Hyaluronic acid-based shape memory cryogel scaffolds for focal cartilage defect repair

Abstract

Purpose: Traumatic joint injuries can result in focal cartilage defects that regenerate poorly due to a lack of blood supply and low chondrocyte density. Cell-laden hydrogel scaffolds are under study but provide little flexibility in modulating their pore size, interconnectivity, swelling and stiffness, properties required for a conducive biomimetic environment. In this study, we engineered an injectable and biomimetic shape memory hyaluronic acid (HA)-based cryogel scaffolds with a macroporous and highly interconnected network in three-dimensions seeded with chondrocytes. Due to their unique ability to withstand up to 90% strains and rapidly recover their shape back, they can be arthroscopically delivered to fill up focal chondral defects for cartilage repair. Here we studied whether chondrocytes encapsulated within cryogels remain viable and metabolically active following injection and provide a superior micro-environment than conventional (nanoporous) hydrogels for cartilage regeneration. We also investigated how RGD-functionalized cryogel affects chondrocyte adhesion, migration and proliferation. Methods: Synthesis of HA-based Cryogels and Hydrogels: HA cryogels were prepared using HA modified with glycidyl methacrylate (HAGM) and either Acryl-PEG-G4RGDS to synthesis gels functionalized with RGD (Cryo RGD+) or Acryl-PEG-methoxy to synthesize gels without RGD (Cryo RGD-) in deionized (DI) water. The mixture was cast in 6 mm diam x 1.5 mm cylindrical discs using APS/TEMED as our initiator system. The samples were frozen at -20°C and allowed to cryopolymerize overnight. Next, cryogels were brought to RT and washed/sanitized prior to cell seeding. HA hydrogels (Hydro RGD+) were fabricated via photopolymerization by mixing HAGM, RGD, and lithium phenyl-2,4,6- trimethylbenzoylphosphinate (LAP) photo-initiator in DI water. Chondrocyte seeding and in-vitro culture:Chondrocytes harvested from calf knee femoral condyle cartilage were seeded onto each scaffold and cultured for 15 days. Injectability test: Cell-laden cryogels were incubated overnight to enable complete cell adhesion and subsequently syringe injected through a 16-gauge needle before incubation for another 24h following which cell viability and metabolism were quantified. Biochemical analysis: At the end of 15-day culture, gel samples were digested to quantify the cell metabolism (Alamar Blue), DNA (PicoGreen) and GAG (DMMB) content. Cell viability and cytoskeleton staining: Gel samples were fixed and stained for nucleus, actin, dead cells and confocal imaged at 40x. Results: Shape memory HA-based cryogels fully recovered their shape back following syringe injection through a 16-gauge needle without impacting cell metabolism or viability (Figs 1A-C). Scanning electron microscopy (SEM) images showed an average pore size of 49.2 ± 15 μm (Fig 1D) rendering a macroporous structure with high interconnectivity, which was 7x higher than hydrogels (Fig 1E). This unique macrostructural architecture enables efficient transport of nutrients and metabolites thereby offering a superior microenvironment for chondrocytes; both cell metabolism and GAG content was 2x higher in cryogels than hydrogels after 15-day culture (Figs 1F-G). Additionally, a significant amount of GAG (∼70 μg) was released from both cryogels into media owing to their macroporous structure; no released GAG was measured in media from Hydro RGD+ (Fig 1G). The presence of RGD (cell-adhesion peptide) did not improve cell adhesion or biosynthesis rates. Chondrocytes formed larger organoid-like structures in Cryo RGD- while remaining homogenously dispersed in the presence of RGD in Cryo RGD+ (Fig 1H). Conclusions: Syringe injectable HA-based shape memory cryogels provide a conducive microenvironment for chondrocyte adhesion, proliferation and matrix biosynthesis for use in repair of cartilage defects.