research, with significant potential for future applications in regenerative medicine and dynamic drug delivery.
Applications	Tissue engineering, drug delivery, bone regeneration, and wound healing (Javaid et al., 2018).	Personalized tissue engineering, custom implants, bone and cartilage regeneration, and drug delivery systems (Javaid et al., 2018).	Advanced tissue engineering, dynamic implants, self-assembling scaffolds, and stimuli-based drug delivery systems (Javaid et al., 2018).

APPLICATION OF 4DP IN ENT IMPLANTS

In recent years, the development of new techniques, smart materials, and viable cell sources

Ear implants

Patients experience severe psychological stress because of hereditary and chronic ear problems. Superior autogenous rib cartilage has been the gold standard for ear reconstruction over the last 50 years. 3D bioprinting technology may simplify surgery and make patient-specific casts more practical. Zhou et al. (2018) accomplished a clinical breakthrough using chondrocytes and polyglycolic/polylactic acid and created a tissue-engineered ear. Microtia chondrocytes were introduced into an ear scaffold, resulting in a substantial increase in the cell count from 4.5 million to over 450 million. After 12 weeks, the regenerated ear resembled the original scaffold shape by more than 90%. This finding suggests a targeted modification of the unique geometry of the cartilage, as intended in the design. Brennan et al. (2021) developed a 3D ear cartilage method using one-stage and two-stage auricular PCL scaffolds via laser sintering of polycaprolactone. The scaffolds were implanted in thymus-free animals and showed stable proportions during an 8-week in vivo assessment. The spherical pore design reduced the total stiffness by over 81%, which affected skin ulceration and dehiscence issues (Brennan et al., 2021). Rose et al. (2015), built temporal bone models by replicating the bone structure based on a patient’s CT image. These models were utilized for realistic simulations and specialized training in performing tympanomastoidectomy for complex cases of recurrent cholesteatoma (Rose et al., 2015). 3D printing (3DP) systems offer a more accurate reconstruction of anatomical structures because of the use of multiple colors and materials. This method shows promise in the field of hearing reconstruction, encompassing both partial and whole reconstruction, and could produce ossicular prostheses tailored to individual patients for ossiculoplasty procedures. Kozin et al. (2015) conducted a successful experiment in which they evaluated a personalized 3D-printed prosthesis for correcting bony superior canal abnormalities on a cadaveric temporal bone (Kozin et al., 2015). Watson et al. (2014) conducted a study to show the efficacy of 3D printing (3DP) in reconstructing ears. They treated three patients who had unilateral ear defects by employing polycaprolactone auricular prostheses. This treatment successfully restored the trust of patients (Watson, Hatamleh, 2014). Three-dimensional technology can also be utilized for surgical planning in complex circumstances. In 2017, Mukherjee et al. demonstrated the effectiveness of utilizing 3D models with an artificial temporal bone to enhance surgical preparation and increase surgical precision while reducing intraoperative challenges. A noninvasive in vivo 3D printing technique has been developed by Chen et al. (2020), using digital near-infrared photopolymerization (DNP) to print cultured chondrocytes in the shape of the ear in mice. This technique has shown promising results in the development of relevant noninvasive in vivo biomedical applications, demonstrating the potential of noninvasive methods in medical research. The process of preparing 3D implants for ears is explained in Figure 1 below.

FIGURE 1
Schematic diagram of digital near-infrared (NIR) photopolymerization (DNP)-based noninvasive 3D bioprinting. Permission is not needed.

Nasal implants

A multifaceted scaffold aimed at addressing the need for nasal cartilage and subchondral bone regeneration. This scaffold was constructed through the utilization of biomaterials and advanced 3D printing techniques. The utilization of a multilayer scaffold has the potential for reconstructing nasal cartilage and subchondral bone. Nasal defects caused by facial tumors, infections, and trauma can be addressed via 3D printing technology for personalized prostheses and preoperative simulations. Möller et al. (2017) designed 3D structures using biopolymers and cells, demonstrating their structural integrity after 60 days.

Yi et al. (2019) used tissue engineering techniques in conjunction with 3D printing to create a personalized nasal implant using human adipose-derived stem cells. The customized cartilage showed remarkable cartilaginous tissue formation after 12 weeks. Kim et al. (2018) examined the efficacy and safety of bioresorbable nasal implants made via 3D printing technology. An investigation revealed that these implants possess distinctive mechanical support, thinness, and surgical manipulability. Xia et al. (2018) successfully fabricated a three-dimensional (3D)-printed scaffold utilizing indigenous polymers to facilitate the regeneration of nasal cartilage. An optimization study on the pore size of the scaffolds revealed that scaffolds with 50% infill density were superior in terms of effectiveness in terms of cell transplantation and achieved a more uniform dispersion of cells (Xia et al., 2018).

In 2019, Zhuo et al. created 3D paranasal sinus and nose models using CT scan data of patients, achieving superior accuracy in comparison with the use of dissection in cadavers. Alrasheed et al. (2017) conducted a study on creating a 3D-printed model of the stomatal complex and frontal sinus designed for training in endoscopic sinus surgery.

3DP systems enable the fabrication of operative templates tailored to the anatomy of patients, such as an osteoplastic flap for septal prostheses or frontal surgery for irregular septal gaps. A 30-year-old male underwent secondary-close reduction with augmentation rhinoplasty using a 3D-printed silicone implant. (Khan et al., 2018). Daniel et al. (2011) successfully used frontal sinus osteoplastic flaps in a 10-patient series. The object was formed using CT data and has an accuracy of 1-2 mm for facial reconstruction with craniofacial fibrous dysplasia, achieving functional and aesthetic results (Daniel et al., 2011).

Nasal implant surgery requires extreme care and accuracy, as it is a part of the face. The common steps are explained in Figure 2. In the first step, face scanning is performed to obtain a clear image of the nasal bone, which is then converted to a 3D design via CAD software with accurate dimensions. The file CAD file is then converted into an STL file for 3D printing of the implant via one of the many 3D printing techniques (stereolithography). A 3D-printed nasal implant, prepared using popular bioink (epoxy resin), is trimmed to the exact size and then implanted at the desired location (Onerci Altunay et al., 2016).

