Imperial College London


Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Tribology and Mechanical Engineering Design



+44 (0)20 7594 7066m.masen




Mrs Chrissy Stevens +44 (0)20 7594 7064




668City and Guilds BuildingSouth Kensington Campus





Publication Type

79 results found

Yap KK, Fukuda K, Vail JR, Wong J, Masen MAet al., 2022, Spatiotemporal mapping for in-situ and real-time tribological analysis in polymer-metal contacts, Tribology International, Vol: 171, Pages: 1-16, ISSN: 0301-679X

Spatiotemporal mapping (SMA) is a graphical technique to visualise the evolution of data with time and space during a process. This paper discusses the benefits of SMA in the field of polymer tribology via two highly different polymer/metal sliding systems. The SMA is found useful for the qualitative and quantitative characterisation and analysis of the transfer phenomena at the contact interface during repeated sliding, e.g., the slide-roll mechanism of transfer lumps, the severe-to-mild wear transition due to the tribo-chemical reaction of PTFE, the accumulation of wear debris, and the formation of friction-reducing back-transfer polyimide films. Additionally, the SMA helps spot various abnormal tribological behaviours, such as the local removal of oxides on a misaligned disc that would otherwise be overlooked.

Journal article

Bikos D, Samaras G, Charalambides M, Cann P, Masen M, Hartmann C, Vieira J, Sergis A, Hardalupas Yet al., 2022, Experimental and numerical evaluation of the effect of micro-aeration on the thermal properties of chocolate, Food and Function, Vol: 13, Pages: 4993-5010, ISSN: 2042-6496

Thermal properties, such as thermal conductivity, specific heat capacity and latent heat, influence the melting and solidification of chocolate. The accurate prediction of these properties for micro-aerated chocolate products with varying levels of porosity ranging from 0% to 15% is beneficial for understanding and control of heat transfer mechanisms during chocolate manufacturing and food oral processing. The former process is important for the final quality of chocolate and the latter is associated with sensorial attributes, such as grittiness, melting time and flavour. This study proposes a novel multiscale Finite Element Model to accurately predict the temporal and spatial evolution of temperature across chocolate samples. The model is evaluated via heat transfer experiments at temperatures varying from 16 °C to 45 °C. Both experimental and numerical results suggest that the rate of heat transfer within the micro-aerated chocolate is reduced by 7% when the 15% micro-aerated chocolate is compared to its solid counterpart. More specifically, on average, the thermal conductivity decreased by 20% and specific heat capacity increased by 10% for 15% micro-aeration, suggesting that micro-pores act as thermal barriers to heat flow. The latter trend is unexpected for porous materials and thus the presence of a third phase at the pore’s interface is proposed which might store thermal energy leading to a delayed release to the chocolate system. The developed multiscale numerical model provides a design tool to create pore structures in chocolate with optimum melting or solidifying response.

Journal article

Desai N, Masen M, Cann P, Hanson B, Tuleu C, Orlu Met al., 2022, Modernising Orodispersible Film Characterisation to Improve Palatability and Acceptability Using a Toolbox of Techniques, PHARMACEUTICS, Vol: 14

Journal article

Zhou X, Masen MA, Li YY, Yap KK, Murali M, Jin ZMet al., 2022, Influence of different fluid environments on tactile perception and finger friction., Journal of the Royal Society Interface, Vol: 19, Pages: 20210783-20210783, ISSN: 1742-5662

Human beings often explore and perceive the characteristics of objects by touching with their fingers. During this process, the contact pressure and shear stress acting on the skin also modulate the tactile sensation. The external environment is an important factor that influences tactile perception as well as the finger friction characteristics. The purpose of this study was to investigate the effects of fluid environments, such as air, deionized water (DW) and thickened water (TW), on perceived roughness and relevant friction behaviour during finger movement. Two studies were performed to analyse the effect of fluid environment as well as the influence of lubricant viscosity on finger tactile friction behaviour. Participants conducted perception and sliding friction tests with their index finger in air and submerged in DW and TW, respectively. Perception tests were performed using a pairwise comparison, scoring the perceived roughness difference between a reference sample and the test sample. The statistical analysis showed that there was no significant difference in the roughness perception between air and DW, while the sensitivity of perception reduced with increasing lubricant viscosity. An approximate calculation of the film thickness was combined with classical lubrication theory to investigate the relationship between perception and friction. In TW, the thick film formed between the finger and the polytetrafluoroethylene plate changed the contact of the asperities with the skin, thus changing the subjective judgement and friction.

