Preparation and characterization of hydrogels with potential for use as biomaterials

Preparation and characterization of hydrogels with potential for use as biomaterials

Mônica Beatriz Thürmer^*; Carlos Eduardo Diehl; Fábio José Bento Brum; Luís Alberto dos Santos

Materials Department, Federal University of Rio Grande do Sul – UFRGS, Av. Bento Gonçalves, 9500, CEP 91509-900, Porto Alegre, RS, Brazil

ABSTRACT

Hydrogels have been extensively explored for biomedical applications due to their ability to absorb high water content in its structure, which gives excellent biocompatibility. This work aims at obtaining biocompatible hydrogels with potential for use in increasing the mechanical strength of bone substitutes, or controlled drug release. Poly (N-vinyl-2-pyrrolidone) hydrogels were prepared by free radical polymerization with and without the addition of acrylic acid. Azobisisobutyronitrile and ammonium persulfate were used as initiator and N,N-methylenebisacrylamide was used as the crosslinking agent. The characterization of the hydrogels was performed by thermogravimetric analysis, differential scanning calorimetry, infrared spectroscopy and swelling properties. The results obtained demonstrate different degrees of crosslinking and swelling of up to 490 ± 30%. The different properties of the hydrogels suggest different applications.

Keywords: biocompatible, hydrogel, PVP, PAA, polymerization

1. Introduction

Polymers are today the largest group of materials used for biomedical purposes. They are used separately and/ or in combination with other materials. The main areas of today's applications of hydrogels materials include wound dressing, drug delivery systems, transdermal systems, dental applications, injectable polymers, implants and ophthalmic applications¹. Hydrogels are polymer-based three dimension network systems that are formed by chemical or physical crosslinking of individual polymer chains, which can absorb large amounts of water and biological fluids in their structure^2,3. Limited water-absorption capabilities (ca. 5-10 wt%) of certain polymeric networks, exhibit the ability to retain the drugs for longer periods^4,5.

The hydrogels have been extensively exploited for biomedical applications due to their high water content which imparts excellent biocompatibility^6,7. Drug delivery systems developed for many different purposes have been based on hydrogels, mostly the neutral hydrogels. In addition, hydrogels have been applied in gel casting approach to reinforce α-tricalcium phosphate-based calcium phosphate cement during the first stage of setting, for use as biomaterial^8,9.

N-vinyl-2-pyrrolidone (VP) is a water-soluble, non-ionic monomer that can be used to prepare hydrogels¹⁰. VP-based materials have been of interest in, for example, biomedical applications^11,12. In recent years, polyacrylic acid (PAA) and its copolymers have often been used as carriers in drug release systems, because of their multifunctional nature, unique properties and good biocompatibility^13,14.

Several studies of the water swelling and drug release behaviours of poly(AA-co-VP) hydrogels have been reported^15-17. In these hydrogels strong hydrogen bonds can be formed between the AA and VP constituents¹⁶ . These copolymer hydrogels have been shown to display properties ranging between those of the two homopolymers and reported that the extent of swelling depends on the copolymer composition and crosslink density.

In the current investigation, we have attempted to develop a biodegradable and biocompatible hydrogel with potential for use in increased resistance of calcium phosphate cement implants or as controlled drug delivery. New formulations of hydrogels have been developed using N-Vinyl-2-pyrrolidone with and without acrylic acid, employing azobisisobutyronitrile or ammonium persulfate as initiator, by the free radical polymerization method. N,N-methylenebisacrylamide was used as crosslinking agent. The aim was to compare the properties obtained by the different formulations prepared. This comparison is not reported in the literature and can be used for the development of future work. The two monomers used have very fair reputation as non-toxic and biocompatible materials¹⁸. Hydrogels based on poly (vinylpyrrolidone) have been applied successfully as local dressings on wound treatments, such as burns, skin's ulceration and postoperative dressings¹⁹ or controls release system for drug delivery^20,21 because of their biocompatibility with the human body. Moreover, polyacrylic acid is known to be a good mucoadhesive and may increase the transit time of formulation, in drug delivery systems²². These hydrogels were characterized by TGA, DSC, FTIR and equilibrium swelling studies.