FIGURE 2
Schematic representation of the development and implantation of 3D-printed nasal structures.

Craniofacial bone implants

In laryngology, 3DP is used to increase tissue repair and surgical training, as well as to create medical equipment for vocal fold regeneration and to design preintervention procedures (Romero, Colton, Thomson, 2021).

In 2019, Richard et al. generated three-dimensional models of two subglottic stenosis instances via CT scan data. The models were utilized for patient education and surgical simulation. A functional 3DP laryngeal model can be developed that resembles human tissue for surgical simulation (Kavanagh et al., 2017). Grolman et al., (1995) developed a laryngeal model that replicates vocal fold injection and simulates surgical disorders such as subglottic cysts, laryngomalacia, subglottic stenosis, and laryngeal clefts in children.

The research conducted by Roskies et al. (2017) provided evidence of the efficacy of utilizing 3D-printed polyether ketone ketone (PEKK) scaffolds loaded with the adipose tissue stromal vascular fraction (ADSCs) for bone regeneration. This was achieved through the examination of a critical-sized bone defect in the jaw of a rabbit model. Research has revealed that the incorporation of ADSC/PEKK composites into lesions of the periphery mandible in rabbits led to significant increases in both the bone-to-tissue volume and the thickness of the trabeculae. According to the research, compressive resistance of a composite material significantly increased by fifteen times compared with the compressive resistance of bone in isolation. Kuss et al. (2017), investigated the application of 3D bioprinted bone constructs to restore craniofacial deformities resulting from cancer resection, trauma, and congenital anomalies. This study revealed that, despite prolonged exposure to low oxygen levels and heightened activity of genes associated with the formation of blood vessels, the resulting structures had encouraging outcomes in terms of both vascularization and the development of bone tissue, as observed in both living organisms and laboratory settings (Kuss et al., 2017). A maxillofacial bone implant is shown in Figure 3.

FIGURE 3
Representation of the process flow of 3D bioprinting of maxillofacial bone and its implant.

Tracheal implants

3D-printed tracheal implants are promising advancements in regenerative medicine, as they use biocompatible materials and intricate designs to create patient-specific scaffolds for tissue regeneration and airway function restoration. Despite being in its developmental stages, this approach offers hope for individuals with complex tracheal injuries or diseases, overcoming limitations associated with traditional treatment methods (Kim et al., 2020).

An artificial trachea was developed by Park et al. (2019) that uses autologously isolated epithelial cells and chondrocytes and has a significantly increased survival rate. The successful integration of the prosthetic trachea was observed in partially excised tracheas, leading to the formation of epithelial tissue and the production of cartilage clusters.

A 3D-printed tracheal scaffold can be prepared using polyurethane (PU), which is specifically designed at the microscale level for the required biomechanical properties that enable it to survive the physiological conditions of the trachea. According to Jung et al. (2016), the implanted scaffolds underwent re-epithelialization after four weeks and exhibited ciliary activity after 8 weeks.

A synthetic trachea was fabricated by Kim et al. (2020) via 3D-printed PCL microfibers and electrospun PCL nanofibers, in which iPSC-derived mesenchymal stem cells, chondrocytes, and human bronchial epithelial cells were used to enhance cartilage regeneration and tracheal mucosa in vivo in a rabbit with a damaged trachea (Figure 4).

FIGURE 4
Schematic representation of the bioprinting of tracheal implants and transplantation along with real images of transplantation and endoscopic analysis.

A 3D-printed tracheal scaffold containing bone marrow-derived mesenchymal stem cells (MSCs) was developed for successful airway reconstruction in animal models. The scaffold promoted MSC differentiation into epithelial cells and chondrocytes, demonstrating functional respiratory properties and integration with host tissue (Kim et al., 2020).Jang et al. (2014) conducted a study on a scaffold used in a rabbit model and demonstrated its effectiveness in promoting tracheal regeneration. The hybrid material supported cartilage and epithelial tissue formation, facilitating tissue regeneration over time. The scaffold gradually degrades, allowing natural tissue to replace it. The study also highlighted the ability of the scaffold to maintain airway patency and support functional respiratory activity.

The development of newer techniques and novel smart bioinks for 3D printing has opened doors to surgical options for bioengineered implants to address ENT problems. Table III compiles some recent research work available regarding 3D/4D printing, materials employed as bioinks, cell sources, and animal models.

TABLE III
Application of 3D/4D printing in ENT implant regeneration

Anatomic region	3DP techniques	Polymeric materials used	Cell source	Animal model	References
Auricle	ME	poly(ε-caprolactone) (PCL)	Adipose-derived stem cells (ASCs) cocultured with chondrocytes	Rat	(Zhou et al., 2018)
Auricle	ME	PCL	Chondrocytes	Mouse	(Yin et al.,2020)
Auricle	D-NIR	Gelatin methacrylate	Articular chondrocytes from newborn rat	Subcutaneous implantation in BALB/c mice	(Jang, et al., 2020)
Nasal cartilage	SL	PCL	Human nasal septal cartilage chondrocytes	Human	(Kim et al., 2018)
Calvarial bone	SLS	Polyetherketoneketone (PEKK)	mesenchymal stem cells (MSC)	Sheep	(Adamzyk et al., 2016)
Calvarial bone	SLS	PEKK	Mesenchymal stem cells	Rabbit	(Lin et al., 2019)
Calvarial bone	-	-PCL and TCP along with Pluronic F127	Human amniotic fluid stem cells (hAFSCs)	Rat	(Kang et al.,2016)
Trachea	-	Polylactic acid (PLA)	chondrocytes	Rabbit	(Goldstein et al., 2015)
Trachea	ME	Poly (l-lactic acid) (PLLA)	Rabbit auricular chondrocytes	Rabbit	(Gao et al., 2019)
Trachea	SL	PCL	Human mesenchymal stem cells (hMSCs)	Pig	(El-Husseiny et al., 2022)

REGULATORY AND ETHICAL PERSPECTIVE FOR ENT IMPLANTS

Regulatory and ethical perspectives for ENT (Ear, Nose, and Throat) implants are critical for ensuring patient safety, efficacy, and the overall success of these medical devices. Regulations and ethical perspectives are discussed further.