Journal article

Charalambides M, Bikos D, Samaras G, Cann P, Masen M, Hartmann C, German J, Vieira J, Hardalupas Iet al., 2022, Effect of structure on the mechanical and physical properties of chocolate considering time scale phenomena occuring during oral processing, Food Structure, Vol: 31, Pages: 1-14, ISSN: 2213-3291

Micro-aeration has been employed by the chocolate industry as a texture and flavour modifier. However, the impact of micro-aeration on oral processing is still not well understood. This study quantifies the mechanical, thermal and tribological behaviour of chocolate materials of different porosity levels. These material properties were then linked to sensory data considering the temporal phenomena of the oral process. In-vivo mastication tests were utilised to define the level of fragmentation of chocolate and coupled with heat transfer numerical models to simulate the melting during oral processing. Micro-aeration affects all material properties resulting in lower fracture stresses, rapid melting and a lower friction coefficient. The sensory results showed that micro-aeration creates a perception of a softer, less sticky chocolate which melts fast inside the mouth, without compromising the sweetness perception. This research adopts an innovative multidisciplinary approach to the physics of chocolates, bringing together the fields of solid mechanics, heat transfer, tribology, and sensory analysis and employing engineering experimental and numerical approaches to provide a link between chocolate structure, material properties and sensory perception. The outcome can contribute a powerful design tool for controlling the perception of sensory attributes for specific chocolate composition.

Journal article

Jobanputra R, Royyuru S, Hayes J, Masen Met al., 2021, A numerical analysis of skin-PPE interaction to prevent facial tissue injury, Scientific Reports, Vol: 11, Pages: 1-10, ISSN: 2045-2322

The use of close-fitting PPE is essential to prevent exposure to dispersed airborne matter, including the COVID-19 virus.The current pandemic has increased pressure on healthcare systems around the world, leading to medical professionalsusing high-grade PPE for prolonged durations, resulting in device-induced skin injuries. This study focuses oncomputationally improving the interaction between skin and PPE to reduce the likelihood of discomfort and tissuedamage. A finite element model is developed to simulate the movement of PPE against the face during day-to-day tasks.Due to limited available data on skin characteristics and how these vary interpersonally between sexes, races and ages,the main objective of this study was to establish the effects and trends that mask modifications have on the resultingsubsurface strain energy density distribution in the skin. These modifications include the material, geometric andinterfacial properties. Overall, the results show that skin injury can be reduced by using softer mask materials, whilstfriction against the skin should be minimised, e.g. through use of micro-textures, humidity control and topical creams.Furthermore, the contact area between the mask and skin should be maximised, whilst the use of soft materials withincompressible behaviour (e.g. many elastomers) should be avoided.

Journal article

Yap KK, Murali M, Tan Z, Zhou X, Li L, Masen Met al., 2021, Wax-oil lubricants to reduce the shear between skin and PPE, Scientific Reports, Vol: 11, Pages: 1-11, ISSN: 2045-2322

Prolonged use of tight-fitting PPE, e.g., by COVID-19 healthcare workers leads to skin injuries. An important contributor is the shear exerted on the skin due to static friction at the skin-PPE interface. This study aims to develop an optimised wax-oil lubricant that reduces the friction, or shear, in the skin-PPE contact for up to four hours. Lubricants with different wax-oil combinations were prepared using beeswax, paraffin wax, olive oil, and mineral oil. In-vivo friction measurements involving seven participants were conducted by sliding a polydimethylsiloxane ball against the volar forearms to simulate the skin-PPE interface. The maximum static coefficient of friction was measured immediately and four hours after lubricant application. It was found that the coefficient of friction of wax-oil lubricants is mainly governed by the ratio of wax to oil and the thermal stability and morphology of the wax. To maintain long-term lubricity, it is crucial to consider the absorption of oil into the PPE material. The best performing lubricant is a mixture of 20 wt% beeswax, 40 wt% olive oil, and 40 wt% mineral oil, which compared to unlubricated skin, provides 87% (P = 0.0006) and 59% (P = 0.0015) reduction in instantaneous and 4-h coefficient of friction, respectively.