2. Experiment

2.1. Material

The hydrogels polymerization occurred by free radical polymerization using N-vinyl-2-pyrrolidone (NVP) (Merck, Germany) and acrylic acid anhydrous (AA) (Aldrich, Germany). As initiator was used azobisisobutyronitrile (AIBN) (Aldrich, Germany) or ammonium persulfate (AP) (Merck, Brazil). The bifunctional compound N,N'-methylenebisacrylamide (MBAM) (Aldrich, Germany) were used as crosslinking agent and N,N,N',N'tetramethylethylenediamine (TEMED) was used as catalyst.

2.2. Preparation of hydrogels

The composition of the hydrogels is shown in Table 1. The solutions were warm to 80 ºC washed and lyophilized for subsequent characterization.

2.3. Hydrogels characterization

2.3.1. Thermogravimetric Analysis (TGA)

Thermal stability was evaluated in a thermogravimetric analyzer TGA 2050 (TA Instruments). The samples were heated to 970 ºC at the rate of 10 ºC.min^–1, under flowing nitrogen of 20 ml.min^–1.

2.3.2. Differential Scanning Calorimetry (DSC)

DSC measurements were performed using a DSC Q20 (TA Instruments) at 10 ºC.min^–1. Nitrogen was the carrier gas, and its flow rate was 50 mL.min^–1. The second heating cycles of these runs were used to determine the Tg.

2.3.3. Infrared spectroscopic analysis (FTIR)

Infrared spectra of hydrogels were obtained using Spectrum 1000 (PerkinElmer) spectrometer in the range of 400-4000 cm^–1 using KBr discs.

2.3.4. Swelling behavior studies of hydrogels The equilibrium swelling of the hydrogels was determined by swelling the hydrogels samples (lyophilized previously) in water distilled at room temperature until equilibrium was attained. Three samples were used for each swelling determination. The swollen weight of the pellet was determined by blotting of the pellet every hour

until equilibrium was attained. The percent swelling was calculated by the Equation 1.

where W_o is the initial weight and the W_t is the final weight of the sample.

3. Results and Discussion

The hydrogel system consists of Poly (N-vinyl-2pyrrolidone) which is slowly degradable and has a high glass transition temperature. Polyacrylic acid was also included in the system which imparts pH sensitivity. This pendent acidic moiety changes ionization in response to the changes in pH. The hydrogels also displays pH sensitive nature which can be exploited for site specific controlled drug delivery.

The composition used in the M3 formulation sample didn't result in polymerization, therefore was not possible to characterize this sample.

3.1. Thermogravimetric Analysis (TGA)

The thermal analysis of hydrogels by TGA (Figure 1) demonstrates the stability of the polymeric matrix to 200 ºC. For the M1 sample there was initial weight loss due to loss of moisture in the gel from 60-120 ºC. The discontinuous TGA curve also indicates the not formation of interpolymeric matrix.

Since the proposed hydrogel system is intended to be used for the release of drug from the varying medium or as a booster for use as a biomaterial, and the release process is normally carried out at physiological temperature 37 ºC, the polymer is totally stable in the vicinity of room temperature which normally varies between 27 and 38 ºC.

3.2. Differential Scanning Calorimetry (DSC)

Figure 2 shows the thermograms of the samples prepared, obtained by differential scanning calorimetric and the Table 2 presents the thermal characteristics of hydrogels. The glass transition temperature of each sample was calculated as the midpoint of the transition. Three DSC runs of each sample were taken to determine the average Tg value and the standard derivation. From the literature, the Tg of PVP having an Mw value of 1.2×10⁶ is 178 ºC²³ and the experimental value of M1 sample (without AA) is 173.51 ºC, which agrees well with the literature value. Wu and Freeman²⁴ found that, in the temperature range of 0-350 ºC, was not observed Tg in a polymerized MBAM sample, probably due to its very high crosslink density. Generally, in highly crosslinked polymers, Tg may be difficult or impossible to observe due to restriction on mainchain motion by the crosslinks.

Based upon these results and the large difference in reactivity ratio between NVP and MBAA, two regions (NVP-rich and MBAA-rich) form during polymerization. Due to the dense crosslinking of MBAA-rich regions, there was no Tg related to crosslinked MBAA observed in the polymer network, and only one Tg, corresponding to that of the NVP-rich regions, was observed.

Analyzing the thermal characteristics of hydrogels, we can verify that the samples showed a single Tg and this is affected by the presence of the AA and also by the initiator used. The presence of AA resulted in the decrease of Tg in both cases. The reduction was more significant when used Ammonium Persulfate as initiator.