Regulations related to 3D/4D printing

Before any new medical technology, including 3D/4D printing, can be used, it must undergo thorough testing and receive permission from regulatory organizations. This process guarantees that the devices and structures produced are both safe and effective. The US FDA has issued Technical Considerations for Additive Manufactured Medical Devices, providing guidance on manufacturing involving 3D printing. The document outlines technical considerations for the additive manufacturing process, including the design and manufacturing process, environmental conditions, material controls, software workflow, and process validation. The goal is to balance patient safety with innovation in new processes (U. S. F. D. Administration, 2018).

The clearance procedure for 4D-printed biomedical equipment is complicated and includes preclinical research, clinical trials, and postmarket supervision. The novelty of 4D printing technology may create regulatory gaps or new standards, demanding collaboration between academics and producers. However, new medical technology regulations often emerged years after their first use, as seen in a clinical trial involving 3D bioprinted ears for surgical reconstruction. There is no clear regulatory pathway for 3D printing, and products may have different risk profiles. Other challenges are material properties, combinations, and potential inconsistencies between production and use (Ramezani, Mohd Ripin, 2023).

Regulations related to ethical considerations

The healthcare sector is facing a severe shortage of donor organs due to the growing global population and aging demographics. As of 2024, more than 100,000 people in the United States were awaiting solid organ transplantations. 3D and 4D bioprinting can address this shortage by improving in vitro drug screening accuracy and reducing the need for animal testing. The FDA recently announced that animal testing is no longer required before starting human drug trials, a significant shift away from conventional animal use. This shift has the potential to reduce costs, address ethical concerns, and avoid potential side effects in human clinical trials. 3D and 4D bioprinting fills the gap between traditional bench and in vitro studies and human clinical trials, allowing for increased clinical relevance and patient-specific therapeutic response. This is particularly important for drug screening in areas such as cancer, where immediate clinical need and therapy selection are critical (Ramezani, Mohd Ripin, 2023).

4D printing in biomedical engineering involves ethical problems such as patient privacy, informed consent, and equal access to equipment. Customized devices frequently need sensitive medical data, demanding privacy and data security measures throughout the design and production processes. Patient consent is critical in clinical studies employing 4D-printed devices since patients must be fully educated about the possible dangers, advantages, and alternatives to using these devices. The high costs of developing and producing 4D-printed biomedical devices may limit their accessibility to patients, necessitating initiatives by researchers and policymakers to minimize these prices and assure universal access to this novel healthcare technology (Ramezani, Mohd Ripin, 2023).

CURRENT STATUS AND FUTURE PROSPECTS

The utilization of 3D printing in the manufacturing sector for industrial medical purposes has undergone notable development. Prominent firms such as Ultimateker, Organovo, and Helisys have emerged as significant players in the production of living human tissue with this technology. Revotek successfully implanted arteries that were fabricated via 3D printing techniques in simian test animals. Similarly, Poietis used D laser-assisted bioprinting technology to generate bioprinted skin prototypes and hair follicles. 3DBio Therapeutics (3DBio) is a clinical-stage biotechnology company in regenerative medicine, and it was the first company to implant 3D-bioprinted living tissues in patients. A significant milestone has been achieved by effectively using 3D/4D bioprinting technology to manufacture the initial animal thyroid gland. Additionally, 3DBio has collaborated with Russia’s URC to develop artificial tissues under the specific conditions of the International Space Station. The company aims to create synthetic thyroid and renal tissue. Research and exploration of 3D/4D printing have been ongoing for the past two decades, resulting in the granting of numerous patents (Table IV).

TABLE IV
Current patents regarding ENT implants with 3D printing technology

Sr. No	Patent no and year of publication	Description
1	US20230010971A1 (2023)	A nasal tissue implant uses a biocompatible polymeric scaffold with open voids for cell development. This component is used for individual nasal tissue reconstruction and engineering. Adipose manufacturing allows for implants that match each patient’s anatomy, ensuring a secure fit and preventing patient distress.
2	US10799696B2 (2020)	Hydrogel formulations prepared through UV cross-linking, stimulate lacrimal gland, nasal, or sinus tissue to increase tear production and treat dry eye.
3	US20200360281A1 (2020)	A thermoresponsive hydrogel, formed by cross-linking chitosan and genipin, can effectively incorporate chemotherapeutic drugs into solid tumor cancers without losing thermoresponsiveness.
4	EP3270821B1 (2018)	A biocompatible artificial tympanic membrane device composed of polyglycolic acid or polylactic acid was developed to address the issues related to the fixation, replacement, or patching of the tympanic membrane.
5	EP3151872B1 (2017)	Wound dressing was crafted lyophilized hydrogel with implanted devices, including chitosan and hypromellose, that absorb water and exudates, maintaining a moist wound site for angiogenesis and healing.
6	WO2017152112A2 (2017)	Hydrogel systems were used for skeletal interfacial tissue regeneration in epiphyseal growth plate repair, utilizing biomaterials and methods to guide the regeneration of similar interfacial tissue structures.
7	US10639175B2 (2016)	A porous bidirectional bellowed tracheal reconstruction device was fabricated as an implantable splinting device that fits a patient’s deficient passageway, containing an acellularised tissue matrix and support structures, and can be designed for implantation between the trachea and esophagus.
8	US20230010971A1 (2023)	A nasal tissue implant uses a biocompatible polymeric scaffold with open voids for cell development. This component is used for individual nasal tissue reconstruction and engineering. Adipose manufacturing allows for implants that match each patient’s anatomy, ensuring a secure fit and preventing patient distress.
9	US20190374336 (2019)	A nasal tissue implant is a device used for reconstructing and engineering nasal tissue in a subject. It includes a tissue scaffold component made of a biocompatible polymeric material with open pores for cell growth. This component conforms to the subject’s nasal region and defines at least a portion of their nasal anatomy. A method for creating an implantable nasal tissue implant involves laser sintering and 3D printing of biocompatible polymeric material, forming a tissue scaffold component that conforms to the specific nasal region of the subject.
10	CN109793565A (2019)	The invention relates to a porous tantalum bone plate made using 3D printing and osteoinductive energy. The plate has an interconnected pore structure suitable for bone ingrowth. The preparation process involves using medical-grade spherical tantalum powder, blasting treatment to remove excess metal powder, and overheating treatment to create a smooth surface. This method allows for the formation of an integrated implant with bone tissue, ensuring permanent biology.
11	CN113456302B (2021)	The invention outlines a titanium mesh for an auxiliary implant, which is integrally molded to a preset angle, forming a bone-forming space. The titanium mesh body includes an edge, a pore structure, and two holes for penetration and connection. The first hole is used for fixing the mesh body on the alveolar bone, while the second hole connects the upper surface of the base station. The edge and pore structure are integrally formed through 3D printing.
12	US20180315348 (2018)	The otologic surgical skills training system includes a housing with a simulated ear canal working port for transcanal surgery and a larger port for lower spatial constraints. The system can be 3D printed and includes surgical instruments like endoscopic and microscopic instruments. The exercise module, which can be attached to a platform or self-contained, offers exercises like placing rings, stacking nuts, and navigating mazes. It allows for minimal depth perception, prostheses placement, soft release, and instrument feel and control.