Journal article

Charalambides M, Bikos D, Masen M, Hardalupas I, Cann P, Samaras G, Hartmann C, Vieira Jet al., 2021, Effect of micro-aeration on the mechanical behaviour of chocolates and implications for oral processing, Food and Function, Vol: 12, Pages: 4864-4886, ISSN: 2042-6496

Aeration in foods has been widely utilised in the food industry to develop novel foods with enhanced sensorial characteristics. Specifically, aeration at the micron-sized scale has a significant impact on the microstructure where micro-bubbles interact with the other microstructural features in chocolates. This study aims to determine the effect of micro-aeration on the mechanical properties of chocolate products, which are directly correlated with textural attributes such as hardness and crumbliness. Uniaxial compression tests were performed to determine the mechanical properties such as Poisson's ratio, Young's modulus and macroscopic yield strength together with fracture tests to estimate the fracture toughness. In vivo mastication tests were also conducted to investigate the link between the fracture properties and fragmentation during the first two chewing cycles. The uniaxial stress–strain data were used to calibrate a viscoplastic constitutive law. The results showed that micro-aeration significantly affects mechanical properties such as Young's modulus, yield and fracture stresses, as well as fracture toughness. In addition, it enhances the brittle nature of the chocolate, as evidenced by lower fracture stress but also lower fracture toughness leading to higher fragmentation, in agreement with observations in the in vivo mastication tests. As evidenced by the XRT images and the stress–strain measurements micro-aeration hinders the re-arrangement of the microscopic features inside the chocolate during the material's deformation. The work provides a new insight of the role of bubbles on the bulk behaviour of complex multiphase materials, such as chocolates, and defines the mechanical properties which are important input parameters for the development of oral processing simulations.

Journal article

Jobanputra R, Boyle C, Dini D, Masen Met al., 2020, Modelling the effects of age-related morphological and mechanical skin changes on the stimulation of tactile mechanoreceptors, Journal of The Mechanical Behavior of Biomedical Materials, Vol: 112, Pages: 1-10, ISSN: 1751-6161

Our sense of fine touch deteriorates as we age, a phenomenon typically associated with neurological changes to the skin. However, geometric and material changes to the skin may also play an important role on tactile perception and have not been studied in detail. Here, a finite element model is utilised to assess the extent to which age-related structural changes to the skin influence the tactile stimuli experienced by the mechanoreceptors. A numerical, hyperelastic, four-layered skin model was developed to simulate sliding of the finger against a rigid surface. The strain, deviatoric stress and strain energy density were recorded at the sites of the Merkel and Meissner receptors, whilst parameters of the model were systematically varied to simulate age-related geometric and material skin changes. The simulations comprise changes in skin layer stiffness, flattening of the dermal-epidermal junction and thinning of the dermis. It was found that the stiffness of the skin layers has a substantial effect on the stimulus magnitudes recorded at mechanoreceptors. Additionally, reducing the thickness of the dermis has a substantial effect on the Merkel disc whilst the Meissner corpuscle is particularly affected by flattening of the dermal epidermal junction. In order to represent aged skin, a model comprising a combination of ageing manifestations revealed a decrease in stimulus magnitudes at both mechanoreceptor sites. The result from the combined model differed from the sum of effects of the individually tested ageing manifestations, indicating that the individual effects of ageing cannot be linearly superimposed. Each manifestation of ageing results in a decreased stimulation intensity at the Meissner Corpuscle site, suggesting that ageing reduces the proportion of stimuli meeting the receptor amplitude detection threshold. This model therefore offers an additional biomechanical explanation for tactile perceptive degradation amongst the elderly. Applications of the develo

Journal article

Samaras G, Bikos D, Vieira J, Hartmann C, Charalambides M, Hardalupas Y, Masen M, Cann Pet al., 2020, Measurement of molten chocolate friction under simulated tongue-palate kinematics: effect of cocoa solids content and aeration, Current Research in Food Science, Vol: 3, Pages: 304-313, ISSN: 2665-9271

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.

