TLT: What triggered your interest in this field?
Masen: I am a mechanical engineer by training, and I completed my undergraduate project on the elastohydrodynamic lubrication of ball bearings, which was supervised by Kees Venner, in collaboration with John Tripp and Piet Lugt at the SKF Engineering and Research Centre. I enjoyed the research side of my work there and decided to continue my education by pursuing a doctorate. My research was on wear mechanisms in metalforming.

After my doctorate, I continued working in that field as part of the R&D team at Hydro Aluminium Extrusion. The work was very interesting, but I missed academic challenge. When my former professor at the University of Twente, Dirk-Jan Schipper, contacted me to ask if I was interested in a position as an assistant professor, the decision was easily made. In the years during my doctorate, we had often discussed the fact that much in the medical world revolves around solving tribological issues. So, when I rejoined, we quickly agreed that “bio” would be interesting and would fulfill a need. Biotribology, at that point in time, was still strongly linked to studies of joint replacements, while the rest of the field was, more or less, in its infancy.

TLT: What projects are you currently working on?
Masen: Whilst I like working on bio-related projects, I strongly believe it is also beneficial to be involved in engineering tribology work, so I try to combine the two. On the bio-side of things, we work on the tribology of perception, both for touch/feel and for mouth/food perception. Together with Philippa Cann, here at Imperial College, I investigate the frictional characteristics of foods and relate this to texture perception, and we develop lab-based screening tests that can reduce the reliance on panel testing. We also focus on understanding how to increase comfort and reduce skin damage for people who use prosthetics.

On the engineering side of things, we work on grease lubrication, and we supervise research investigating the fundamental wear and degradation mechanisms of rubber materials and of extreme-performance polymers. Currently I am spending most of my time on research into providing a solution to the skin injuries that many medics experience while working extended shifts during the COVID-19 pandemic. Wearing personal protective equipment (PPE), such as face masks, causes shear forces to act on the skin, and its use for extended periods can cause tissue breakdown. With a team of 25 volunteers from Imperial College London, we have been testing on live human skin (in-vivo testing) to understand the friction behavior of a wide range of commercially available creams and ointments. The objective was to reduce the friction in a skin-silicone rubber contact for up to four hours. We obtained insightful results, showing that some products provide excellent, long-lasting low friction. In contrast, products that had been recommended for the purpose were counterproductive. We are currently writing up our findings for peer-review, after which we will be providing advice to frontline medical staff.



TLT: How is soft material tribology different from common industrial materials—metals such as steel/aluminum or ceramics such as glass?
Masen: Soft material tribology requires you to question things that you have always taken for granted as a tribologist. The measurement principle of many tribometers relies on small deformations of surfaces in contact in combination with relatively high forces, or contact pressures. Most tribometers were originally designed with bearings and gears in mind, but for soft materials, these basic assumptions are not valid anymore. We need to accommodate large deformations and be able to apply rather low contact pressures. In addition, the contact area for soft material contacts can be very large, meaning that, for example, in a pinon-disk experiment, you get a large velocity gradient inside the contact. Modeling these materials is also a challenge; often we do not really know the material properties of tissue such as skin, and they are scale-dependent, directional, time-dependent, nonhomogeneous and highly variable. From an academic point of view, that is highly fascinating because it means that there is always a new aspect to research.

TLT: How do you translate your experience studying elastomers to biological materials, and what are the similarities between human tissue, hydrogels and elastomers?
Masen: As an engineer, my starting point would be to simplify the material I work with as much as possible, and then add complexity when and where needed. So, to a first degree, skin behaves just like a rubber material, which means we can use similar measurement methods. Human tissue is very sensitive to minor variations of the testing conditions, which is something we also experience when testing hydrogels, so even though these materials are not the same, we can learn much from their similarities. Of course, when performing experiments on people, you need to be extra careful and really think before you act. There is no margin of error. The process of applying for ethics approval really helps with that; it forces you to think, focus and structure your experimental program. This approach also helps me with my engineering tribology work; it makes lab time much more efficient.

