Development of Photocurable Degradable Elastomers for Biomedical Applications

1. Jeffrey M. Karp, Christiaan L.E. Nijst,Joost P. Bruggeman, Lino S. Ferreira, Andreas Zumbuehl, Christopher J. Bettinger, Robert Langer Development of Photocurable Degradable Elastomers for Biomedical Applications • Summary • There is considerable need and interest to develop tough biodegradable elastomers, which exhibit mechanical properties similar to that of soft tissue. Typical degradable elastomers are cured at 120 oC in vacuo for 24h, which limits their applications. Herein we demonstrate the ability to rapidly form complex structures from acrylated poly(glycerol sebacate) (PGS) under ambient conditions. The mechanical and degradation properties of this new photocurable elastomer can be precisely controlled by varying the density of acrylate moieties in the matrix of the polymer, and through incorporation of acrylated polyethylene glycol (PEG). • 1H-NMR confirms acrylation of PGS polymer matrix, tensile tests showed control of Young’s modulus between 0.05 to 1.38 MPa, ultimate strength from 0.06 to 0.47 MPa and elongation at break between 42 and 189% strain. Combining di-acrylated PEG with acrylated-PGS resulted in a ten fold increase in Young’s modulus and ultimate strength compared to a typical PEG hydrogel, while maintaining its elongation at break. The development of multifunctional photocurable degradable elastomers with precisely controlled material properties is an important step towards multiple medical applications. • Objectives • Incorporate acrylate groups in PGS pre-polymer • Characterize photocurable degradable elastomers 5. Copolymerization of PEGDA and PGSA One of the main advantages of starting with PGSA is that it can be easily combined with acrylated hydrogel precursors to impart mechanical, biodegradable, and swelling properties that are not normally associated with typical hydrogel materials (Fig. 5) [5]. Most hydrogel materials are very fragile and have poor mechanical properties. For example, a hydrogel formed from 20% (w/w) poly(ethylene glycol) diacrylate (700 Da) in water exhibits an elongation of 14%, Young’s modulus of 0.54 MPa and ultimate strength of 0.063 MPa. Through combining PEGDA with PGSA (DA=0.34), the Young’s modulus, ultimate strength, elongation and swelling ratio can be precisely controlled (Fig. 5). Specifically, through increasing the concentration of PGSA, the elongation ranged from 4 to 60%, Young’s modulus from 20 to 0.6 MPa and ultimate strength from 0.890 to 0.270 MPa. The networks formed by the copolymerization of PEGDA with PGSA (DA=0.34) (50:50) showed a ten fold higher Young’s modulus and ultimate strength than the typical PEGDA hydrogel while maintaining its elongation. Furthermore, the swelling behavior of these networks can be tuned from 40% to 10% through changing the concentration of PGSA from 10% to 90%. The chemical composition was determined by calculating the signal intensities of –COCH2CH2-CH2- at 1.2, 1.5, 2.2 ppm for the sebacic acid, -CH2-CH- at 3.7, 4.2 and 5.2 ppm for glycerol and –CH=CH2 at 5.9 ppm, 6.1 ppm and 6.5 ppm for the protons on the acrylate groups. The signal intensity of the methylene groups of the sebacic acid (1.2 ppm) and the acrylate groups (average signal intensity of 5.9, 6.1 and 6.5 ppm) were used to calculate the degree of acrylation (DA). 3. Mechanical properties of PGSA The mechanical properties of the photocured PGSA spanned from soft to relatively stiff as determined by the tensile Young's modulus of the polymer, which varied from 0.05 MPa (DA=0.17) to 1.38 MPa (DA=0.54). This demonstrates the potential to achieve the mechanical compliance of the peripheral nerve which has a Young’s modulus of approximately 0.45 MPa [2] and of the thoracic aorta which has a Young’s modulus of 0.53 MPa [3]. The ultimate tensile strength ranged from 0.06 MPa to 0.47 MPa whereas the strain to failure of photocured PGSA ranged from 189% to 42% with increasing DA. 6. Fabrication of different morphologies PGSA was used to manufacture microparticles (Fig. 6), which was previously not possible with PGS due to the processing conditions (thermal curing). These elastic microparticles might potentially be useful for the controlled release of drugs in joints or other mechanically dynamic environments (36). Thin walled tubes (Fig. 6), inner diameter of 1 mm and 0.20 mm wall thickness for small-diameter vascular and nerve guide grafts were made. Also, micropatterned surfaces (Fig. 6), and porous scaffolds (Fig. 6) were made with existing methods [6, 7]. These may be useful for the development of prefabricated vascular networks and tissue engineered scaffolds, respectively. 