Biomimetic Scaffolds for Ligament Tissue Engineering Applications

Abstract

Ligament is the soft tissue that connects bone to bone and, in case of severe injury or rupture, it cannot heal itself mainly because of its poor vascularity and dynamic nature. High failure rates of surgical treatment and significant drawbacks of currently available medical approaches brought about a need for alternative treatment approaches such as tissue engineering. It carries the potential to restore the injured tissue functions by utilization of scaffolds mimicking the structure of native ligament tissue. As the initial structural unit of the ligament, collagen fibrils have a diameter ranging from 20 to 150 nm, which defines the cellular topography, physical, and mechanical properties of the tissue. Currently, ability to fabricate scaffolds with relevant fiber diameter in this range is a significant challenge. The literature review showed a scarcity in terms of bioscaffolds that mimic the foundational unit of tissue consisting of ultra-thin nanofibers with diameter not exceeding 200 nm. To cover the gap, this work aims at: i) investigating the conditions for the fabrication of sub-100 nm fibers, and ii) fabricating aligned scaffolds with bimodal diameter distribution (with two-peaks) resembling the healthy Anterior Cruciate Ligament (ACL) tissue structure, and unaligned scaffolds with unimodal diameter distribution (with a single peak) representing structure of injured ACL tissue. It is hypothesized that such scaffolds can be produced from electrospun polycaprolactone (PCL) solutions in the form of unimodal and bimodal diameter distributions. For testing this hypothesis, various PCL solutions were formulated in acetone and formic acid in combination with pyridine, and electrospun to generate sub-100 nm fibers. Next, this formulation was adjusted for the production of nanofibers with a diameter of greater than 100 nm. Finally, these solutions were combined in the co-electrospinning, i.e., two-spinneret electrospinning, process to fabricate biomimetic scaffolds with bimodal and unimodal diameter distribution. Findings revealed that electrospinning of 8% and 15% PCL solutions, respectively, resulted in the production of fibers with diameters below and above 100 nm. The combined scaffold exhibited bimodal distribution of aligned fibers with the peaks around 60-80 and 160-180 nm, thus mimicking the collagen fibril diameter distribution seen in healthy ACL tissue. To the best of my knowledge, this is the first study using the system of acetone and formic acid in combination with pyridine for the production of PCL nanofibers, especially sub-100 nm ultra-thin nanofibers. Another novelty is the fabrication of scaffold with a bimodal distribution to both qualitatively and quantitatively match the distribution of collagen fibrils seen in healthy ACL tissue. The biomaterial scaffold fabricated here could be used as a foundation for the development of grafts and has the potential to move the ligament tissue engineering field forward. Moreover, the study outcomes can be applied to the design of not only ligament but also other soft tissue grafts such as tendons and muscles. Therefore, this research is expected to have a society-wide impact because it aims at healing, enhancement of the health condition and life quality of a wide range of patients.