Wearable-device geometry and tissue mechanical variability determine sacral soft-tissue loading and pressure injury risk

Abstract

Wearable sensors and monitoring systems are increasingly used in clinical and home-care settings, often requiring prolonged contact with skin over bony prominences such as the sacrum. Tissue deformation beneath sustained loading is recognized as a key mechanical factor in pressure-induced tissue damage. However, device geometry influence on stress and strain distribution within underlying soft tissues remains insufficiently characterized. A three-dimensional finite element model of the sacral region was developed, incorporating skin, adipose tissue, muscle, and sacral bone. Two wearable-device geometries were simulated: a circular skin-mounted sensor and an elongated cable segment. Static pressures of 2, 6, 8, and 10 kPa were applied. Tissue stiffness was varied by +10% and +20% to represent inter-individual variability. Mechanical exposure was quantified using layer-resolved stress and strain distributions and a normalized risk index derived from cumulative histograms within a defined region of interest. Sensor loading produced predominantly superficial stress concentrations in skin, whereas cable loading redistributed mechanical exposure toward deeper tissues. At 10 kPa, strain-based risk indices in adipose tissue and muscle approached maximal values under cable loading across stiffness conditions, while stress-based risk remained highest in skin. Increasing tissue stiffness elevated stress-based risk but had limited influence on strain-based risk, especially in deeper tissues. We show that device geometry strongly determines tissue-level mechanical exposure. Sensor-like devices concentrate stress superficially, whereas cable-like geometries produce persistent strain in deeper tissues, which is relatively insensitive to moderate stiffness changes. Our results emphasize the need for geometry-aware, deformation-based assessment of wearable-device loading risk in pressure-vulnerable anatomical regions.