Journal article

Katsikouli P, Ferraro P, Richardson H, Cheng H, Anderson S, Mallya MD, Timoney D, Masen M, Shorten Ret al., 2020, Distributed ledger enabled control of tyre induced particulate matter in Smart Cities, Frontiers in Sustainable Cities, Vol: 2, ISSN: 2624-9634

The link between transport related emissions and human health is a major issue for municipalities worldwide and one of the main challenges to address in the context of Smart Cities. Specifically, Particulate Matter (PM) emissions from exhaust and non-exhaust sources are one of the main worrying contributors to air-pollution. In this paper, we challenge the notion that a ban on internal combustion engine vehicles will result in clean and safe air in our cities, since emissions from tyers and other non-exhaust sources are expected to increase in the near future. We support this claim through simple calculations, based on publicly available data from the city of Dublin, and we present a high level solution to this problem, in the form of a control mechanism and ride-sharing scheme to limit the number of vehicles and therefore maintain the amount of transport-related PM to safe levels. Thanks to the use of Distributed Ledger Technology our proposal is entirely distributed, fair and privacy preserving, which makes it ideal for application in the Smart City domain.

Journal article

Porte E, Cann P, Masen M, 2020, A lubrication replenishment theory for hydrogels, Soft Matter, Vol: 16, Pages: 10290-10300, ISSN: 1744-683X

Hydrogels are suggested as less invasive alternatives to total joint replacements, but their inferior tribological performance compared to articular cartilage remains a barrier to implementation. Existing lubrication theories do not fully characterise the friction response of all hydrogels, and a better insight into the lubrication mechanisms must be established to enable optimised hydrogel performance. We therefore studied the lubricating conditions in a hydrogel contact using fluorescent imaging under simulated physiological sliding conditions. A reciprocating configuration was used to examine the effects of contact dimension and stroke length on the lubricant replenishment in the contact. The results show that the lubrication behaviour is strongly dependent on the contact configurations; When the system operates in a ‘migrating’ configuration, with the stroke length larger than the contact width, the contact is uniformly lubricated and shows low friction; When the contact is in an ‘overlapping’ configuration with a stroke length smaller than the contact width, the contact is not fully replenished, resulting in high friction. The mechanism of non-replenishment at small relative stroke length was also observed in a cartilage contact, indicating that the theory could be generalised to soft porous materials. The lubrication replenishment theory is important for the development of joint replacement materials, as most physiological joints operate under conditions of overlapping contact, meaning steady-state lubrication does not necessarily occur.

Journal article

Masen MA, Chung A, Dawczyk JU, Dunning Z, Edwards L, Guyott C, Hall TAG, Januszewski RC, Jiang S, Jobanputra RD, Karunaseelan KJ, Kalogeropoulos N, Lima MR, Mancero Castillo CS, Mohammed IK, Murali M, Paszkiewicz FP, Plotczyk M, Pruncu CI, Rodgers E, Russell F, Silversides R, Stoddart JC, Tan Z, Uribe D, Yap KK, Zhou X, Vaidyanathan Ret al., 2020, Evaluating lubricant performance to reduce COVID-19 PPE-related skin injury, PLoS One, Vol: 15, Pages: e0239363-e0239363, ISSN: 1932-6203

BackgroundHealthcare 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.MethodsA 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.FindingsMost 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.InterpretationTalcum 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.

Journal article

Zhou X, Mo JL, Li YY, Xiang ZY, Yang D, Masen MA, Jin ZMet al., 2020, Effect of Finger Sliding Direction on Tactile Perception, Friction and Dynamics, TRIBOLOGY LETTERS, Vol: 68, ISSN: 1023-8883

Journal article

Korbeld K, Klaassen M, Jobanputra R, de Vries E, Masen Met al., 2020, Effects of sebum properties on skin friction: investigation using a bench test, Biosurface and Biotribology, Vol: 6, Pages: 43-47, ISSN: 2405-4518

The hydro lipid film is an emulsion of sweat and sebum that covers the surface of the human skin and affectsthe tribological properties of the human skin. This paper investigates the effects of the composition of thesebum on the average coefficient of friction. A range of simplified sebums was developed and the frictionbehaviour was investigated. Five realistic sebums showed a strong variation in friction results, indicatingthat interpersonal differences in frictional behaviour might have their origin in differences in sebumcomposition. A more detailed investigation employing controlled variations of individual ingredientsshowed that friction is highly sensitive to the amount of squalene in the sebum. The amount of fatty acids inthe sebum also showed some effects, whilst the amount of cholesterol does not appear to be relevant forthe friction behaviour. The main new outcome from this study is that the composition of sebum has asignificant effect on the friction response of skin in ways that are currently not yet fully understood.