Currently, several clinical trials are being conducted in the United States and Europe, as indicated in Table V. The present invention involves a biocompatible artificial tympanic membrane device composed of polyglycolic or polylactic acid, which is suitable for repairing, substituting, or mending the tympanic membrane in implants. Bioinks facilitate cell proliferation, especially in tissue engineering and regenerative medicine. Nevertheless, it should be noted that bioinks, compared with natural tissues, offer a low level of complexity,

TABLE V
Current clinical trials regarding ENT implants with 3D printing technology

Sr. No	Clinical trial ID.	Topic	Sponsored by (Status)
1.	NCT05273060	Biomedicine approach to nasal implant	Mayo Clinic (Recruiting)
2.	NCT03348293	Safety study of 3D printing personalized biodegradable implant for breast reconstruction	Xijing Hospital (Recruiting)
3.	NCT04098146	Examining the treatment process and outcomes of patients undergoing lower jaw reconstruction after oral squamous cell carcinoma removal	AO Innovation Translation Center (Recruiting)
4.	NCT03607227	Continuous popliteal block for managing postoperative pain in patients undergoing major ear, nose, and throat surgery with microvascular free flap reconstruction	Region Skane (Completed)
5.	NCT02559050	Nasal implant using a personalized 3DP nasal stent for congenital arhinia	Seoul St. Mary’s Hospital(Completed)
6.	NCT06118554	3D Printing to Improve Nasal Irrigation Outcome	Ohio State University(Active, not recruiting)
7.	NCT06224816	Clinical Trial for the Evaluation of Customized 3D Printed NIV Nasal Masks in Premature Infants	Hospital Universitario (Recruiting)
8.	NCT02914431	Personalized Titanium Plates vs CAD/CAM Surgical Splints in Maxillary Repositioning of Orthognathic Surgery	Shanghai Ninth People’s Hospital Affiliated to Shanghai Jiao Tong University (Completed)
9.	NCT04635865	3D-Printed Patient-Specific Surgical Plates Versus Conventional Surgical Plates in Jaw Reconstruction	The University of Hong Kong (Recruiting)
10.	NCT03890575	Airway Segmented Stent Modified With 3D Printing for Malignant Stricture Involving Carina and Distal Bronchi	Ruijin Hospital (Completed)
11.	NCT06051747	Patient-Customized Bioprinting Technology for Practical Regeneration of the Respiratory Tract (Trachea)	Ja Seong Bae, MD, PhD (Active, not recruiting)

and frequently lack the essential attributes required for facilitating cell proliferation and differentiation. The existing printing technologies are limited in their ability to fabricate intricate structures resembling natural tissues, hence posing a significant obstacle in the field of bioprinting for the development of tissues and organs that function comparably to their native equivalents. The safety and efficacy of bioprinted tissues and organs must be thoroughly established before their use in clinical conditions.

CONCLUSION

Bioprinting in ENT implants can revolutionize tissue regeneration and the management of diseases affecting the ENT. Bioprinted biocompatible tissues that resemble natural tissue can increase the effectiveness of tissue regeneration and reduce rejection rates. However, many medical institutions struggle to access bioprinting technology because of the high costs and complexity of printers. Owing to its ability to create customized devices, 3DP is becoming increasingly important in tissue engineering and the development of biomaterials. 4D printing enables evolving, flexible systems for tissue engineering applications. However, developing materials with multiple sensitivities for device dynamicity remains a challenge. Bioprinted tissues must pass strict regulatory standards before being used in humans, and the intricate design of ENT tissues makes reproduction difficult via current bioprinting processes. Despite these challenges, bioprinting holds significant potential for tissue regeneration in ENT surgeries.

