Introduction to Cartilage repair using electrospinning

Electrospinning has been used to construct scaffolds for various tissue regeneration applications since the early 2000s. While electrospun scaffolds have been tested widely in bone regeneration, the cartilage has proven to be more challenging. As with many other tissues and organs, electrospun membrane has been shown to be able to support proliferation and differentiation of stem cells into chondrocyte lineage under the right conditions [Li et al 2005; Wise et al 2009, Xin et al 2007]. For clinical application, there are many other factors to consider. One important area of consideration is the interface between the bone and cartilage.

In osteochondral repair, part of the graft lies within the bone below the cartilage region while the other part remains on the surface for cartilage regeneration. To cater for the requirements of both regions, a hybrid graft comprising two different materials may be used. The bone interface area is typically made out of a harder material while the upper cap is made out of softer material for cartilage regeneration. Liu et al (2015) constructed a nanofiber yarn-collagen type I/hyaluronate hybrid/TCP biphasic scaffold and tested its potential in osteochondral repair using a rabbit model with the defect at the patellar groove of the distal femur. Comparison was made with sponge-cap which is made out of cartilage extracellular matrix components by freeze-drying. Bone marrow derived stem cells which were undifferentiated or differentiated in chondrogenic medium were seeded in both nanofiber yarn cap or sponge cap scaffolds as part of the study groups. Nanofiber yarn caps with differentiated cells seeded showed cell morphology in the regenerated cartilage almost identical to that of native host cartilage. The sponge cap scaffold also showed a smooth repaired surface that is well integrated with the host cartilage. However, the nanofiber yarn cap showed three-fold greater compressive strength than the sponge cap and the presence of GAG in the middle of the defects is low.

Another use of electrospun membrane in cartilage regeneration is to form a barrier between cartilage and bone interface. A typical electrospun membrane has very small pore size between the interfiber spaces. This restricts cell migration across the electrospun membrane which can be used for cell separation. Mellor et al (2020) used multi-phasic 3D-bioplotting to construct the main body of the scaffold for cartilage and bone repair. In the part meant for cartilage regeneration, decellularized bovine cartilage extracellular matrix (dECM) hydrogel was injected to the 3D-bioplotted poly(ε-caprolactone) (PCL) scaffold. For bone regeneration, 20% β-tricalcium phosphate (TCP)/ 80% PCL was 3D-bioplotted. An electrospun PCL membrane layer was added between the two 3D-bioplotted scaffolds. This electrospun PCL membrane layer is needed to inhibit cell migration between the scaffold layers. In a clinical setting, this layer will also prevent blood vessels from invading the chondrogenic portion of the scaffold. In vitro study using human adipose-derived stem cells (hASC) showed that the electrospun membrane was able to effectively separate the cell populations while evidence of site-specific hASC osteogenesis and chondrogenesis was observed.

Electospun membrane as a barrier may be used in the construction of niches. The relative ease of loading drugs into electrospun fibers may also be utilized to release drugs within the niche. Ji et al (2021) constructed an avascular niche made of axitinib-loaded PCL/collagen nanofibrous membrane for implantation into subcutaneous environments in order to maintain cartilaginous phenotype of mesenchymal stromal cells (MSCs). The loading and release of axitinib in the niche is vital to inhibit angiogenic activity since vascular invasion is a prerequisite for endochondral ossification. In vivo study of the niche seeded with MSCs implanted subcutaneously into nude mice showed that samples maintained their cartilage features even after 24 weeks of subcutaneous implantation in the 3% Axitinib and 6% Axitinib groups. However, samples in the groups with 1% Axitinib and 0% Axitinib begin to show bone-like tissues with partial fibrosis after 12 weeks. Investigations into the gene expression of 3% Axitinib group at 24 weeks in vivo showed expression of genes related to chondrogenesis (Col2a1 and Aggrecan) and the expression of antiangiogenic gene (Chm-I) was upregulated and others that favor the stabilization of chondrocyte phenotype.

Micro-CT evaluation and chondrogenesis assessment of BEC at 24 weeks postimplantation. a) Representative 2D and 3D images of BEC by micro-CT in different groups. Scale bar = 1 mm. The bone volume fraction b) and mineral density c) of BEC. Values represented mean + SD, n = 3, **P < 0.01, ***P < 0.001. Quantification of cartilage-specific extracellular matrix GAG d) and Collagen II e) in different groups. BEC before implantation was used as a basic control, and native auricular cartilage was used as a positive control. Values represented mean + SD, n = 3, **P < 0.01, ***P < 0.001 [Ji et al 2021].