I think there is a strong need for reciprocity between biotribology and engineering tribology as these are not separate subjects. I also see that the doctorate students in the tribology group who are normally very focused on their own projects, once they get talking to each other, they begin to understand that the problems they are experiencing have previously been solved by others. Thinking along the same lines, the International Conference on Biotribology, which I organize together with Markus Wimmer, will join forces with the Wear of Materials Conference in April 2021. Our goal is to introduce “bio” to many tribologists who normally work on engineering aspects and to also introduce the challenges of engineering tribology to those who are more bio-focused.

TLT: What makes skin a special soft material?
Masen: Skin is such a complex and responsive material, and this behavior varies for everyone. There are obvious differences between people, such as those with dry skin or greasy skin, but it goes much further. Our in-vivo measurements show an enormous variation in friction responses between people, even if we keep all other settings and conditions the same. On some people the response is very stable, while for others, the repeatability of experiments is very poor as their skin immediately reacts to mechanical stimuli (e.g., by sweating profusely). We are only slowly finding out the causes of these differences, and this research requires experiments ranging from the cell-scale to the human level, with collaboration from experts in engineering, biology, medicine, chemistry and many other areas.



TLT: You work on a wide range of projects on skin, from touch to skin creams and pressure ulcers. What are the various similarities?
Masen: All of my projects on skin are focused on better understanding what actually happens on the surface of and inside the skin. For tactile applications and creams, as well as for food-based investigations, this includes the use of finite element models to assess local stresses and strains in the tissue and quantify how the mechanoreceptors in the skin are triggered. In that way, we can start relating the outcomes of perception tests—which are often performed using costly trials—to the tribological input. This enables us to better understand the role of contact and friction in tactile perception. For projects focused on preventing skin injury, we follow a similar approach. In collaboration with biologists and clinicians, we model the skin and try to connect that to measurements of skin irritation (e.g., by the release of pro- and anti-inflammatory proteins after mechanical and tribological loading) with the aim of understanding the fundamental aspects involved.

TLT: What are the key limitations in conducting experimental measurements with biological materials?
Masen: We cannot perform any destructive testing, including wear tests, at least not in-vivo. In addition, at present there is no good simulant material or accurate reference material. This makes setting up a reliable experimental program a challenge. We do not do tests on animals in our laboratory, and I also would strongly question if results obtained on animals are representative anyway. If you do testing using volunteers, you normally need to perform many repeats on a large number of volunteers. The anatomical location of testing also needs careful consideration; too often we do measurements on the hand or the arm because that is the only body part you can easily access with a tribometer. It is well known that the properties of the skin vary strongly over the body. However, it is accepted that this is sometimes the only viable approach to get results. There does, however, need to be awareness of the limited applicability of results and care taken not to over-extrapolate them.

TLT: Please explain the health benefits coming out of your research. Where do you see the utility of skin friction data, and what current applications can be improved by better tribological characterization of the interaction of human skin?
Masen: We recently completed a project where we investigated the role of shear forces in the development of pressure ulcers. Our findings show that skin is very robust against normal loads but easily breaks down when there is a friction or shear component in the load. We also showed that there are two types of people: those whose skin strongly respond to mechanical loading and those who show only a minor reaction.

To summarize, my aims could be best described as ensuring that products are designed to optimize how they interface with skin and, thus, minimize the risk of injury. This varies widely, for example, from prosthetic sockets to AstroTurf and bed sheets to face masks. In collaboration with colleagues in bioengineering and medicine, I am now pursuing the possibility of developing cell therapy to make the skin more resistant to mechanical loading, which could be a game changer for users of prosthetics. In the short term, I hope that our research into skin lubricants will provide frontline medical staff with some relief when wearing PPE.

You can reach Marc Masen at m.masen@imperial.ac.uk.