3.3. Infrared spectroscopic analysis (FTIR)

The hydrogels were characterized by FT-IR spectroscopy as showed in Figure 3. Száraz and Forsling²⁵ reported two very strong bands in the IR spectrum of pure liquid NVP that are sensitive to extent of reaction. The first, at 1629cm^#150;1, is ascribed to C=C bond stretching vibrations. The second, attributed to carbonyl stretching (C=O), is located at 1706cm^#150;1. Greever et al.²⁶ also identified characteristic IR absorptions for NVP at 1623 cm^–1 (C=C) and 1700 cm^–1 (C=O). The non-appearance of these bands indicates polymerization of the NVP monomer, and a broad C=O bond appears at 1650cm^#150;1 in polyvinylpyrrolidone (PVP). The characteristic IR absorption at ~1630 cm^–1 is assigned to the C=C bond stretching vibration of MBAM²⁷. The infrared region of 3800-3000 cm^–1 is attributed to OH stretch mode of water.

Consistent with the literature discussed above, the disappearance of the characteristic NVP and MBAA absorptions in the FTIR spectra of the crosslinked NVP films and the appearance of a broad C=O absorption, characteristic of PVP, at 1650 cm^#150;1 both suggest that the polymerization conditions were sufficient to essentially completely react the double bonds in the NVP and MBAA. The appearance of an absorption in the region of 1550cm^#150;1 (amide II band) suggests that the MBAM was successfully crosslinked with NVP during polymerization.

According to Devine and Higginbotham²⁸, the carbonyl group of PVP exhibits a stretching vibration peak between 1650 and 1680 cm^–1 and the carbonyl group of carboxylic acid group on the PAA chain exhibits a peak at approximate 1750 cm^–1. When the carbonyl group forms intermolecular bond (such as hydrogen bond), there is a negative shift exhibited in the FTIR Spectrum, that can be observed in M4 sample. It is also evidence that a small shoulder appeared in this sample at about 1730 cm^–1 corresponding to stretching vibration of carbonyl group of carboxylic acid group on the PAA chain, which further illustrated that some intermolecular hydrogen bond did occur and the complexation formed between PVP bead and PAA. Therefore, this suggests that, for the M4 sample, may have been the formation of a semi-interpenetrating network. However this did not occur in this sample, because there was a decreased in the Tg and was totally dissolved in water shortly after immersion, as shown below.

3.4. Swelling behavior studies of hydrogels

In the equilibrium swelling studies was possible to observe different swelling capacities. The M1 sample presented swelling capacities very high, around 490 ± 30%, however the hydrogel integrity was lost with the time and the M2 sample presented around 90 ± 2% swelling at equilibrium indicating a high degree of crosslinking. We know that the increase in the crosslinked density result in a lower equilibrium swelling degree²⁹. The M4 sample showed a low degree of crosslinking, and dissolved in contact with water. This may be associated with the formation of oligomeric products due to low initiation efficiency compared to the AIBN.

Literature reports suggest that hydrogels containing NVP require high concentrations (5-20 wt%) of crosslinker to produce a material with useful mechanical properties¹². Ordinarily, higher crosslinker concentrations would lead to higher crosslink density and, thus, better mechanical properties.

4. Conclusion

The new formulations of hydrogels allowed to obtain different properties which leads to different applications. The product of polymerization of N-vinyl-2-pyrrolidone initiated only with ammonium persulfate, was totally soluble in water, this may be associated with the formation of oligomeric products due to low initiation efficiency compared to the AIBN. The hydrogels showed good thermal stability around 200 ºC and a single Tg, indicating possibly the formation of random copolymers. The M1 sample showed a low degree of crosslinking may lead to use as absorbable material or controlled delivery of drugs. The M2 sample had a higher degree of crosslinking may potentially strengthen structure of bone cement. The M4 sample can also be used in controlled delivery of drugs, but for short periods of time, since it dissolves easily. The hydrogels studied are known for their biocompatibility, but no more specific tests were carried out to confirm such biocompatibility, which will be performed posteriorly.

