Introduction: To mimic, in a synthetic material, the interface between distinctly different biological tissues, the mechanical and chemical properties of a hydrogel must be spatially controlled. Three-dimensional (3D) printing is a technique often employed to gain this control, but it is difficult to print both mechanically robust (high modulus) and biocompatible (low modulus) structures due to fast species diffusion within hydrogels[1]. Here we report on a biocompatible 3D structured hydrogel with spatially varying mechanical properties using a single photoclickable poly(ethylene glycol) (PEG) precursor solution[2]. We obtain spatially distinct material properties using only a single precursor through a sequential process of repeated precursor in-diffusion and patterned light exposures.
Materials and Methods: Hydrogels were formed using 5, 10, and 20 wt% monomer [8-arm PEG norbornene (10kDa) and PEG dithiol (1kDa) at 0.5:1 thiol:ene ratio] and biocompatible 0.05 wt% LAP photoinitiator via photopolymerization (6 minutes, 405 nm, 13 mW/cm2, room temperature)[3]. To characterize bulk properties, 3mm by 2mm cylindrical hydrogels were swollen to equilibrium in a fresh precursor solution and then subsequently exposed to light. The process was repeated up to 10 cycles, referred to as a diffusion and exposure (DE) cycle. The hydrogels (n=3-4) were subjected to unconfined compression until failure and the compressive modulus of each gel was determined at 15% strain. The area under the stress-strain curve was measured to determine toughness. The hydrogel were spatially patterned using a stereolithography printer.
Results: The modulus increases after each DE cycle, reaching a plateau after 2-4 cycles, depending on monomer concentration. Hydrogel toughness shows a similar behavior, exhibiting a distinct maximum after 2-4 cycles. Fracture stress (not shown) was increased up to a factor of five over hydrogels with the same compressive modulus that did not undergo repeated DE cycles. After bulk properties were established, spatially photopatterned hydrogels were fabricated with using the same DE process via a stereolithographic printer and achieved 200µm transverse resolution imaged on a DIC microscope (not shown).
Discussion and Conclusion: This study demonstrates that repeated precursor in-diffusion and exposure cycles affect both modulus and toughness of PEG hydrogels through a process spatially controlled with optical exposure. Because the hydrogels are formed at a high solvent concentration and off stoichiometry, there will be non-idealities in the network structure that result in free thiols and free ‘enes’. We hypothesize during the first 2-4 DE cycles, the remaining thiols and ‘enes’ in the first network further react creating a more densely crosslinked network, evident by the increase in modulus, while simultaneously a second network begins to form, leading to toughness increase. Beyond 2-4 DE cycles, the compressive modulus stabilizes whereas toughness declines. The latter is attributed to the stretching of bonds within the gel network, reducing their ability to absorb and dissipate additional strain energy[4]. Patterned gels were fabricated and their material properties are being characterized using a Hysitron nanoindenter. Photopatterning mechanical properties using a single biocompatible material will enable the study of cellular reaction to local changes to their extracellular environment.
National Science Foundation Graduate Research Fellowship (DGE 1144083); NSF 0847390; NIH SBIR 1R43MH102946-01
References:
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