bioengineering Review Collagen-Based Electrospun Materials for Tissue Engineering: A Systematic Review Britani N. Blackstone 1, Summer C. Gallentine 2 and Heather M. Powell 1,2,3,* 1 Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA; [email protected] 2 Department of Biomedical Engineering, The Ohio State University, Columbus, OH 43210, USA; [email protected] 3 Research Department, Shriners Hospitals for Children, Cincinnati, OH 45229, USA * Correspondence: [email protected] Abstract: Collagen is a key component of the extracellular matrix (ECM) in organs and tissues throughout the body and is used for many tissue engineering applications. Electrospinning of collagen can produce scaffolds in a wide variety of shapes, fiber diameters and porosities to match that of the native ECM. This systematic review aims to pool data from available manuscripts on electrospun collagen and tissue engineering to provide insight into the connection between source material, solvent, crosslinking method and functional outcomes. D-banding was most often observed in electrospun collagen formed using collagen type I isolated from calfskin, often isolated within the laboratory, with short solution solubilization times. All physical and chemical methods of crosslinking utilized imparted resistance to degradation and increased strength. Cytotoxicity was observed at high concentrations of crosslinking agents and when abbreviated rinsing protocols were utilized. Collagen and collagen-based scaffolds were capable of forming engineered tissues in vitro Citation: Blackstone, B.N.; and in vivo with high similarity to the native structures. Gallentine, S.C.; Powell, H.M. Collagen-Based Electrospun Keywords: collagen; electrospinning; tissue engineering Materials for Tissue Engineering: A Systematic Review. Bioengineering 2021, 8, 39. https://doi.org/ 10.3390/bioengineering8030039 1. Introduction Collagen, specifically type I, is a major constituent of many tissues and organs, includ- Academic Editor: Gou-Jen Wang ing skin, bone, tendon, blood vessels, and cardiac tissue. As a result, collagen type I matrices are often used as a surrogate extracellular matrix (ECM) for in vitro tissue engineering and Received: 22 February 2021 in vivo tissue regeneration or repair [1–5]. Given the fibrous nature of the native ECM, Accepted: 12 March 2021 electrospinning, a technique that creates matrices comprised of nanometric or micron-sized Published: 18 March 2021 fibers, is commonly utilized to generate scaffolds for tissue engineering [6–9]. Scaffold properties, such as fiber diameter, porosity, interfiber distance, and fiber organization can Publisher’s Note: MDPI stays neutral be altered via changes in spinning parameters (polymer, solvent, solution concentration, with regard to jurisdictional claims in published maps and institutional affil- needle-ground distance, applied voltage), leading to a wide variety of scaffold architec- iations. tures that can be manufactured to mimic the native ECM structure [10,11]. Additionally, mechanical properties and resistance to degradation can be tuned via crosslinking and/or through the incorporation of higher strength materials to the spinning process [12–16]. With the vast literature available on electrospun collagen materials, it can be challeng- ing to determine the most critical fabrication parameters, the connection between these Copyright: © 2021 by the authors. parameters and functional outcomes, and subsequently, the level of biomimicry required Licensee MDPI, Basel, Switzerland. for successful tissue regeneration. In addition, there are conflicting reports regarding the This article is an open access article ultrastructure of electrospun collagen materials [17,18], the toxicity of chemical crosslinkers distributed under the terms and conditions of the Creative Commons used to stabilize the matrices [19,20] and mechanical properties of these matrices [21,22]. Attribution (CC BY) license (https:// Thus, the objective of this systematic review was to examine the literature on electrospun creativecommons.org/licenses/by/ collagen materials for tissue engineering to examine trends in source materials, electro- 4.0/). Bioengineering 2021, 8, 39. https://doi.org/10.3390/bioengineering8030039 https://www.mdpi.com/journal/bioengineering Bioengineering 2021, 8, x FOR PEER REVIEW 2 of 17 Bioengineering 2021, 8, 39 2 of 16 on electrospun collagen materials for tissue engineering to examine trends in source ma- terials, electrospinning parameters and crosslinking techniques and to determine if any spinning parameters and crosslinking techniques and to determine if any connections exist connections exist between these parameters and functional outcomes. between these parameters and functional outcomes. 