Understanding of weight distribution and compliance characteristics of support surfaces is vital to reducing occurrences of pressure injuries among immobilized individuals due to physical and neurological conditions. Key factors influencing pressure injury formation are pressure and shear at the body-support surface interface. Commercial interface pressure mats are typically used to evaluate the interface pressure. However, studies show that shear is an important factor that contributes significantly in pressure injury formation. Evaluating shear at the interface will enable implementing better pressure injury prevention strategies. However, there are no commercial devices available for measuring shear at the body-support surface interface. This paper investigates numerical simulation methodologies to evaluate shear stresses at the interface. The simulation model and parameters are developed and validated based on experimental data on interface pressure. In this preliminary work, we employ an air cell based cushion to generate experimental data to develop the simulation environment. We have previously developed an air cell based smart seat cushion that can measure and modulate interface pressure through closed loop control of internal pressure of air cells.In this paper, a finite element simulation model development is carried out by a comparison of static analysis with respect to interface pressure map, internal pressure map, immersion, and interaction forces. A rigid cushion loading indenter (solid model of human buttocks) is used in the experiment to apply known immersion into the cushion. The experimental data was generated from three sources: 1) A commercial seating pressure mat yields the interface pressure map, 2) The automated smart seat cushion yields a map of internal pressures of all the air cells, and 3) The rigid cushion loading indenter yields the immersion into the cushion and the force applied on the cushion. This experimental data is used to develop and validate the simulation model, which consists of models of the cushion and the rigid cushion loading indenter. Material properties for the simulation model were obtained through mechanical testing of the experimentally evaluated cushion material. The validated simulation model is used to obtain shear stress data which is not experimentally available due to the lack of shear sensors.Both experiment and simulation were carried out in two scenarios, namely isolated (all air cells are isolated from each other) and equalized modes (all air cells are interconnected so that internal pressures of all cells are equal regardless of loading). Simulation model and parameters were developed based on the experimental data obtained from the isolated mode. Later, the experimental data from equalized mode were used to validate the simulation model. The validated model was then used to evaluate shear stress at the body-support surface interface. This study provides a basic framework to evaluate shear for widely used air cell array support surfaces which can aid in providing an insight into pressure and shear relationship.

Numerical Modeling of Air Cell Cushion and Estimation of Shear Force Distribution at Sitting Interface

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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