Journal article

Klaassen M, De Vries EG, Masen MA, 2020, Interpersonal differences in the friction response of skin relate to FTIR measures for skin lipids and hydration, COLLOIDS AND SURFACES B-BIOINTERFACES, Vol: 189, ISSN: 0927-7765

Journal article

Boyle C, Carpanen D, Pandelani T, Higgins C, Masen M, Masouros Set al., 2020, Lateral pressure equalisation as a principle for designing support surfaces to prevent deep tissue pressure ulcers, PLoS One, Vol: 15, ISSN: 1932-6203

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.

Journal article

Khafidh M, Schipper DJ, Masen MA, Vleugels N, Dierkes WK, Noordermeer JWMet al., 2019, Validity of Amontons' law for run-in short-cut aramid fiber reinforced elastomers: The effect of epoxy coated fibers, FRICTION, Vol: 8, Pages: 613-625, ISSN: 2223-7690

Journal article

Boyle C, Plotczyk M, Fayos Villalta S, Patel S, Hettiaratchy S, Masouros S, Masen M, Higgins Cet al., 2019, Morphology and composition play distinct and complementary roles in the tolerance of plantar skin to mechanical load, Science Advances, Vol: 5, Pages: 1-13, ISSN: 2375-2548

Plantar skin on the soles of the feet has a distinct morphology and composition that is thought to enhance its tolerance to mechanical loads, although the individual contributions of morphology and composition have never been quantified. Here, we combine multiscale mechanical testing and computational models of load bearing to quantify the mechanical environment of both plantar and nonplantar skin under load. We find that morphology and composition play distinct and complementary roles in plantar skin’s load tolerance. More specifically, the thick stratum corneum provides protection from stress-based injuries such as skin tears and blisters, while epidermal and dermal compositions provide protection from deformation-based injuries such as pressure ulcers. This work provides insights into the roles of skin morphology and composition more generally and will inform the design of engineered skin substitutes as well as the etiology of skin injury.

Journal article

Masen M, de Vries EG, 2019, Relating dermal friction to water and skin moisture content, ACS Fall National Meeting and Exposition, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

Khafidh M, Schipper DJ, Masen MA, Vleugels N, Dierkes WK, Noordermeer JWMet al., 2019, Friction and wear mechanism of short-cut aramid fiber and silica reinforced elastomers, Wear, Vol: 428-429, Pages: 481-487, ISSN: 0043-1648

© 2019 Elsevier B.V. Important phenomena during sliding contact of elastomeric materials are friction and wear. Wear reduction of elastomers can be achieved by minimizing the propagation of cracks in the elastomer during sliding contact. Adding fillers like silica and fibers is a way to reduce the propagation of cracks and as a result reduction of wear. In the present study, the wear processes of short-cut aramid fiber reinforced elastomers as a function of sliding distance and their relation to friction are investigated. Two different types of systems are considered, i.e. (1) elastomers reinforced by solely short-cut aramid fibers and (2) elastomers reinforced by short-cut aramid fibers and silica. A pin-on-disc tribometer and a microscope are used to analyze the friction and wear mechanisms of the elastomeric composites in sliding contact with a granite counter surface. The results show that the coefficient of friction of the composites consists of different stages, these stages are influenced by the wear processes during sliding. For elastomers which are reinforced by short-cut aramid fibers and silica, a higher energy input is needed to achieve all stages since the presence of silica in the elastomer matrix increases the resistance of matrix particle detachment. A general friction behavior of short-cut aramid fiber and silica reinforced elastomers is proposed.

Journal article

Klaassen M, de Vries EG, Masen MA, 2019, Friction in the contact between skin and a soft counter material: Effects of hardness and surface finish, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 92, Pages: 137-143, ISSN: 1751-6161

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.

