Temperature modulation of skin tolerance to applied mechanical loading and shear

Pressure ulcers (PUs) are a localised damage to the skin resulting from prolonged periods of pressure and shear when lying, sitting, or wearing a medical device such as a face mask. PUs can affect any individual, particularly those who experience prolonged immobility due to hospitalisation, ranging from babies to elderly adults. PUs worsen people's quality of life and they are also very costly to healthcare providers such as the NHS. Indeed, in the UK, the annual cost of treating skin wounds, including PUs, has been estimated to be approximately £8.3 billion a year. Accordingly, improving our understanding of how the human skin respond to pressure could lead to the development of cost-effective, personalised solutions to maintain skin health.

Cooling the skin is a promising approach to increase the skin's resilience to damage induced by prolonged periods of pressure (e.g. when one is bedridden) or repeated rubbing against a hard surface (i.e. shear stress). Early animal studies showed that cooling the skin reduces the risk of developing a PU, and this has been more recently confirmed in preliminary human studies. Yet, we still do not know enough about why and how cooling makes the skin more resilient to pressure and shear. Furthermore, cooling the skin can be uncomfortable and this can limit how acceptable the therapy is, particularly amongst vulnerable people at risk of developing PUs such as the elderly and those with a spinal cord injury. This knowledge gap raises a series of basic biological questions, which have clear implications for the prevention of PUs, and for the development of new devices that control skin temperature and help maintaining skin health across the life course. These questions are:

- How does cooling change the vascular, immunological, structural, and perceptual responses of the skin to a mechanical insult?
- Do the responses to cooling change with ageing and with the presence of a spinal cord injury, both of which increase the risk of acquiring a PU?
- Can we identify a level of cooling that is both beneficial and comfortable to protect the skin from developing pressure and shear damage?

Our project will answer these questions and fill these knowledge gaps. Our goal is to better understand how cooling impact the function and comfort of the skin when this is subjected to pressure and shear, and how this varies with age and clinical status. We have designed a series of experiments in both young and older participants, and in those with a spinal cord injury, to answer our research questions. Our first objective is to investigate how different levels of cooling alter the skin' vascular, inflammatory, structural, and perceptual responses to sustained pressure and repeated shear stress. Our second objective is to determine how ageing and the presence of spinal cord injury modulate the skin' physiological and perceptual responses underlying the beneficial effects of cooling on skin tolerance to pressure and shear.

Our project will develop new basic knowledge on the role of temperature in reducing the risk of skin damage. This will support innovation in the design of healthcare and user-centred technologies, such as support mattresses, clothing, and medical devices that can control skin temperature. This will unlock the potential of cooling to the skin that will help maintain skin health across the life course.

Thermodynamic conditions around skin tissues, commonly termed microclimate, strongly influence the risk of soft tissue damage and the development of chronic wounds, e.g. pressure ulcers (PUs). Early animal studies show that reduced surface temperature minimises the risk of mechanically induced skin damage, highlighting the potential therapeutic role of cooling for maintaining tissue health. Yet, the mechanisms by which cooling enhances skin tolerance to pressure and shear remain poorly understood. This project will examine the effects of cooling on the physiological and perceptual tolerance of human skin to mechanical loading in younger and older healthy participants. In addition, a sub-cohort of patients at high risk of PU (spinal cord-injured, SCI), will be recruited. This programme of research includes an innovative combination of non-invasive techniques including: 1) microvascular Laser Doppler Flowmetry; 2) inflammatory biomarker sampling from skin sebum; 3) structural and functional imaging via Optical Coherence Tomography; 4) biophysical modelling of skin friction; 5) quantitative sensory testing. Leveraging unique thermal stimulators and friction rigs, we have designed a clinically relevant set of experiments in healthy young participants and in groups at-risk of PUs, to determine how different levels of cooling alter the skin' microvascular, inflammatory, structural, and perceptual responses to a) sustained pressure-induced ischemia; b) post-occlusive reactive hyperaemia; c) repeated shear stress. The outcomes of this project will help identifying the metabolic, immunological, biophysical, and perceptual pathways underlying the potential beneficial effects of cooling on skin tolerance to loading in distinct cohorts to fundamentally change our understanding of normal and pathological skin function. This knowledge will be translated to support innovation of assistive thermal technologies that maintain skin health across the life course.