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

84 results found

Zhou X, Masen MA, Mo J, Shi X, He Y, Jin Zet al., 2023, Investigation of Experimental Devices for Finger Active and Passive Tactile Friction Analysis, Chinese Journal of Mechanical Engineering (English Edition), Vol: 36, ISSN: 1000-9345

Complicated tribological behavior occurs when human fingers touch and perceive the surfaces of objects. In this process, people use their exploration style with different conditions, such as contact load, sliding speed, sliding direction, and angle of orientation between fingers and object surface consciously or unconsciously. This work addressed interlaboratory experimental devices for finger active and passive tactile friction analysis, showing two types of finger movement. In active sliding experiment, the participant slid their finger freely against the object surface, requiring the subject to control the motion conditions themselves. For passive sliding experiments, these motion conditions were adjusted by the device. Several analysis parameters, such as contact force, vibration acceleration signals, vibration magnitude, and fingerprint deformation were recorded simultaneously. Noticeable friction differences were observed when comparing active sliding and passive sliding. For passive sliding, stick-slip behavior occurred when sliding in the distal direction, evidenced by observing the friction force and the related deformation of the fingerprint ridges. The employed devices showed good repeatability and high reliability, which enriched the design of the experimental platform and provided guidance to the standardization research in the field of tactile friction.

Journal article

Zhou X, Li Y, Tian Y, Masen MA, Li Y, Jin Zet al., 2023, Friction and neuroimaging of active and passive tactile touch., Sci Rep, Vol: 13

Two types of exploratory touch including active sliding and passive sliding are usually encountered in the daily life. The friction behavior of the human finger against the surface of objects is important in tactile perception. The neural mechanisms correlating to tribological behavior are not fully understood. This study investigated the tactile response of active and passive finger friction characterized with functional near-infrared spectroscopy (fNIRS). The friction test and fNIRS test were performed simultaneously using the tactile stimulus of polytetrafluoroethylene (PTFE) specimens. Results showed that the sliding modes did not obviously influence the friction property of skin. While three cortex regions were activated in the prefrontal cortex (PFC), showing a higher activation level of passive sliding. This revealed that the tribological performance was not a simple parameter to affect tactile perception, and the difference in cortical hemodynamic activity of active and passive touch was also recognised. The movement-related blood flow changes revealed the role of PFC in integrating tactile sensation although there was no estimation task on roughness perception.

Journal article

Samaras G, Bikos D, Skamniotis C, Cann P, Masen M, Hardalupas Y, Vieira J, Hartmann C, Charalambides Met al., 2023, Experimental and computational models for simulating the oral breakdown of food due to the interaction with molar teeth during the first bite, Extreme Mechanics Letters, Vol: 62, Pages: 1-11, ISSN: 2352-4316

The first bite involves the structural breakdown of foods due to the interaction with teeth and is a crucial process in oral processing. Although in vitro experiments are useful in predicting the oral response of food, they do not facilitate a mechanistic understanding of the relationship between the intrinsic food mechanical properties and the food behaviour in the oral cavity. Computer simulations, on the other hand, allow for such links to be established, offering a promising design alternative that will reduce the need for time consuming and costly in vivo and in vitro trials. Developing virtual models of ductile fracture in soft materials, such as food, with random and non-predefined crack morphology imposes many challenges. One of the most important is to derive results that do not depend on numerical parameters, such as Finite Element (FE) mesh density, but only physical constants obtained through independent standard mechanical tests, such as fracture strain and/or critical energy release rate. We demonstrate here that this challenge can be overcome if a non-local damage approach is used within the FE framework. We develop a first bite FE modelling methodology that provides mesh independent results which are also in agreement with physical first bite experiments performed on chocolate. The model accounts for key features found in chocolate and a wide range of compliant media, such as rate dependent plasticity and pressure dependent fracture initiation strain. As a result, our computational methodology can prove valuable in studying food structure-function relationships that are essential in product development.

Journal article

Bikos D, Samaras G, Cann P, Masen M, Hardalupas I, Vieira J, Hartmann C, Huthwaite P, Lan B, Charalambides Met al., 2023, Destructive and non-destructive mechanical characterisation of chocolate with different levels of porosity under various modes of deformation, Journal of Materials Science, Vol: 58, Pages: 5104-5127, ISSN: 0022-2461

Chocolate exhibits a complex material response under the varying mechanical loads present during oral processing. Mechanical properties such as Young’s modulus and fracture stress are linked to sensorial attributes such as hardness. Apart from this link with hardness perception, these mechanical properties are important input parameters towards developing a computational model to simulate the first bite. This study aims to determine the mechanical properties of chocolate with different levels of micro-aeration, 0–15%, under varying modes of deformation. Therefore, destructive mechanical experiments under tension, compression, and flexure loading are conducted to calculate the Young’s modulus, yield, and fracture stress of chocolate. The values of Young’s modulus are also confirmed by independent ultrasonic mechanical experiments. The results showed that differences up to 35% were observed amongst the Young’s modulus of chocolate for different mechanical experiments. This maximum difference was found to drop with increasing porosity and a negligible difference in the Young’s modulus measurements amongst the different mechanical experiments is observed for the 15% micro-aerated chocolate. This phenomenon is caused by micro-pores obstructing the microscopic inelastic movement occurring from the early stages of the material’s deformation. This work provides a deeper understanding of the mechanical behaviour of chocolate under different loading scenarios, which are relevant to the multiaxial loading during mastication, and the role of micro-aeration on the mechanical response of chocolate. This will further assist the food industry’s understanding of the design of chocolate products with controlled and/or improved sensory perception.

Journal article

Bikos D, Samaras G, Charalambides M, Cann P, Masen M, Hartmann C, Vieira J, Sergis A, Hardalupas Iet al., 2023, A micromechanical based finite element model approach to accurately predict the effective thermal properties of micro-aerated chocolate, Innovative Food Science and Emerging Technologies, Vol: 83, ISSN: 1466-8564

Micro-aeration is a method to modify the sensorial attributes of chocolate but also affects the material properties of chocolate, which in turn, determine its material response during manufacturing and oral processes. This study aims to define the effect of micro-aeration on the thermal properties of chocolate by considering the changes of chocolate microstructure due to micro-aeration. Micro-aeration was found to alter the chocolate microstructure creating a layer of a third phase at the porous interfaces, which is argued to consist of cocoa butter of higher melting properties. A multiscale Finite Element Model is developed, which was confirmed by macroscale heat transfer measurements, to parametrically simulate the structural changes of micro-porous chocolates at the microscale level and estimate their effective properties, such as thermal conductivity and specific heat capacity. The developed multiscale computational model simulates the porous chocolate as a two-phase (chocolate- pores) or three-phase material (chocolate-cocoa butter layer- pores). The investigation identified a new, complex transient thermal mechanism that controls the behaviour of micro-aerated chocolate during melting and solidification. The results showed a maximum 13% reduction of keff and 15% increase of Cpeff with 15% micro-aeration resulting to a slower transient heat transfer through the micro-aerated chocolate. The reason is that the micro-aerated chocolate can store a larger amount of thermal energy than its solid counterpart. This effect slows down the transient heat transfer rate in the chocolate and modifies melting/solidification rate and impacts sensorial attributes during oral processing and cooling during manufacturing.

Journal article

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

<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

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