REFERENCES

• Adamzyk C, Kachel P, Hoss M, Gremse F, Modabber A, Hölzle F, et al. Bone tissue engineering using polyetherketoneketone scaffolds combined with autologous mesenchymal stem cells in a sheep calvarial defect model. Journal of cranio-maxillo-facial surgery: official publication of the European Association for Cranio-Maxillo-Facial Surgery. 2016;44(8):985-94.
• Alrasheed AS, Nguyen LHP, Mongeau L, Funnell WRJ, Tewfik MA. Development and validation of a 3D-printed model of the ostiomeatal complex and frontal sinus for endoscopic sinus surgery training. International forum of allergy & rhinology. 2017;7(8):837-41.
• Álvarez E, Estévez M, Gallo-Cordova A, González B, Castillo RR, Morales MD, et al. Superparamagnetic Iron Oxide Nanoparticles Decorated Mesoporous Silica Nanosystem for Combined Antibiofilm Therapy. Pharmaceutics [Internet]. 2022;14(1).
• Arakawa CK, Badeau BA, Zheng Y, DeForest CA. Multicellular Vascularized Engineered Tissues through User-Programmable Biomaterial Photodegradation. Advanced Materials 2017;29.
• Brennan JR, Cornett A, Chang B, Crotts SJ, Nourmohammadi Z, Lombaert I, et al. Preclinical assessment of clinically streamlined, 3D-printed, biocompatible single-and two-stage tissue scaffolds for ear reconstruction. J Biomed Mater Res Part B: Applied Biomaterials. 2021;109(3):394-400.
• Chen Q-Z, Li Y, Jin L-Y, Quinn JMW, Komesaroff PA. A new sol-gel process for producing Na2O-containing bioactive glass ceramics. Acta Biomaterialia. 2010;6(10):4143-53.
• Chen QZ, Thompson ID, Boccaccini AR. 45S5 Bioglass-derived glass-ceramic scaffolds for bone tissue engineering. Biomaterials. 2006;27(11):2414-25.
• Chen Y, Zhang J, Liu X, Wang S, Tao J, Huang Y, et al. Noninvasive in vivo 3D bioprinting. Sci Adv. 2020;6(23):eaba7406.
• Conoscenti G, Schneider T, Stoelzel K, Carfì Pavia F, Brucato V, Goegele C, et al. PLLA scaffolds produced by thermally induced phase separation (TIPS) allow human chondrocyte growth and extracellular matrix formation dependent on pore size. Mater Sci Eng C Mater Biol Appl. 2017;80:449-59.
• Cui H, Miao S, Esworthy T, Lee S-j, Zhou X, Hann SY, et al. A novel near-infrared light responsive 4D printed nanoarchitecture with dynamically and remotely controllable transformation. Nano Res. 2019;12(6):1381-8.
• Daniel M, Watson J, Hoskison E, Sama A. Frontal sinus models and onlay templates in osteoplastic flap surgery. J Laryngol Otol. 2011;125(1):82-5.
• Du X, Fu S, Zhu Y. 3D printing of ceramic-based scaffolds for bone tissue engineering: an overview. J Mater Chem B. 2018;6(27):4397-412.
• El-Husseiny HM, Mady EA, Hamabe L, Abugomaa A, Shimada K, Yoshida T, et al. Smart/stimuli-responsive hydrogels: Cutting-edge platforms for tissue engineering and other biomedical applications. Mater Today, Today Bio. 2022;13:100186.
• Elsner JJ, Zilberman M. Novel antibiotic-eluting wound dressings: an in vitro study and engineering aspects in the dressing’s design. Journal of tissue viability. 2010;19(2):54-66.
• Eltom A, Zhong G, Muhammad A. Scaffold Techniques and Designs in Tissue Engineering Functions and Purposes: A Review. Adv Mater Sci Eng. 2019;2019(1):3429527.
• Gao B, Jing H, Gao M, Wang S, Fu W, Zhang X, et al. Long-segmental tracheal reconstruction in rabbits with pedicled Tissue-engineered trachea based on a 3D-printed scaffold. Acta biomaterialia. 2019;97:177-86.
• Gladman AS, Matsumoto EA, Nuzzo RG, Mahadevan L, Lewis JA. Biomimetic 4D printing. Nature Materials 2016;15:413-8.
• Goldstein TA, Smith BD, Zeltsman D, Grande D, Smith LP. Introducing a 3-dimensionally Printed, Tissue-Engineered Graft for Airway Reconstruction: A Pilot Study. Otolaryngology-head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery. 2015;153(6):1001-6.
• Goyanes A, Det-Amornrat U, Wang J, Basit AW, Gaisford S. 3D scanning and 3D printing as innovative technologies for fabricating personalized topical drug delivery systems. J Control Release. 2016;234:41-8.
• Green R, Abidian MR. Conducting Polymers for neural prosthetic and neural interface applications. Adv Mater. 2015;27(46):7620-37.
• Grolman W, Schouwenburg PF, Verbeeten Jr B, de Boer MF, Meeuwis CA. Three-dimensional models of the tracheostoma using stereolithography. ORL; Journal for oto-rhino-laryngology and its related specialties. 1995;57(6):338-42.
• Gualtieri T, Taboni S, Ferrari M, Gilbert R. Bioengineering for head and neck reconstruction: the role of customized flaps. Curr Opin Otolaryngol Head Neck Surg. 2021;29(2):156-60.
• Güney E, Emir C, Altan D, Yücel S. Development of biocomposite tissue scaffolds of collagen/gelatin/ borondoped bioactive glass prepared through solvent casting/particulate leaching method for bone tissue engineering. J Indian Chem Soc. 2020;97:2006-12.
• Harris LD, Kim BS, Mooney DJ. Open pore biodegradable matrices formed with gas foaming. J Biomed Mater Res. 1998;42(3):396-402.
• Hinton TJ, Jallerat Q, Palchesko RN, Park JH, Grodzicki MS, Shue H-J, et al. Three-dimensional printing of complex biological structures by freeform reversible embedding of suspended hydrogels. Science Advances. 2015;1(9):e1500758.
