Skin morphology and composition play complementary roles in protecting plantar skin from stress- and deformation-induced injury.

The perception of some food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact during mastication. This paper reports a new bench test to measure friction in the simulated tongue-palate contact. The test consists of a flat PDMS disk, representing the tongue loaded and reciprocating against a stationary lower glass surface representing the palate. The test was applied to molten chocolate samples with and without artificial saliva. Friction was measured over the first few rubbing cycles, simulating mechanical degradation of chocolate in the tongue-palate region. The effects of chocolate composition (cocoa solids content ranging between 28 ​wt% and 85 ​wt%) and structure (micro-aeration/non-aeration 0–15 ​vol%) were studied. The bench test clearly differentiates between the various chocolate samples. The coefficient of friction increases with cocoa solids percentage and decreases with increasing micro-aeration level. The presence of artificial saliva in the contact reduced the friction for all chocolate samples, however the relative ranking remained the same.

The interaction behaviour of skin with a counter surface depends strongly on the surface roughness of the counter surface. For relatively hard surfaces this effect is described in various literature, but for soft, or compliant, materials this is much less studied. Inside the contact, the protuberances on the surface will deform substantially. In order to gain insights into the effect of surface roughness and hardness on the frictional behaviour between skin and a soft counter surface a range of experiments were performed using artificial skin and various silicone compounds which are commonly used in medical devices that interact with the human skin. Using these results, a 'friction map' was created that shows the friction behaviour as a function of the elastic modulus and the surface roughness. When the surface roughness is increased the friction coefficient decreases due to the reduction in the real area of contact, which weakens the adhesion between the two surfaces. A minimum coefficient of friction was observed at a surface roughness of approximately 4 µm. For the softest compounds tested there was minimal effect of surface roughness on friction because the roughness protuberances inside the contact will be flattened. Silicone compounds with increased hardness showed a larger sensitivity of the friction to the surface roughness, because these harder surface roughness protuberances are more resistant against deformation. The friction map provides a tool when designing products that require certain frictional properties: for products that are required to adhere to skin a smooth and soft material is recommended, whereas for products that require a low coefficient of friction a harder compound with a surface roughness of approximately 4 µm is recommended.

Healthcare workers around the world are experiencing skin injury due to the extended use of personal protective equipment (PPE) during the COVID-19 pandemic. These injuries are the result of high shear stresses acting on the skin, caused by friction with the PPE. This study aims to provide a practical lubricating solution for frontline medical staff working a 4+ hours shift wearing PPE. A literature review into skin friction and skin lubrication was conducted to identify products and substances that can reduce friction. We evaluated the lubricating performance of commercially available products in vivo using a custom-built tribometer. Most lubricants provide a strong initial friction reduction, but only few products provide lubrication that lasts for four hours. The response of skin to friction is a complex interplay between the lubricating properties and durability of the film deposited on the surface and the response of skin to the lubricating substance, which include epidermal absorption, occlusion, and water retention. Talcum powder, a petrolatum-lanolin mixture, and a coconut oil-cocoa butter-beeswax mixture showed excellent long-lasting low friction. Moisturising the skin results in excessive friction, and the use of products that are aimed at ‘moisturising without leaving a non-greasy feel’ should be prevented. Most investigated dressings also demonstrate excellent performance.

When immobile or neuropathic patients are supported by beds or chairs, their soft tissues undergo deformations that can cause pressure ulcers. Current support surfaces that redistribute under-body pressures at vulnerable body sites have not succeeded in reducing pressure ulcer prevalence. Here we show that adding a supporting lateral pressure can counter-act the deformations induced by under-body pressure, and that this ‘pressure equalisation’ approach is a more effective way to reduce ulcer-inducing deformations than current approaches based on redistributing under-body pressure. A finite element model of the seated pelvis predicts that applying a lateral pressure to the soft tissue reduces peak von Mises stress in the deep tissue by a factor of 2.4 relative to a standard cushion (from 113 kPa to 47 kPa)—a greater effect than that achieved by using a more conformable cushion, which reduced von Mises stress to 75 kPa. Combining both a conformable cushion and lateral pressure reduced peak von Mises stresses to 25 kPa. The ratio of peak lateral pressure to peak under-body pressure was shown to regulate deep tissue stress better than under-body pressure alone. By optimising the magnitude and position of lateral pressure, tissue deformations can be reduced to that induced when suspended in a fluid. Our results explain the lack of efficacy in current support surfaces and suggest a new approach to designing and evaluating support surfaces: ensuring sufficient lateral pressure is applied to counter-act under-body pressure.