Journal article

Boyle CJ, Carpanen D, Pandelani T, Higgins CA, Masen MA, Masouros SDet al., 2019, Lateral pressure equalisation as a principle for designing support surfaces to prevent deep tissue pressure ulcers, Publisher: Cold Spring Harbor Laboratory

<jats:title>Abstract</jats:title><jats:p>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.</jats:p><jats:p>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 — a greater effect than that achieved by using a more conformable cushion. 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.</jats:p><jats:p>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.</jats:p>

Working paper

Limbert G, Masen MA, Pond D, Graham HK, Sherratt MJ, Jobanputra R, McBride Aet al., 2019, Biotribology of the ageing skin—Why we should care, Biotribology, Vol: 17, Pages: 75-90, ISSN: 2352-5738

Ageing of populations has emerged as one of the most pressing societal, economic and healthcare challenges currently facing most nations across the globe. The ageing process itself results in degradation of physiological functions and biophysical properties of organs and tissues, and more particularly those of the skin. Moreover, in both developed and emerging economies, population ageing parallels concerning increases in lifestyle-associated conditions such as Type 2 diabetes, obesity and skin cancers. When considered together, these demographic trends call for even greater urgency to find clinical and engineering solutions for the numerous age-related deficits in skin function.From a tribological perspective, detrimental alterations of skin biophysical properties with age have fundamental consequences on how one interacts with the body's inner and outer environments. This stems from the fact that, besides being the largest organ of the human body, and also nearly covering its entirety, the skin is a multifunctional interface which mediates these interactions.The aim of this paper is to present a focused review to discuss some of the consequences of skin ageing from the viewpoint of biotribology, and their implications on health, well-being and human activities. Current and future research questions/challenges associated with biotribology of the ageing skin are outlined. They provide the background and motivation for identifying future lines of research that could be taken up by the biotribology and biophysics communities.

Journal article

Khafidh M, Schipper DJ, Masen MA, 2019, The formation of a modified surface layer on elastomeric materials, Tribology Letters, Vol: 67, Pages: 27-27, ISSN: 1023-8883

Surface modification of an elastomer may be formed during sliding contact with a rigid counter surface. This alteration leads to a change of mechanical properties at the surface and as a result a change in frictional behavior. Therefore, investigations related to the formation of a modified surface layer on elastomers and its effect on friction are of importance. In the present study, the formation of a modified surface layer on elastomer reinforced by silica is studied. Sliding friction is performed using a pin-on-disc tribometer. Several parameters are varied, namely contact pressure, velocity, and roughness of the counter surface. The existence of a modified surface layer is investigated by using a scanning electron microscope. The results show that the existence of a modified surface layer depends on the competition between the formation rate of the layer and the wear rate. The formation of the layer depends on the contact pressure, velocity, and sliding distance. A general formulation to calculate the volume of formation is proposed. Furthermore, a map of the formation of a modified surface layer is developed.

Journal article

Wang L, Hu Y, Zheng Y, Politis D, Masen M, Cui Jet al., 2019, Development of an interactive friction model to predict aluminum transfer in a pin-on-disc sliding system, Tribology International, Vol: 130, Pages: 216-228, ISSN: 0301-679X

In aluminum forming processes, it is observed that the coefficient of friction increases and a transfer layer is formed on the tool surfaces. In the current paper, this phenomenon is studied via pin-on-disc dry sliding tests with aluminum alloy 6082 sliding against cast iron G3500. The results showed that the aluminum transfer layers generated at the sliding interface were identified as the origin of this behaviour that affects both friction and wear. To model this phenomenon, an interactive friction model was developed enabling the prediction of friction and the evolution of the transfer layer from the running in to the steady state. This mechanism based model can be used for representing friction variations and material transfer in sliding systems.

Journal article

Porte E, Cann P, Masen M, 2019, Fluid load support does not explain tribological performance of PVA hydrogels, Journal of The Mechanical Behavior of Biomedical Materials, Vol: 90, Pages: 284-294, ISSN: 1751-6161

The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stress-relaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.