Received: June 26, 2013;

Revised: June 23, 2014

• 1. Fechine GJM, Barros JAG and Catalani LH. Poly(N-vinyl)2-pyrrolidone) hydrogels production by ultraviolet radiation: new methodologies to accelerate crosslinking. Polymer 2004; 45(14):4705-4709. http://dx.doi.org/10.1016/j.polymer.2004.05.006
• 2. Tanaka T, Fillmore D, Sun S, Nishio I, Swislow G and Shah A. Phase transition in ionic gels. Physical Review Letters 1980; 45:1636-1639. http://dx.doi.org/10.1103/PhysRevLett.45.1636
• 3. Lin CC and Metters AT. Hydrogels in controlled release formulations: network design and mathematical modeling. Advanced Drug Delivery Reviews 2006; 58(12-13):1379-1408. PMid:17081649. http://dx.doi.org/10.1016/j.addr.2006.09.004
• 4. O'Hagan DT, Jeffery H and Davis SS. The preparation and characterization of poly(lactide-co-glycolide) microparticles: III. Microparticle/polymer degradation rates and the in vitro release of a model protein. International Journal of Pharmaceutics 1994; 103(1):37-45. http://dx.doi.org/10.1016/0378-5173(94)90201-1
• 5. Rafati H, Coombes AGA, Adler J, Holland J and Davis SS. Protein-loaded poly(-lactide-co-glycolide) microparticles for oral administration: formulation, structural and release characteristics. Journal of Controlled Release 1997; 43(1):89-102. http://dx.doi.org/10.1016/S01683659(96)01475-7
• 6. Kost J and Langer R. Equilibrium swollen hydrogels in Controlled release applications. In: Peppas NA, editor. Hydrogels in medicine and pharmacy Boca Raton: CRC Press; 1986. p. 95-108.
• 7. Pitt CG. The controlled parenteral delivery of polypeptides and proteins. International Journal of Pharmaceutics 1990; 59(3):173-196. http://dx.doi.org/10.1016/03785173(90)90108-G
• 8. Santos LA, Oliveira LC, Rigo ECS, Carrodeguas RG, Boschi AO and Arruda ACF. Influence of polymeric additives on the mechanical properties of α-tricalcium phosphate cements. Bone 1999; 25(2):99S-102S. http://dx.doi.org/10.1016/S87563282(99)00143-X
• 9. Davidenko N, Carrodeguas RG, Sastre R and San Román J. Photopolymerization of dual-setting α-tricalcium phosphate cements. International Journal of Polymeric Materials 2002; 51(7):577-589. http://dx.doi.org/10.1080/714975796
• 10. Kroschwitz JI. Polymers: biomaterials and medical applications. Nova Jersey: John Wiley & Sons; 1989.
• 11. Lai YC. A novel crosslinker for UV copolymerization of N-vinyl pyrrolidone and methacrylates to give hydrogels. Journal of Polymer Science: Part A: Polymer Chemistry 1997; 6:1039-1046. http://dx.doi.org/10.1002/(SICI)10990518(19970430)35:6<1039::AID-POLA7>3.0.CO;2-
• 12. Peppas NA, Huang Y, Torres-Lugo M, Ward JH and Zhang J. Physicochemical foundations and structural design of hydrogels in medicine and biology. Annual Review of Biomedical Engineering 2000; 2:9-29. PMid:11701505. http://dx.doi.org/10.1146/annurev.bioeng.2.1.9
• 13. Dittgen M, Durrani M and Lehmann K. Acrylic polymers: a review of pharmaceutical applications. STP Pharma Science 1997; 7:403-437.
• 14. Devine DM, Devery SM, Lyons JG, Geever LM, Kennedy JE and Higginbotham CL. Multifunctional polyvinylpyrrolidinone polyacrylic acid copolymer hydrogels for biomedical applications. International Journal of Pharmaceutics 2006; 326(1-2):50-59. PMid:16926073. http://dx.doi.org/10.1016/j.ijpharm.2006.07.008
• 15. Ozyurek C, Caykara T, Kantoglu O and Guven O. Characterization of network structure of poly(N-vinyl 2-pyrrolidone/acrylic acid) polyelectrolyte hydrogels by swelling measurements. Journal of Polymer Science: Part B: Polymer Physics 2000; 38(24):3309-3317. http://dx.doi.org/10.1002/1099-0488(20001215)38:24<3309::AIDPOLB130>3.0.CO;2-H
