materials Review Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities Andrea Karen Persons 1,2 , John E. Ball 2,3 , Charles Freeman 4 , David M. Macias 5,6 , Chartrisa LaShan Simpson 1 , Brian K. Smith 7 and Reuben F. Burch V. 2,7, *   Citation: Persons, A.K.; Ball, J.E.; Freeman, C.; Macias, D.M.; Simpson, C.L.; Smith, B.K.; Burch V., R.F. Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities. Materials 2021, 14, 4070. https://doi.org/10.3390/ ma14154070 Academic Editor: Zhe Zhang Received: 11 June 2021 Accepted: 16 July 2021 Published: 21 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Agricultural and Biological Engineering, Mississippi State University, 130 Creelman Street, Starkville, MS 39762, USA; akp4@msstate.edu (A.K.P.); clsimpson@abe.msstate.edu (C.L.S.) 2 Human Factors and Athlete Engineering, Center for Advanced Vehicular Systems, Mississippi State University, 200 Research Boulevard, Starkville, MS 39759, USA; jeball@ece.msstate.edu 3 Department of Electrical and Computer Engineering, Mississippi State University, 406 Hardy Road, Starkville, MS 39762, USA 4 School of Human Sciences, Mississippi State University, 255 Tracy Drive, Starkville, MS 39762, USA; cfreeman@humansci.msstate.edu 5 Department of Kinesiology, Mississippi State University, P.O. Box 6186, Starkville, MS 39762, USA; dmacias@columbusortho.com 6 Columbus Orthopaedic Clinic, 670 Leigh Drive, Columbus, MS 39705, USA 7 Department of Industrial and Systems Engineering, Mississippi State University, 479-2 Hardy Road, Starkville, MS 39762, USA; smith@ise.msstate.edu * Correspondence: burch@ise.msstate.edu; Tel.: +1-662-325-1677 Abstract: Standards for the fatigue testing of wearable sensing technologies are lacking. The majority of published fatigue tests for wearable sensors are performed on proof-of-concept stretch sensors fabricated from a variety of materials. Due to their flexibility and stretchability, polymers are often used in the fabrication of wearable sensors. Other materials, including textiles, carbon nanotubes, graphene, and conductive metals or inks, may be used in conjunction with polymers to fabricate wearable sensors. Depending on the combination of the materials used, the fatigue behaviors of wearable sensors can vary. Additionally, fatigue testing methodologies for the sensors also vary, with most tests focusing only on the low-cycle fatigue (LCF) regime, and few sensors are cycled until failure or runout are achieved. Fatigue life predictions of wearable sensors are also lacking. These issues make direct comparisons of wearable sensors difficult. To facilitate direct comparisons of wearable sensors and to move proof-of-concept sensors from “bench to bedside”, fatigue testing standards should be established. Further, both high-cycle fatigue (HCF) and failure data are needed to determine the appropriateness in the use, modification, development, and validation of fatigue life prediction models and to further the understanding of how cracks initiate and propagate in wearable sensing technologies. Keywords: fatigue testing; cyclic testing; low-cycle fatigue; high-cycle fatigue; wearables; lead failure; stretch sensor; hysteresis; cyclic softening; fatigue testing standards 1. Introduction Interest in wearable stretch sensors has increased due to their potential uses in medical applications to monitor the health of a patient [1–10], to assess biomechanics, [11–21], and as drug delivery systems in pharmaceutical applications [22,23]. Wearable sensors may also have applications in athletics. [11,18,21,24–28], soft robotics [21,29,30], ergonomic assessments [19] and deep space exploration [31]. This interest is especially timely as the SARS-CoV-2 (COVID-19) epidemic has led to decreased in-person office visits to medical professionals while concomitantly increasing the number of virtual visits via telemedical platforms [32–34]. The increased use of telemedicine has led to an increased interest in the use of wearable sensors to monitor the health of patients outside of the Materials 2021, 14, 4070. https://doi.org/10.3390/ma14154070 https://www.mdpi.com/journal/materials