1. Polymer synthesis To fabricate photocurable biodegradable elastomers at room temperature, a three step process was employed. In the first step, a pre-polymer from glycerol and sebacic acid was created as previously described (Fig. 1A) [1]. In a second step, functional hydroxyl groups on backbone of the PGS pre-polymer were acrylated and subsequently purified (Fig. 1B). In a third step, the PGS-acrylate (PGSA) was polymerized with UV light in the presence of a photoinitiator. This elastomer is referred to as photocured PGSA. 4. In vitro biocompatibility and degradation In vitro cell culture show that the photocured elastomers support similar levels of cell adhesion to thermally cured PGS. 59 12% of the human foreskin fibroblasts cells seeded on photocured PGSA and 56  14% on PGS attached as demonstrated by nuclei staining at 4 hours (Fig 4). Although cell adhesion was lower than the one observed for polystyrene (PS) culture dishes, the results show that photocured PGSA can function as a cell adhering biomaterial. Ultimately, the presence of functional hydroxyls could permit the modulation of cell response through conjugation with cell adhesive peptides such as RGD. Photocured PGSA elastomeric networks are degradable at physiologic conditions. All polymers displayed a linear hydrolytic degradation profile over time in PBS, with 9-12% degradation occurring after 10 weeks (Fig 4) (2). This is slower than the degradation of thermally cured PGS as previously reported (17% degradation over 9.5 weeks) [1]. Given that the crosslink density is proportional to the mechanical strength (Fig. 3) and to the degradation rate (Fig 4A), decoupling the mechanical and degradation properties presents a challenge. Likely, this can be overcome by copolymerization with other acrylated (hydrophilic) macromolecules [4]. Conclusion We describe the creation of photocurable elastomers which encompasses functional hydroxyl groups, easily tunable mechanical properties, and rapid room temperature UV-curing. The liquid acrylated polymer precursor can be combined with other acrylated molecules to improve mechanical properties and control swelling behavior of typical hydrogel materials. The development of multifunctional photocurable degradable elastomers with precisely controlled material properties could lead to new materials for potential multiple medical applications. • Acknowledgements • C.N. acknowledges the financial support of Dr. Saal van Zwanenberg stichting, Vreede stichting, Shell and KIVI. J.M.K. is supported by a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship. J.P.B. acknowledges J.F.S. Esser Stichting and Stichting Michael-Van Vloten Fonds, L.F acknowledges the financial support of Fundação para a Ciência e a Tecnologia (SFRH/ BPD/14502/2003), A.Z. acknowledges the financial support from the Swiss National Science Foundation. This work was funded by NIH R01 DE13023, NIH HL060435 and NSF NIRT 0609182. • References • [1]. Wang Y, Ameer GA, Sheppard BJ, Langer R. A tough biodegradable elastomer. Nat Biotechnol 2002;20(6):602-6. • [2]. Rydevik BL, Kwan MK, Myers RR, Brown RA, Triggs KJ, Woo SL, et al. An in vitro mechanical and histological study of acute stretching on rabbit tibial nerve. J Orthop Res 1990;8(5):694-701. • [3]. O'Rourke MF, Staessen JA, Vlachopoulos C, Duprez D, Plante GE. Clinical applications of arterial stiffness; definitions and reference values. Am J Hypertens 2002;15(5):426-44. • [4]. Yang J, Webb AR, Ameer GA. 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