• Hosseinkhani H, Domb AJ, Sharifzadeh G, Nahum V. Gene therapy for regenerative medicine. Pharmaceutics [Internet]. 2023;15(3).
• Jamal M, Kadam SS, Xiao R, Jivan F, Onn TM, Fernandes R, et al. Bio-origami hydrogel scaffolds composed of photocrosslinked PEG bilayers. Adv healthcare mater. 2013;2(8):1142-50.
• Jang CH, Koo Y, Kim G. ASC/chondrocyte-laden alginate hydrogel/PCL hybrid scaffold fabricated using 3D printing for auricle regeneration. Carbohydrate polymers. 2020;248:116776.
• Jang YS, Jang CH, Cho YB, Kim M, Kim GH. Tracheal regeneration using polycaprolactone/collagen-nanofiber coated with umbilical cord serum after partial resection. International journal of pediatric otorhinolaryngology. 2014;78(12):2237-43.
• Javaid M, Haleem A. 4D printing applications in medical field: A brief review. Clin Epidemiol Global Health. 2018;7.
• Jeong HY, An S-C, Lim Y, Jeong MJ, Kim N, Jun YC. 3D and 4D Printing of Multistable Structures. Applied Sciences [Internet]. 2020; 10(20).
• Jiang J, Chen X, Mei Z, Chen H, Chen J, Wang X, et al. Review of Droplet Printing Technologies for Flexible Electronic Devices: Materials, Control, and Applications. Micromachines [Internet]. 2024;15(3).
• Jung S, Lee SJ L, Kim HY, Park HS, Wang Z, Kim HJ, et al. 3D printed polyurethane prosthesis for partial tracheal reconstruction: A pilot animal study. Biofabrication. 2016;8:045015.
• Kang HW, Lee SJ, Ko IK, Kengla C, Yoo JJ, Atala A. A 3D bioprinting system to produce human-scale tissue constructs with structural integrity. Nature biotechnology. 2016;34(3):312-9.
• Kavanagh KR, Cote V, Tsui Y, Kudernatsch S, Peterson DR, Valdez TA. Pediatric laryngeal simulator using 3D printed models: A novel technique. The Laryngoscope. 2017;127(4):E132-e7.
• Khan G, Choi YS, Park ES, Choi YD. The Application of Three-Dimensional Simulation Program and Three-Dimensional Printing in Secondary Rhinoplasty. The Journal of craniofacial surgery. 2018;29(8):e774-e7.
• Khoo Z, Teoh J, Liu Y, Chua C, Yang S, An J, et al. 3D printing of smart materials: A review on recent progresses in 4D printing. Virtual and Physical Prototyping. 2015;10:103-22.
• Kim DH, Yun WS, Shim JH, Park KH, Choi D, Park MI, et al. Clinical Application of 3-Dimensional Printing Technology for Patients With Nasal Septal Deformities: A Multicenter Study. JAMA otolaryngology-head & neck surgery. 2018;144(12):1145-52.
• Kim IG, Park SA, Lee S-H, Choi JS, Cho H, Lee SJ, et al. Transplantation of a 3D-printed tracheal graft combined with iPS cell-derived MSCs and chondrocytes. Scientific Reports. 2020;10(1):4326.
• Kim KO, Lee Y, Hwang JW, Kim H, Kim SM, Chang SW, et al. Wound healing properties of a 3-D scaffold comprising soluble silkworm gland hydrolysate and human collagen. Colloids and surfaces B, Biointerfaces. 2014;116:318-26.
• King SN, Hanson SE, Hematti P, Thibeault SL. Current applications of mesenchymal stem cells for tissue replacement in otolaryngology-head and neck surgery. Am J Stem Cells. 2012;1(3):225-38.
• Kozin ED, Remenschneider AK, Cheng S, Nakajima HH, Lee DJ. Three-Dimensional Printed Prosthesis for Repair of Superior Canal Dehiscence. Otolaryngology-head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery . 2015;153(4):616-9.
• Kuss MA, Harms R, Wu S, Wang Y, Untrauer JB, Carlson MA, et al. Short-term hypoxic preconditioning promotes prevascularization in 3D bioprinted bone constructs with stromal vascular fraction derived cells. RSC Advances. 2017;7(47):29312-20.
• Li T, Chen L, Yuan Y, Shi R. The Current Status, Prospects, and Challenges of Shape Memory Polymers Application in Bone Tissue Engineering. Polymers [Internet]. 2023; 15(3).
• Lin Y, Umebayashi M, Abdallah M-N, Dong G, Roskies MG, Zhao YF, et al. Combination of polyetherketoneketone scaffold and human mesenchymal stem cells from temporomandibular joint synovial fluid enhances bone regeneration. Scientific Reports . 2019;9(1):472.
• Lu T, Li Y, Chen T. Techniques for fabrication and construction of three-dimensional scaffolds for tissue engineering. International journal of nanomedicine. 2013;8:337-50.
• Luo Y, Lin X, Chen B, Wei X. Cell-laden four-dimensional bioprinting using near-infrared-triggered shape-morphing alginate/polydopamine bioinks. Biofabrication . 2019;11(4):045019.
• Lyons FG, Al-Munajjed AA, Kieran SM, Toner ME, Murphy CM, Duffy GP, et al. The healing of bony defects by cell-free collagen-based scaffolds compared to stem cell-seeded tissue engineered constructs. Biomaterials . 2010;31(35):9232-43.
• McGivern S, Boutouil H, Al-Kharusi G, Little S, Dunne N, Levingstone T. Translational Application of 3D Bioprinting for Cartilage Tissue Engineering. Bioengineering. 2021;8:144.
• Mei S, Wang J, Li Z, Ding B, Li S, Chen X, et al. 4D printing of polyamide 1212 based shape memory thermoplastic polyamide elastomers by selective laser sintering. Journal of Manufacturing Processes. 2023;92:157-64.
• Mironov V, Boland T, Trusk T, Forgacs G, Markwald RR. Organ printing: computer-aided jet-based 3D tissue engineering. Trends in biotechnology. 2003;21(4):157-61.