Journal article

Jean-Fulcrand A, Masen M, Bremner T, Wong Jet al., 2019, Effect of temperature on tribological performance of polyetheretherketone-polybenzimidazole blend, Tribology International, Vol: 129, Pages: 5-15, ISSN: 0301-679X

Polyetheretherketone (PEEK) is one of the most commonly used High Performance Polymers (HPP) although its high temperature performance is poor. In this study, polybenzimidazole (PBI), a HPP with one of the highest glass transition temperatures currently available, is blended to PEEK to form a 50:50 blend (TU60). Tribological performance of the blend (TU60) was investigated by rubbing it against steel at temperatures up to 280 °C. Results obtained are compared to those from neat PEEK and neat PBI. All three polymers were thermally stable during the duration of tests. However chemical analyses on polymeric transfer layers on steel surfaces and polymer debris suggest polymer degradation. The degradation observed is shear-assisted, possibly promoted by shear heating. Indeed the estimated interfacial temperature based on Jaeger model was above the melting point of PEEK in some cases. TU60 outperforms PEEK in all test conditions and PBI at 280 °C. TU60 formed transfer layers on steel similar to that of PEEK. When contact temperature is closed to the melting point of PEEK, PEEK in the TU60 creates a low strength transfer layer which acts as an interfacial lubricant. This reduces friction which in turn reduces PBI degradation in TU60 at high temperature. This work provides a strategy for creating interfacial layers to improve polymer tribological performance while maintaining the integrity of the polymer.

Journal article

Masen MA, Veijgen N, Klaassen M, 2019, Experimental Tribology of Human Skin, Studies in Mechanobiology, Tissue Engineering and Biomaterials, Pages: 281-295

The interaction behavior of the human skin is of relevance for the functional performance of a wide range of products and, as a result, the topic is widely studied in both industry and academia. However, the key underlying mechanisms determining the interaction behavior of skin are at present not well understood. Skin is a living material and thus will respond and may adapt to mechanical interaction, for instance by producing sweat, releasing biomarkers and even developing a blister or a wound. In addition, the properties of skin strongly depend on personal traits and characteristics. This makes predictive modelling of the interaction behaviour of skin challenging, and therefore there is a continued need for experimental investigations. In literature a large range of experimentally obtained friction values have been reported. These have been measured using a wide variety of tribometers. When commencing tribological testing it is essential to ensure that the investigations are performed using the appropriate tribo-system, meaning that contact conditions such as pressures, sliding velocities and environmental conditions are representative for the final application, as any of these factors will have a significant effect on the obtained tribological result. Additionally, many studies use the volar forearm as measurement site; whilst this area provides ease of measurement, it may not always be highly representative of the actual skin site of interest. Because of the complex nature of skin interactions, much of the underlying fundamental physical mechanisms remain to be discovered. Focused in-depth experimental investigations will be key to achieving a better understanding in skin tribology.

Book chapter

Khafidh M, Setiyana B, Jamari J, Masen MA, Schipper DJet al., 2018, Understanding the occurrence of a wavy wear track on elastomeric materials, Wear, Vol: 412-413, Pages: 23-29, ISSN: 0043-1648

Elastomeric materials are used in daily applications, such as tires and conveyor belts. Sliding friction of elastomeric materials often produces a unique periodic wear pattern. In general, the periodic wear pattern has to be prevented because it generates surface instabilities and, as a result, vibration. Sliding contact between an elastomer and a rigid ball may cause the development of a periodic wear pattern like a wave on the elastomer surface, called a wavy wear track. The present study has investigated the occurrence of the wavy wear track on the elastomers under several operating conditions. The elastomers used were a Styrene-Butadiene Rubber (SBR) and Butadiene Rubber (BR) reinforced with two types of fillers, i.e., highly dispersible silica and short-cut aramid fiber. A pin-on-disc tribometer was used to study the wavy wear track of the materials under several operating conditions. An analytical model was used to predict the wave length of the wear pattern and to study the occurrence of the wavy wear track. The results show that the occurrence of the wavy wear track depends on the mechanical properties of the elastomer, the operating conditions (such as sliding velocity and force), the inertia mass of the counter surface frame and the circumferential length of the wear track. The analytical model is in good agreement with the experimental results.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00741641&limit=30&person=true