• 16. Chun M, Cho C and Choi H. Characteristics of poly(vinyl pyrrolidone)/poly (acrylic acid) intermolecular complex prepared by template polymerization of acrylic acid: effect of reaction solvent and molecular weight of template. Journal of Applied Polymer Science 2004; 94(6):2390-2394. http://dx.doi.org/10.1002/app.21176
• 17. Lee W and Chiang W. Swelling and drug-release behaviour of the poly(AA-co- vinyl pyrrolidone)/chitosan interpenetrating polymer network hydrogels. Journal of Applied Polymer Science 2004; 91(4):2135-2142. http://dx.doi.org/10.1002/app.13353
• 18. Ameye D, Voorespoels J, Foreman P, Jasi J, Richardson P, Garesh S, et al. Ex vivo bioadhesion and in vivo testosterone bioavailability study of different bioadhesive formulations based on starch-g-poly(acrylic acid) copolymers and starch/ poly(acrylic acid) mixtures. Journal of Controlled Release 2002; 79(1-3):173-182. http://dx.doi.org/10.1016/S01683659(01)00539-9
• 19. Razzak MT, Darwis D and Zainuddin S. Irradiation of polyvinyl alcohol and polyvinyl pyrrolidone blended hydrogel for wound dressing. Radiation Physics and Chemistry 2001; 62(1):107-113. http://dx.doi.org/10.1016/S0969-806X(01)00427-3
• 20. Risbud MV, Hardikar AA, Bhat SV and Bhonde RR. pHsensitive freeze-dried chitosanpolyvinyl pyrrolidone hydrogels as controlled release system for antibiotic delivery. Journal of Controlled Release 2000; 68(1):23-30. http://dx.doi.org/10.1016/S0168-3659(00)00208-X
• 21. Marsano E, Bianchi E, Vicini S, Compagnino L, Sionkowska A, Skopinska J, et al. Stimuli responsive gels based on interpenetrating network of chitosan and poly(vinylpyrrolidone). Polymer 2005; 46(5):1595-1600. http://dx.doi.org/10.1016/j.polymer.2004.12.017
• 22. Hornof M, Weyenberg W, Ludwing A and Sehnurch AB. Mucoadhesive ocular insert based on thiolated poly(acrylic acid): development and in vivo evaluation in humans. Journal of Controlled Release 2003; 89(3):419-428. http://dx.doi.org/10.1016/S0168-3659(03)00135-4
• 23. Mark JE. Polymer data handbook Oxford: Oxford University Press; 1999.
• 24. Wu YH and Freeman BD. Structure, water sorption, and transport properties of crosslinked N-vinyl-2-pyrrolidone/ N,N'-methylenebisacrylamide films. Journal of Membrane Science 2009; 344(1-2):182-189. http://dx.doi.org/10.1016/j.memsci.2009.07.050
• 25. Száraz I and Forsling W. A spectroscopic study of the solvation of 1-vinyl-2- pyrrolidone and poly(1-vinyl-2-pyrrolidone) in different solvents. Polymer 2000; 41(13):4831-4839. http://dx.doi.org/10.1016/S0032-3861(99)00705-3
• 26. Geever LM, Devine DM, Nugent MJD, Kennedy JE, Lyons JG and Higginbotham CL. The synthesis, characterization, phase behavior and swelling of temperature sensitive physically crosslinked poly(1-vinyl-2-pyrrolidinone)/ poly(N-isopropylacrylamide) hydrogels. Europe Polymer Journal 2005; 42(1):69-80. http://dx.doi.org/10.1016/j.eurpolymj.2005.09.027
• 27. Roeges NPG. A guide to the complete interpretation of infrared spectra of organic structure Nova Jersey: John Wiley & Sons; 1994.
• 28. Devine DM and Higginbotham CL. Synthesis and N-vinylpyrrolidinone (NVP) based hydrogels. European Polymer Journal 2005; 41(6):1272-1279. http://dx.doi.org/10.1016/j.eurpolymj.2004.12.022
• 29. Sen M and Yakar A. Enhancement of copolymerization of Itaconic acid with N-vinyl 2-pyrrolidone chemically and by radiation induced cross-linking. Nuclear Instruments and Methods in Physics Research B 2005; 234(3):226-234. http://dx.doi.org/10.1016/j.nimb.2005.01.002

Publication Dates

• Publication in this collection
  25 Aug 2014
• Date of issue
  Aug 2014

History

• Received
  26 June 2013
• Accepted
  23 June 2014