• Möller T, Amoroso M, Hägg D, Brantsing C, Rotter N, Apelgren P, et al. In Vivo Chondrogenesis in 3D Bioprinted Human Cell-laden Hydrogel Constructs. Plastic and reconstructive surgery Global open. 2017;5(2):e1227.
• Moshikur RM, Carrier RL, Moniruzzaman M, Goto M. Recent Advances in Biocompatible Ionic Liquids in Drug Formulation and Delivery. Pharmaceutics [Internet]. 2023;15(4).
• Mukherjee P, Cheng K, Flanagan S, Greenberg S. Utility of 3D printed temporal bones in prepre-surgical planning for complex BoneBridge cases. European Arch Oto-Rhino-Laryngol. 2017;274(8):3021-8.
• Mulakkal M, Trask R, Ting V, Seddon A. Responsive cellulose-hydrogel composite ink for 4D printing. Materials & Design. 2018;160.
• Murphy SV, Atala A. Organ engineering-combining stem cells, biomaterials, and bioreactors to produce bioengineered organs for transplantation. BioEssays : news and reviews in molecular, cellular, and developmental biology. 2013;35(3):163-72.
• Nan W, Wang W, Gao H, Liu W. Fabrication of a shape memory hydrogel based on imidazole-zinc ion coordination for potential cell-encapsulating tubular scaffold application. Soft Matter. 2013;9(1):132-7.
• Öblom H, Cornett C, Bøtker J, Frokjaer S, Hansen H, Rades T, et al. Data-enriched edible pharmaceuticals (DEEP) of medical cannabis by inkjet printing. International journal of pharmaceutics. 2020;589:119866.
• Onerci Altunay Z, Bly JA, Edwards PK, Holmes DR, Hamilton GS, O’Brien EK, et al. Three-dimensional printing of large nasal septal perforations for optimal prosthetic closure. American journal of rhinology & allergy. 2016;30(4):287-93.
• Park J-H, Yoon J-K, Lee JB, Shin YM, Lee K-W, Bae S‒W,S-W, et al. Experimental Tracheal Replacement Using 3-dimensional Bioprinted Artificial Trachea with Autologous Epithelial Cells and Chondrocytes. Scientific Reports . 2019;9(1):2103.
• Pham DT, Ji C. Design for stereolithography. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science. 2000;214(5):635-40.
• Pham QP, Sharma U, Mikos AG. Electrospinning of polymeric nanofibers for tissue engineering applications: a review. Tissue engineering. 2006;12(5):1197-211.
• Pita-López ML, Fletes-Vargas G, Espinosa-Andrews H, Rodríguez-Rodríguez R. Physically cross-linked chitosan-based hydrogels for tissue engineering applications: A state-of-the-art review. European Polymer Journal. 2021;145:110176.
• Potyondy T, Uquillas JA, Tebon PJ, Byambaa B, Hasan A, Tavafoghi M, et al. Recent advances in 3D bioprinting of musculoskeletal tissues. Biofabrication . 2021;13(2).
• Ramezani M, Mohd Ripin Z. 4D Printing in Biomedical Engineering: Advancements, Challenges, and Future Directions. J Funct Biomater. [Internet]. 2023;14(7).
• Rao F, Yuan Z, Li M, Yu F, Fang X, Jiang B, et al. Expanded 3D nanofibre sponge scaffolds by gas-foaming technique enhance peripheral nerve regeneration. Artif Cells, Nanomed, Biotechnol. 2019;47(1):491-500.
• Raucci M, Guarino V, Ambrosio L. Hybrid composite scaffolds prepared by sol-gel method for bone regeneration. Compos Sci Technol. 2010;70:1861-8.
• Richard Z, Jackson E, Jung JP, Kanotra SP. Feasibility, and potential of three-dimensional printing in laryngotracheal stenosis. J Laryngol Otol . 2019;133(6):530-4.
• Romero RGT, Colton MB, Thomson SL. 3D-Printed Synthetic Vocal Fold Models. J Voice. 2021;35(5):685-94.
• Rose AS, Webster CE, Harrysson OL, Formeister EJ, Rawal RB, Iseli CE. PrePre-operative simulation of pediatric mastoid surgery with 3D-printed temporal bone models. International journal of pediatric otorhinolaryngology . 2015;79(5):740-4.
• Roseti L, Parisi V, Petretta M, Cavallo C, Desando G, Bartolotti I, et al. Scaffolds for Bone Tissue Engineering: State of the art and new perspectives. Mater Sci Eng C Mater Biol Appl . 2017;78:1246-62.
• Roskies MG, Fang D, Abdallah MN, Charbonneau AM, Cohen N, Jordan JO, et al. Three-dimensionally printed polyetherketoneketone scaffolds with mesenchymal stem cells for the reconstruction of critical-sized mandibular defects. The Laryngoscope . 2017;127(11):E392-e8.
• Saska S, Pilatti L, Blay A, Shibli JA. Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering. Polymers 2021;563. https://doi.org/10.3390/ polym13040563.
  » https://doi.org/https://doi.org/10.3390/ polym13040563
• Stavrakas M, Karkos PD, Markou K, Grigoriadis N. Platelet-rich plasma in otolaryngology. J laryngol Otol. 2016;130(12):1098-102.
• Serna JA, Rueda-Gensini L, Céspedes-Valenzuela DN, Cifuentes J, Cruz JC, Muñoz-Camargo C. Recent Advances on Stimuli-Responsive Hydrogels Based on Tissue-Derived ECMs and Their Components: Towards Improving Functionality for Tissue Engineering and Controlled Drug Delivery. Polymers [Internet]. 2021; 13(19).
• Sin D, Miao X, Liu G, Wei FS, Chadwick G, Yan C, et al., editors. Polyurethane (PU) scaffolds prepared by solvent casting/particulate leaching (SCPL) combined with centrifugation. 2010.
• Sobczak M. Enzyme-Responsive Hydrogels as Potential Drug Delivery Systems-State of Knowledge and Future Prospects. Int J Mol Sci. [Internet]. 2022; 23(8).
• Sultana Phd Ceng Csci N. Mechanical and biological properties of scaffold materials. 2018. p. 1-21.
