Chondroitin Sulfate Conjugated Nanofiber Shish Kebabs as Biomimetic Bone Templates
Authors:   , Yu Tony , Li Christopher Y. , Marcolongo Michele 
Published: July 2021

Bone defects affect over 2.2 million people worldwide through diseases, injuries, aging, or the combination of these. The gold standards for bone defects are autografts harvested generally from the iliac crest or the hip bone of the patient. The use of autografts is limited and require additional surgery, which increases the risk of infection and donor site morbidity. Although there is no immune or compatibility issue, some autografts still fail due to non-union of bone and related complications. There is a medical need to synthesize polymeric bone grafts that can perform similarly if not better than bone autografts. To this end, we have previously fabricated a polycaprolactone (PCL) nanofiber shish kebab (NFSK) template as synthetic bone scaffolds via polymer crystallization of a block copolymer of PCL-b-PAA. The novelty of this work is the ability to control the mineral crystal orientation and spatial location on the nanofiber, which mimics the molecular structure of bone or mineralized collagen fibrils. The objective of this thesis is two-fold: 1) to investigate the cell to biomaterial surface interaction provided by the unique NFSK surface features and 2) to use the NFSK platform to design biomimetic bone templates. The advantage of using NFSK templates is ease of modifying the template surface with different functional groups and biomimetic nature of NSFK-templated mineralization. As a result, the surface roughness and chemistry can be manipulated and designed towards an osteogenic microenvironment. For these reasons, it was hypothesized that the NFSK templates will promote cell differentiation and marker expression of mineralization of MC3T3 E1 pre-osteoblast cells. In the first part of the dissertation, NFSK templates were used to investigate the topological effect, e.g. fiber alignment and kebab size, on pre-osteoblast cell proliferation and ALP activity, which the latter is an important enzyme related to bone mineralization and osteogenesis. Aligned nanofiber with larger kebab size increased both ALP activity and cell proliferation because of the surface roughness of NFSK. NFSK templates were then mineralized in simulated body fluid to mimic mineralized collagen fibrils, which was showed to increase ALP and cell proliferation as well. Interestingly, the kebab period did not influence proliferation or ALP activity when mineralized indicating that surface chemistry played a more dominant role than surface roughness. The NFSK templates was further designed to also mimic the bone matrix by conjugating chondroitin sulfate (CS) onto the PAA region of the kebab. CS is an important glycosaminoglycan that is important in collagen fiber organization in the bony matrix. CS-NFSK templates were characterized using FTIR and mass balance, which showed formation of an amide bone between the CS amine group and the PAA carboxylic acid group and increases in the template mass after conjugation. Thermal analysis and contact angle also showed difference in thermal degradation, heat flow, and hydrophilicity. Lastly, CS-NFSK templates promoted ALP activity and cell proliferation compared to the control. Osteoblast gene expression including RUNX2, ALP, COL1, and BGLAP were also upregulated in the CS-NFSK templates indicating mineralization and formation of matrix. For the first time, CS-NFSK were molecularly engineered to mimic the bone structure and matrix, which showed promise as a biomimetic bone template.