Citation: Smatov, S.; Mukasheva, F.; Erisken, C. Collagen Fibril Diameter Distribution of Sheep Anterior Cruciate Ligament. Polymers 2023, 15, 752. https://doi.org/10.3390/ polym15030752 Academic Editor: Yi-Je Juang Received: 17 December 2022 Revised: 20 January 2023 Accepted: 29 January 2023 Published: 1 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Collagen Fibril Diameter Distribution of Sheep Anterior Cruciate Ligament Smail Smatov, Fariza Mukasheva and Cevat Erisken * Department of Chemical and Materials Engineering, Nazarbayev University, Astana 010000, Kazakhstan * Correspondence: cevat.erisken@nu.edu.kz Abstract: The anterior cruciate ligament (ACL) tissue is a soft tissue connecting the femur and tibia at the knee joint and demonstrates a limited capacity for self-regeneration due to its low vascularity. The currently available clinical procedures are unable to fully restore damaged ACL tissue, and tissue engineering can offer options with a potential of restoring the torn/ruptured ACL by using biomimetic constructs that are similar to native tissue in terms of structure, composition, and functions. However, a model substrate to understand how the ACL cells regenerate the injured tissue is still not available. In this study, it is hypothesized that the nanofiber-based model substrate with bimodal and unimodal fiber diameter distributions will mimic the diameter distribution of collagen fibrils seen in healthy and injured sheep ACL, respectively. The aims were to (i) create an ACL injury in a sheep ACL by applying extensional force to rupture the healthy ACL tissue, (ii) measure the collagen fibril diameter distributions of healthy and injured ACL, (iii) fabricate polycaprolactone (PCL) nanofiber-based model constructs using electrospinning with diameter distributions similar to healthy and injured ACL tissue, and (iv) measure mechanical properties of ACL tissue and PCL electrospun constructs. The results showed that the fiber diameter distributions of PCL electrospun constructs and those of the healthy and injured ACL tissues were similar. The novelty in this investigation is that the collagen fibril diameter distribution of healthy and injured sheep ACL tissues was reported for the first time. The study is significant because it aims to create a model construct to solve an important orthopedic- related clinical problem affecting millions of people globally. The model construct fabricated in this work is expected to have an important impact on ACL regeneration efforts. Keywords: ACL; sheep; collagen fibrils; diameter distribution; PCL; tissue engineering; nanofibers; biomimicry; ligament construct; electrospinning 1. Introduction ACL-related injuries represent a significant portion of musculoskeletal-related injuries, with around 4 million annual cases worldwide [1,2]. Additionally, this number shows an increasing trend, especially among the professional sportsmen including football players, skiers, and athletes [3,4]. Prior reports demonstrate that ACL injuries can potentially cause osteoarthritis [1] and secondary complications such as structural changes in cartilage and meniscus because the ACL has a limited potential of healing itself due to its low cellularity and vascularity [5]. The gold standard for ACL repair involves reconstruction of the ruptured ACL with natural and synthetic grafts but only at the expense of added morbidity and pain. Synthetic grafts are plausible options because their structural and morphological properties can be tuned to meet the needs. However, these grafts lack a significant physiologic property of the native ACL tissue, such as the organization and diameter distribution of collagen fibrils. The objective of this study is, therefore, to create a nanofiber-based model to be later used as a construct for investigating ACL regeneration mechanism. ACL tissue consists of water (~65% by weight) and solid materials. The biochem- ical composition of solid structure (~75% w/w) is Col-I (85%), Col-III, -VI, -V, -XI, and -XIV. The other 25% of solids contains proteoglycans (<1%), elastins, and glycoproteins [6]. Polymers 2023, 15, 752. https://doi.org/10.3390/polym15030752 https://www.mdpi.com/journal/polymers