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Review
. 2021 Jul 21;14(15):4070.
doi: 10.3390/ma14154070.

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 , Reuben F Burch V  2   7
Affiliations
Review

Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities

Andrea Karen Persons et al. Materials (Basel). .

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: cyclic softening; cyclic testing; fatigue testing; fatigue testing standards; high-cycle fatigue; hysteresis; lead failure; low-cycle fatigue; stretch sensor; wearables.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Examples of wearable sensing technologies. Wearable sensing technologies include internal (e.g., internal cardioverter-defibrillator and continuous glucose monitor) and external (e.g., smart watch, sleep/fitness tracking ring, and mobile electrocardiogram) wearable sensing technologies.
Figure 2
Figure 2
Simplified cross-section of an implantable cardioverter-defibrillator lead.
Figure 3
Figure 3
Sinusoidal pattern created by fully reversed loading. The sinusoidal pattern results from the alternation of tensile (reversal 1) and compressive (reversal 2) loading during cycling.
Figure 4
Figure 4
Example of a bending load. When a bending load is applied, the object experiences both tensile and compressive forces. During fully reversed cycling, the tensile and compressive forces alternate sides, resulting in the sinusoidal pattern observed in Figure 3.
Figure 5
Figure 5
Theoretical example of hysteresis loops indicative of cyclic softening. Cyclic softening occurs when the peak stress of a material decreases with an increased number of cycles.
Figure 6
Figure 6
Theoretical examples of crack tips. (A) A sharp crack tip promotes rapid propagation of the crack. (B) A blunt crack tip slows the propagation of the crack.
Figure 7
Figure 7
StretchSense™ StretchFABRIC sensor hasa fabric substrate. The polymer housing that protects the sensor is affixed to the substrate via an adhesive.
See this image and copyright information in PMC

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