Reducing the foreign body response (FBR) to implanted biomaterials will enhance their performance in tissue engineering. Poly(ethylene glycol) (PEG) hydrogels are increasingly popular for this application due to their low cost, ease of use, and the ability to tune their compliance via molecular weight and crosslinking densities. PEG hydrogels can elicit chronic inflammation in vivo, but recent evidence has suggested that extremely hydrophilic, zwitterionic materials and particles can evade the immune system. To combine the advantages of PEG-based hydrogels with the hydrophilicity of zwitterions, we synthesized hydrogels with co-monomers PEG and the zwitterion phosphorylcholine (PC). Recent evidence suggests that stiff hydrogels elicit increased immune cell adhesion to hydrogels, which we attempted to reduce by increasing hydrogel hydrophilicity. Surprisingly, hydrogels with the highest amount of zwitterionic co-monomer elicited the highest FBR we observed. Lowering the hydrogel modulus (165 kPa to 3 kPa), or PC content (20 wt% to 0 wt%), mitigated this effect. A high density of macrophages was found at the surface of implants associated with a high FBR, and mass spectrometry analysis of the proteins adsorbed to these gels implicated extracellular matrix, immune response, and cell adhesion protein categories as drivers of macrophage recruitment to these hydrogels. Overall, we show that modulus regulates macrophage adhesion to zwitterionic-PEG hydrogels, and demonstrate that chemical modifications to hydrogels should be studied in parallel with their physical properties to optimize implant design.

This study investigated the effects of introducing hydroxyapatite nanoparticles into a matrix metalloproteinase (MMP) sensitive poly(ethylene glycol) (PEG) hydrogel containing cell adhesion peptides of RGD for bone tissue engineering. MC3T3-E1 pre-osteoblasts were encapsulated in the biomimetic PEG hydrogel, which was formed from the photoclick thiol-norbornene reaction system, cultured for up to 28 d in growth medium or osteogenic differentiation medium, and evaluated by cellular morphology and differentiation by alkaline phosphatase (ALP) activity and bone-like extracellular matrix deposition for mineral and collagen. Hydroxyapatite nanoparticles were incorporated during hydrogel formation and cell encapsulation at 0%, 0.1% or 1% (w/w). Incorporation of hydroxyapatite nanoparticles did not affect the hydrogel properties as measured by compressive modulus and equilibrium swelling. In growth medium, encapsulated MC3T3-E1 cells remained largely round regardless of hydroxyapatite concentration. ALP activity increased by 25% at day 14 and total collagen content increased by 55% at day 28 with increasing hydroxyapatite concentration from 0% to 1%. In differentiation medium, cell spreading was evident regardless of hydroxyapatite indicating that the MC3T3-E1 cells were able to degrade the hydrogel. For the 1% hydroxyapatite condition, ALP activity was 27% higher at day 14 and total collagen content was 22% higher at day 28 in differentiation medium when compared to growth medium. Mineral deposits were more abundant and spatial elaboration of collagen type I was more evident in the 1% (w/w) hydroxyapatite condition with differentiation medium when compared to all other conditions. Overall, osteogenesis was observed in the hydrogels with hydroxyapatite nanoparticles in growth medium but was enhanced in differentiation medium. In summary, a biomimetic hydrogel comprised of MMP-sensitive crosslinks, RGD cell adhesion peptides, and 1% (w/w) hydroxyapatite nanoparticles is promising for bone tissue engineering.