Fabrication of MSC-laden composites of hyaluronic acid hydrogels reinforced with MEW scaffolds for cartilage repair.

Hydrogels are of interest in cartilage tissue engineering due to their ability to support the encapsulation and chondrogenesis of mesenchymal stromal cells (MSCs). However, features such as hydrogel crosslink density, which can influence nutrient transport, nascent matrix distribution, and the stability of constructs during and after implantation must be considered in hydrogel design. Here, we first demonstrate that more loosely crosslinked (i.e., softer, ~2 kPa) norbornene-modified hyaluronic acid (NorHA) hydrogels support enhanced cartilage formation and maturation when compared to more densely crosslinked (i.e., stiffer, ~6-60 kPa) hydrogels, with a >100-fold increase in compressive modulus after 56 days of culture. While soft NorHA hydrogels mature into neocartilage suitable for the repair of articular cartilage, their initial moduli are too low for handling and they do not exhibit the requisite stability needed to withstand the loading environments of articulating joints. To address this, we reinforced NorHA hydrogels with polycaprolactone (PCL) microfibers produced via melt-electrowriting (MEW). Importantly, composites fabricated with MEW meshes of 400 m spacing increased the moduli of soft NorHA hydrogels by ~50-fold while preserving the chondrogenic potential of the hydrogels. There were minimal differences in chondrogenic gene expression and biochemical content (e.g., DNA, GAG, collagen) between hydrogels alone and composites, whereas the composites increased in compressive modulus to ~350 kPa after 56 days of culture. Lastly, integration of composites with native tissue was assessed ex vivo; MSC-laden composites implanted after 28 days of pre-culture exhibited increased integration strengths and contact areas compared to acellular composites. This approach has great potential towards the design of cell-laden implants that possess both initial mechanical integrity and the ability to support neocartilage formation and integration for cartilage repair.