RESOURCES

Abstract
Reinforced hydrogels represent a promising strategy for tissue engineering of articular cartilage. They can recreate mechanical and biological characteristics of native articular cartilage and promote cartilage regeneration in combination with mesenchymal stromal cells. One of the limitations of in vivo models for testing the outcome of tissue engineering approaches is implant fixation. The high mechanical stress within the knee joint, as well as the concave and convex cartilage surfaces, makes fixation of reinforced hydrogel challenging. Methods. Different fixation methods for full-thickness chondral defects in minipigs such as fibrin glue, BioGlue®, covering, and direct suturing of nonenforced and enforced constructs were compared. Because of insufficient fixation in chondral defects, superficial osteochondral defects in the femoral trochlea, as well as the femoral condyle, were examined using press-fit fixation. Two different hydrogels (starPEG and PAGE) were compared by 3D-micro-CT (μCT) analysis as well as histological analysis. Results. Our results showed fixation of below 50% for all methods in chondral defects. A superficial osteochondral defect of 1 mm depth was necessary for long-term fixation of a polycaprolactone (PCL)-reinforced hydrogel construct. Press-fit fixation seems to be adapted for a reliable fixation of 95% without confounding effects of glue or suture material. Despite the good integration of our constructs, especially in the starPEG group, visible bone lysis was detected in micro-CT analysis. There was no significant difference between the two hydrogels (starPEG and PAGE) and empty control defects regarding regeneration tissue and cell integration. However, in the starPEG group, more cell-containing hydrogel fragments were found within the defect area. Conclusion. Press-fit fixation in a superficial osteochondral defect in the medial trochlear groove of adult minipigs is a promising fixation method for reinforced hydrogels. To avoid bone lysis, future approaches should focus on multilayered constructs recreating the zonal cartilage as well as the calcified cartilage and the subchondral bone plate.

Abstract
Recent research has been focusing on the generation of living personalized osteochondral constructs for joint repair. Native articular cartilage has a zonal structure, which is not reflected in current constructs and which may be a cause of the frequent failure of these repair attempts. Therefore, we investigated the performance of a composite implant that further reflects the zonal distribution of cellular component both in vitro and in vivo in a long-term equine model. Constructs constituted of a 3D-printed poly(ϵ-caprolactone) (PCL) bone anchor from which reinforcing fibers protruded into the chondral part of the construct over which two layers of a thiol-ene cross-linkable hyaluronic acid/poly(glycidol) hybrid hydrogel (HA-SH/P(AGE-co-G)) were fabricated. The top layer contained Articular Cartilage Progenitor Cells (ACPCs) derived from the superficial layer of native cartilage tissue, the bottom layer contained mesenchymal stromal cells (MSCs). The chondral part of control constructs were homogeneously filled with MSCs. After six months in vivo, microtomography revealed significant bone growth into the anchor. Histologically, there was only limited production of cartilage-like tissue (despite persistency of hydrogel) both in zonal and non-zonal constructs. There were no differences in histological scoring; however, the repair tissue was significantly stiffer in defects repaired with zonal constructs. The sub-optimal quality of the repair tissue may be related to several factors, including early loss of implanted cells, or inappropriate degradation rate of the hydrogel. Nonetheless, this approach may be promising and research into further tailoring of biomaterials and of construct characteristics seems warranted.

Abstract
Abstract This study investigates the use of allyl-functionalized poly(glycidol)s (P(AGE-co-G)) as cytocompatible cross-linker for thiol-functionalized hyaluronic acid (HA-SH) and the optimization of this hybrid hydrogel as bioink for 3D bioprinting. Chemical cross-linking of gels with 10 wt.% overall polymer concentration was achieved by UV-induced radical thiol-ene coupling between the thiol and allyl groups. Addition of unmodified high molecular weight HA (1.36 MDa) allowed tuning of the rheology for extrusion based bioprinting. Incorporation of additional HA resulted in hydrogels with lower Young’s modulus and higher swelling ratio especially in the first 24 h, but a comparable equilibrium swelling for all gels after 24 h. Embedding of human and equine mesenchymal stem cells (MSCs) in the gels and subsequent in vitro culture showed promising chondrogenic differentiation after 21 d for cells from both origins. Moreover, cells could be printed with these gels, and embedded hMSCs showed good cell survival for at least 21 d in culture. To achieve mechanical stable and robust constructs for the envisioned application in articular cartilage, the formulations were adjusted for double printing with the thermoplastic poly--caprolactone (PCL).

Abstract
In this study we introduce linear poly(glycidol) (PG), a structural analog of poly(ethylene glycol) bearing side chains at each repeating unit, as polymer basis for bioink development. We prepare allyl- and thiol-functional linear PG that can rapidly be polymerized to a three-dimensionally cross-linked hydrogel network via UV mediated thiol-ene click reaction. Influence of polymer concentration and UV irradiation on mechanical properties and swelling behavior was examined. Thiol-functional PG was synthesized in two structural variations, one containing ester groups that are susceptible to hydrolytic cleavage, and the other one ester-free and stable against hydrolysis. This allowed the preparation of degradable and non-degradable hydrogels. Cytocompatibility of the hydrogel was demonstrated by encapsulation of human bone marrow-derived mesenchymal stem cells (hBMSCs). Rheological properties of the hydrogels were adjusted for dispense plotting by addition of high molecular weight hyaluronic acid. The optimized formulation enabled highly reproducible plotting of constructs composed of 20 layers with an overall height of 3.90 mm.