2.2. Materials Materials and and Methods Methods A systematic review of the literature published on electrospun collagen materials for A systematic review of the literature published on electrospun collagen materials for tissuetissue engineering engineering applications waswas conducted. conducted. PubMed PubMed and and Web Web of Science of Science were searchedwere searched for “electrospun collagen” AND “tissue engineering” with duplicates removed. for “electrospun collagen” AND “tissue engineering” with duplicates removed. One Onehundred hundred thirteen thirteen articles articles were were assessed assessed for inclusion for inclusion in the in analysis. the analysis. Articles Articles were excluded were excludedif not available if not available in English. in English. Reviews, Review books, sections, book sections, conference conference proceedings proceedings and patentsand patentswere also were excluded. also excluded. Inclusion Inclusion was determined was determined by relevance by relevance to the to topic the (i.e.,topic must (i.e., examinemust examinecollagen-containing collagen-containing electrospun electrospun materials mate forrials tissue for engineering/regenerative tissue engineering/regenerative medicine medicineapplications) applications) (Figure1 (Figure). After 1). assessment After assessment [ 16,17,19 [16,17,19–102]–102], studies, metstudies the met selection the selec- criteria tionand criteria were considered and were considered in this review in this (Supplemental review (Supplemental Table S1). The Table objective S1). The of thisobjective systematic of thisreview systematic was to review assess was the to role assess of collagenthe role of source, collagen electrospinning source, electrospinni parametersng parame- (solvent, terssolution (solvent, composition) solution composition) and crosslinking and crosslinking method on themethod resultant on the materials resultant properties materials and propertiestheir utility and for their tissue utility engineering. for tissue engin Outcomeseering. for Outcomes each paper for were each assessedpaper were independently assessed independentlyby all authors by (BNB, all authors SCG and (BNB, HMP). SCG Forand quantitative HMP). For quantitative analyses on analyses material on composition, material composition,source, and crosslinkingsource, and crosslinking method, all 86method, papers all were 86 included.papers were If multiple included. collagen If multiple sources, collagencompositions sources, and/or compositions crosslinking and/or methods crosslinking were usedmethods in a singlewere used manuscript, in a single each manu- unique script,feature each was unique included feature individually was included in theindividually total count. in the For total example, count. ifFor one example, manuscript if oneexamined manuscript scaffolds examined electrospun scaffolds using electrospu a blendn using of collagen a blend typeof collagen I and type polycaprolactone I and pol- ycaprolactone(PCL) at multiple (PCL) ratios,at multiple including ratios, aincluding collagen a only collagen group, only it group, was counted it was counted as a citation as a forcitation pure for collagen pure collagen and as aand citation as a citation for PCL-collagen for PCL-collagen blends blends (different (different ratios ofratios the sameof thepolymers same polymers were counted were counted only once). only once). FigureFigure 1. 1.Flow-throughFlow-through diagram diagram for for literature literature sear searchch and and inclusion/exclusion inclusion/exclusion criteria. criteria. 3.3. Results Results 3.1. Sources of Collagen for Electrospinning Collagen can be extracted from a number of different tissues and a wide variety of organisms, including mammals, amphibians, fish, and birds [103]. For use with electro- Bioengineering 2021, 8, x FOR PEER REVIEW 3 of 17 Bioengineering 2021, 8, 39 3.1. Sources of Collagen for Electrospinning 3 of 16 Collagen can be extracted from a number of different tissues and a wide variety of organisms, including mammals, amphibians, fish, and birds [103]. For use with electro- spinning, collagen from bovine and calf dermis dominates the literature, followed by col- spinning,lagen of rat collagen origin (Figure from bovine 2). Theand frequent calfdermis use of bovine dominates and calf the collagen literature, is followedlikely due by to collagen of rat origin (Figure2). The frequent use of bovine and calf collagen is likely due the abundance of the source tissue and commercial availability of the isolated collagen. to the abundance of the source tissue and commercial availability of the isolated collagen. Due to concerns regarding potential allergen or pathogen risk from animal sources, many Due