• Tabriz AG, Hermida MA, Leslie NR, Shu W. Three-dimensional bioprinting of complex cell laden alginate hydrogel structures. Biofabrication . 2015;045012. https://doi.org/10.1088/1758-5090/7/4/045012.
  » https://doi.org/https://doi.org/10.1088/1758-5090/7/4/045012
• Tamay DG, Dursun Usal T, Alagoz AS, Yucel D, Hasirci N, Hasirci V. 3D and 4D Printing of Polymers for Tissue Engineering Applications. Frontiers in Bioengineering and Biotechnology. 2019;1(1);7.
• Tibbits S. 4D Printing: Multi-Material Shape Change. Architectural Design. 2014;84(1):116-21.
• U. S. F. D. Administration. Technical Considerations for AdditiveManufactured Medical Devices. 2018;1(1):1-31.
• Valencia C, Valencia CH, Zuluaga F, Valencia ME, Mina JH, Grande-Tovar CD. Synthesis and Application of Scaffolds of Chitosan-Graphene Oxide by the Freeze-Drying Method for Tissue Regeneration. Molecules [Internet]. 2018;23(10).
• Wang Z, Ye Q, Yu S, Akhavan B. Polyethylene Glycol (PEG)-Based Hydrogels for Drug Delivery in Cancer Therapy: A Comprehensive Review. Advanced Healthcare Materials. 2023;12(18):2300105.
• Watson J, Hatamleh MM. Complete integration of technology for improved reproduction of auricular prostheses. J Prosthet Dent. 2014;111(5):430-6.
• Wei H, Zhang Q, Yao Y, Liu L, Liu Y, Leng J. Direct-Write Fabrication of 4D Active Shape-Changing Structures Based on a Shape Memory Polymer and Its Nanocomposite. ACS applied materials & interfaces. 2017;9(1):876-83.
• Wu H, Zhang X, Ma Z, Zhang C, Ai J, Chen P, et al. A Material Combination Concept to Realize 4D Printed Products with Newly Emerging Property/Functionality. Advanced Science. 2020;7(9):1903208.
• Xia H, Zhao D, Zhu H, Hua Y, Xiao K, Xu Y, et al. Lyophilized scaffolds fabricated from 3D-printed photocurable natural hydrogel for cartilage regeneration. ACS Appl. Mater Interfaces. 2018;10:31704-31715. 10.1021/acsami.8b10926.
• Xu Y, Han J, Chai Y, Yuan S, Lin H, Zhang X. Development of porous chitosan/tripolyphosphate scaffolds with tunable uncross-linking primary amine content for bone tissue engineering. Materials Science & Engineering C, Biomimetic Materials, Sensors, and Systems. 2018;182- 190.
• Yang GH, Kim W, Kim J, Kim G. A skeleton muscle model using GelMA-based cell-aligned bioink processed with an electric-field assisted 3D/4D bioprinting. Theranostics. 2021;11(1):48-63.
• Yi HG, Choi YJ, Jung JW, Jang J, Song TH, Chae S, et al. Three-dimensional printing of a patient-specific engineered nasal cartilage for augmentative rhinoplasty. J Tissue Eng. 2019;10:2041731418824797.
• Yin Z, Li D, Liu Y, Feng S, Yao L, Liang X, et al. Regeneration of elastic cartilage with accurate human-ear shape based on PCL strengthened biodegradable scaffold and expanded microtia chondrocytes. Applied Materials Today. 2020;20:100724.
• Zarek M, Mansour N, Shapira S, Cohn D. 4D Printing of Shape Memory-Based Personalized Endoluminal Medical Devices. Macromol Rapid Commun. 2017;38(2):1600628.
• Zhang J, Wehrle E, Rubert M, Müller R. 3D Bioprinting of Human Tissues: Biofabrication , Bioinks, and Bioreactors. Int J Mol Sci . [Internet]. 2021;22(8).
• Zhang X, Yang Y, Yang Z, Ma R, Aimaijiang M, Xu J, et al. Four-Dimensional Printing and Shape Memory Materials in Bone Tissue Engineering. Int J Mol Sci [Internet]. 2023;24(1).
• Zhao Q, Qi HJ, Xie T. Recent progress in shape memory polymer: New behavior, enabling materials, and mechanistic understanding. Prog Polym Sci. 2015;49-50:79-120.
• Zhao W, Zhang F, Leng J-s, Liu YJCS. Technology. Personalized 4D printing of bioinspired tracheal scaffold concept based on magnetic stimulated shape memory composites. Compos Sci Technol . 2019.
• Zhou G, Jiang H, Yin Z, Liu Y, Zhang Q, Zhang C, et al. In Vitro Regeneration of Patient-specific Ear-shaped Cartilage and Its First Clinical Application for Auricular Reconstruction. EBioMedicine. 2018;28:287-302.
• Zhu P, Yang W, Wang R, Gao S, Li B, Li Q. 4D Printing of Complex Structures with a Fast Response Time to Magnetic Stimulus. ACS Appl Mater Interfaces 2018;36435-36442. https://doi.org/10.1021/acsami.8b12853.
  » https://doi.org/https://doi.org/10.1021/acsami.8b12853
• Zhuo C, Lei L, Yulin Z, Wentao L, Shuangxia W, Chao W, et al. Creation and validation of three-dimensional printed models for basic nasal endoscopic training. International forum of allergy & rhinology . 2019;9(6):695-701.
• Zolfagharian A, Denk M, Bodaghi M, Kouzani AZ, Kaynak A. Topology-Optimized 4D Printing of a Soft Actuator. Acta Mech Solid Sin. 2019;33(3):418-30.
• Zolfagharian A, Kaynak A, Bodaghi M, Kouzani AZ, Gharaie S, Nahavandi S. Control-Based 4D Printing: Adaptive 4D-Printed Systems. Applied Sciences [Internet]. 2020;10(9).
• Zopf DA, Mitsak AG, Flanagan CL, Wheeler M, Green GE, Hollister SJ. Computer aided-designed, 3-dimensionally printed porous tissue bioscaffolds for craniofacial soft tissue reconstruction. Otolaryngol-Head Neck Surg: official journal of American Academy of Otolaryngology-Head and Neck Surgery. 2015;152(1):57-62.

Edited by

• Associated Editor:
  Silvya Stuchi Maria-Engler

Publication Dates

• Publication in this collection
  20 Jan 2025
• Date of issue
  2025

History

• Received
  08 June 2024
• Accepted
  12 Aug 2024