The model includes a medical device, tissue substrate, and stressor object as shown in Figure 2. The

bottom of the medical device is assumed to be adhered to the tissue substrate at the adhesion interface. During the simulation, the stressor object translates toward and collides with the medical device. The resulting mechanical shear stress and shear strain experienced will be calculated. Several simulations will be performed, each representing a design change to the shape or size of the medical device. The geometry of the tissue substrate and stressor object will not change. The maximum stress and strain across all designs will then be compared and results analyzed.

The skin and subcutaneous thickness is 39.9 mm (1.547 in), which represents the upper limit of adipose tissue thickness at the anterior abdomen in a study population of 449 adult males [10]. For this simulation, the stressor object is a rounded, rigid object that collides with the medical device tangentially to the surface plane of the skin. The simulation represents a patient who walks too closely to a solid object, such as a door frame, and contacts it with the device.

The material properties of all objects will remain constant throughout the study. The material selection and related mechanical properties for each object are shown in Table 1.

The medical device is assumed to be fabricated from a thermoplastic; polyethylene is an appropriate representative material. The mechanical properties of human tissue are derived from experimental data [11 - 14]. Tissue composition varies widely between patients. In order to bound the calculations, the minimum Young’s Modulus in the experimental range will be used. Structural steel was chosen for the stressor object to ensure that it behaves like a rigid object, although the results are insensitive to this material choice.

This model will use symmetry to reduce simulation time. Each simulated design iteration maintained symmetry across the XY plane, shown in blue in Figure 3.

The placement of elements and nodes within a model affects the accuracy of the calculations. Additionally, increasing the number of elements within the model correspondingly increases the simulation time. Therefore, an efficient model concentrates element and node placement at the locations of highest interest [15]. In this model, the locations of highest interest are where the maximum stress and strain occur, which is also at the adhesion interface. The resulting mesh is shown in Figure 4.

During the simulation, the stressor object will translate toward and collide with the medical device tangential to the skin surface (Figure 5). This interaction resembles the standardized test methods for shear adhesion according to ASTM D3654 [16]. Expanding on the earlier example of a patient colliding with a solid object such as a door frame, the velocity of the stressor will be 1.43 meters per second – the average adult male walking speed [17]. At that velocity, a simulation end time of 0.0175 seconds will translate the stressor to the midway point of the device. The bottom of the tissue substrate is fixed in place by its connection with deeper-layer tissue. On all free surfaces, a zero-stress condition is applied. At the symmetry plane, the normal gradient of all variables is zero.

The time steps used to integrate the unsteady simulation were ~8e−4 seconds. The potential sensitivity of the results to the value of the time steps will be discussed later.

The maximum shear elastic strain and the maximum shear stress are the target metrics for each simulation. The maximum strain consistently occurs within the tissue substrate directly below the medical device (after the stressor has impacted the medical device and moved halfway across the device). The time at which this maximum occurs varies slightly across simulations, but a representative result is shown in Figure 6. In the figure, the stressor is moved over the device and it has clearly pressed the medical device downwards into the resilient tissue. It is noteworthy that the shear strain is not completely symmetric, because of the direction of motion of the stressor. The colors are keyed to the color legend on the left-hand side of the image. The largest strain is adjacent to the leading and trailing edges of the adhesion interface and therefore is relevant to the issue of whether the device would maintain its on-body adhesion.

The maximum stress consistently occurs within the medical device adjacent to the adhesion interface. The time at which this maximum occurs varies across simulations, but a representative result is shown in Figure 7. As evident from the image, the stress is low in the tissue, partly because of the deformable character of the tissue. The units of stress in Figure 7 are Pa, as indicated by the color legend. As with strain, the largest values of stress are at the adhesion interface and thus are directly relevant to the maintenance of the medical device.

This simulation was performed to elucidate how changes to the mechanical design of the medical device will impact the stress and strain imparted by the stressor object. There are four design characteristics that will be investigated: 1) shape, 2) height, 3) size, and 4) taper. The set of simulations within a design characteristic category is called a “suite”. The simulation suites will be performed in the order previously listed (i.e., shape first, height second, etc.). Upon completion of each suite, the optimal design will be identified through direct comparison or by linear regression analysis. In suites that use linear regression, a coefficient of determination above 0.9 will be considered significantly meaningful [18].

The results allow an identification of an optimal design of a body-adhered medical device. The device should have a square planform area with a thin height, small footprint, and large taper profile. Table 6 presents the data from each simulation compared to the baseline model. The resultant stress and strain of these models is likewise compared to the baseline stress and strain. By comparing the data in this way, it can be seen which design characteristic (height, size, or taper) had the largest impact on the resultant stress and strain.

A 10% reduction in device height resulted in a 21% reduction in stress and 24% reduction in strain. A 20% reduction in device size caused a 7% reduction in stress and 2% reduction in strain. A 20% increase in device taper size resulted in a negligible reduction in stress and a 6% reduction in strain.