Citation: Alharbi, N.; Brigham, A.; Guthold, M. The Mechanical Properties of Blended Fibrinogen:Polycaprolactone (PCL) Nanofibers. Nanomaterials 2023, 13, 1359. https://doi.org/10.3390/ nano13081359 Academic Editors: Mohammad Malikan, Shahriar Dastjerdi, Bekir Akgöz and Ömer Civalek Received: 31 March 2023 Revised: 10 April 2023 Accepted: 11 April 2023 Published: 13 April 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/). nanomaterials Article The Mechanical Properties of Blended Fibrinogen:Polycaprolactone (PCL) Nanofibers Nouf Alharbi, Annelise Brigham and Martin Guthold * Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA; alhana17@wfu.edu (N.A.) * Correspondence: gutholdm@wfu.edu; Tel.: +1-(336)-758-4977 Abstract: Electrospinning is a process to produce versatile nanoscale fibers. In this process, syn- thetic and natural polymers can be combined to produce novel, blended materials with a range of physical, chemical, and biological properties. We electrospun biocompatible, blended fibrino- gen:polycaprolactone (PCL) nanofibers with diameters ranging from 40 nm to 600 nm, at 25:75 and 75:25 blend ratios and determined their mechanical properties using a combined atomic force/optical microscopy technique. Fiber extensibility (breaking strain), elastic limit, and stress relaxation times depended on blend ratios but not fiber diameter. As the fibrinogen:PCL ratio increased from 25:75 to 75:25, extensibility decreased from 120% to 63% and elastic limit decreased from a range between 18% and 40% to a range between 12% and 27%. Stiffness-related properties, including the Young’s modu- lus, rupture stress, and the total and relaxed, elastic moduli (Kelvin model), strongly depended on fiber diameter. For diameters less than 150 nm, these stiffness-related quantities varied approximately as D −2 ; above 300 nm the diameter dependence leveled off. 50 nm fibers were five–ten times stiffer than 300 nm fibers. These findings indicate that fiber diameter, in addition to fiber material, critically affects nanofiber properties. Drawing on previously published data, a summary of the mechanical properties for fibrinogen:PCL nanofibers with ratios of 100:0, 75:25, 50:50, 25:75 and 0:100 is provided. Keywords: electrospinning; fibrinogen; polycaprolactone; mechanical properties; nanofibers; diameter dependence 1. Introduction Electrospun nanofibers have gained prominence in recent years due to their versa- tility and unique properties. Large surface area to volume ratios, nanoscale size, and a wide range of physical and biochemical properties make electrospun fibers an attractive material for various fields such as tissue engineering [1–3], medication delivery [4], textile manufacture [5,6], filtration [7,8], and clean energy (batteries, solar panels, fuel cells) [9,10]. Although several techniques exist to generate ultra-thin fibers, electrospinning is one of the most economical and straightforward processes. Electrospinning offers several ad- vantages, including ease of use, scalability, and adjustability [11]. This technique utilizes a high electric field to produce fibers on the nanoscale using polymer solutions of syn- thetic or natural polymers [12,13]. It allows control of fiber diameter, mesh pore size, and surface morphology [14,15], and, if desired, the fibers may be infused with additional small molecules. Furthermore, electrospinning enables the creation of diverse structures, including hollow [16], core-shell [17], multilayer [18], and nanowires [19], providing great versatility in the nanofiber design for various demands in the applications [20]. Several factors affect electrospinning, including solution composition, processing parameters (flow rate, electric field strength), and ambient conditions (temperature, hu- midity) [21–26]. Understanding and adjusting these parameters allows the production of nanofibers that meet the requirements of specific applications. Over the past years, natural polymers such as collagen, fibrinogen, and elastin were successfully electrospun to nanofibers for potential uses such as tissue engineering scaffolds, Nanomaterials 2023, 13, 1359. https://doi.org/10.3390/nano13081359 https://www.mdpi.com/journal/nanomaterials