How Much Touch?
A high-resolution optical coherence tomography tool reveals new detail into the real contact mechanics between a finger and a touchscreen.
Written by Mark Crawford
UNDERSTANDING THE INTERACTION OF SKIN with surfaces is important for many applications, such as medicine, bioelectronics, and consumer products. Performance is strongly governed by frictional forces, which are intrinsically linked to the real area of contact. The real contact area, in turn, is impacted by factors such as skin hydration and the topographical roughness of the interacting surfaces.
To gain a deeper understanding of these interactions, a group of researchers at Sheffield University and Loughborough University in the U.K., led by Raman Maiti, and using a new high-resolution optical coherence tomography (OCT) tool developed in-house, analyzed the size of real contact area and the stratum corneum thickness in finger-pad interfaces.
“The study investigated the interaction between finger-pad skin and surfaces, focusing on quantifying the real contact area during sliding using advanced optical coherence tomography [OCT] systems and examining the influence of skin thickness and surface roughness on friction,” he said.
His research path was shaped by a growing curiosity about how human skin interacts with surfaces, particularly in situations where grip, touch, and control are critical to performance. “Applications in areas such as medical skin grafts, wearable sensors, and robotic manipulation show how essential it is to understand friction and the real contact area between skin and different materials,” Maiti said.
This figure illustrates (a) the architecture of the new high-resolution OCT. (In: inside, O: outside, CL: collimator, PC: polarization controller, ND: neutral density). It also shows a comparison of a finger-pad loaded at 1 N compressive force and imaged in (b) high resolution and (c) with the VivoSight OCT system. Photo: Maiti, et al.
Instrument Testing and Calibration
Traditionally, OCT relied on indirect measurements or lower-resolution imaging. “However, with the new OCT system, we could directly visualize the skin layers and the true contact interface at an unprecedented level of detail,” Maiti said. “This allowed us to see that the actual contact area was significantly different from what had previously been measured, revealing how strongly it is influenced by deformation, hydration, and surface characteristics. This combination of new measurement capability and the insight it provided into real contact mechanics was the turning point.”
One of the biggest research challenges in developing this idea was both experimental and methodological. Traditional measurement approaches often relied on indirect estimates or lower-resolution tools, which limited the accuracy of the results. A key challenge, therefore, “was developing and implementing a high-resolution OCT system capable of imaging beneath the skin surface, while maintaining sufficient spatial resolution,” Maiti said. “This required careful calibration and validation, particularly when comparing results with existing lower-resolution systems.”
Another major difficulty was dealing with the dynamic nature of the experiments. Capturing the finger pad skin motion under the imaging was difficult and required a lot of capturing and analysis, especially during sliding contact. Even small movements could affect image quality and data consistency, making it challenging to extract reliable measurements of contact area and stratum corneum thickness.
In addition, interpreting the OCT data itself was complex, Maiti noted. The deformation of the skin layers, particularly the observed increase in stratum corneum thickness during sliding, required detailed post-processing and careful analysis to understand how these changes influenced friction and contact mechanics.
Other research considerations included the influence of skin thickness and deformation on contact as well as the relationship between friction, contact pressure, and surface roughness. The team also had to keep in mind the connections between skin morphological parameters and friction, the influence of surface roughness on friction, along with friction coefficient and contact area relationships. Counterface and finger-pad surface roughness both also have an effect on contact mechanics.
Two noninvasive OCT devices were used to quantify the contact area during finger-pad sliding on a smooth glass surface: the in-house high-resolution OCT device and a lower-resolution VivoSight® OCT device. Forty-seven finger-pad sliding tests against smooth glass were performed using three different volunteers with forces ranging from 0.5 to 3 N. Post-test analyses of the OCT images captured revealed that the real measured contact area was 54± 7 percent of the apparent contact area using the high-resolution OCT device in comparison to 63± 10 percent measured with the VivoSight OCT.
Biggest Surprise
The researchers were surprised by the difference between the real and apparent contact area and how much the skin deforms during sliding. “The high-resolution OCT revealed a lower real contact area than previously measured and showed that the stratum corneum thickens under motion due to deformation,” Maiti said. “The finger pad behaving under different loads and roughness was also surprising, highlighting how sensitive and adaptive skin is in real contact conditions.”
Mechanical engineers will be most interested in how contact mechanics and friction impact soft interfaces. Using high-resolution OCT, the researchers were also able to directly measure the real contact area and observe how it evolves under load and sliding, which is critical for accurate modeling of tribological systems. The work also highlights how skin deformation and material properties (like the stratum corneum) influence contact behavior, providing insights for designing better interfaces. This offers more information that can be useful in the development of sports devices, interacting with finger pad design of robotic finger skin.
“These findings have implications for the design of gripping systems and robotic fingers, where roughness, thickness, and contact pressure collectively affect grip performance.”
—Raman Maiti, lecturer, Wolfson School of Mechanical, Electrical, and Manufacturing Engineering, Loughborough University
The high‑resolution OCT setup for imaging the finger pad, with the bottom‑right inset displaying the corresponding OCT‑acquired image. Photo: Raman Maiti, Loughborough University
Next Steps
The results justify using a high-resolution OCT system, as it enables precise visualization of the contact perimeter at the tribological interface, thereby facilitating a more accurate quantification of interfacial geometry and contact mechanics. The thickness of the stratum corneum was also seen to increase during sliding under the high-resolution OCT device. This resulted from the high skin deformation, which in turn influenced the contact area.
Across all three sliding stages, namely, postloading, sliding, and postsliding, the real contact area ranged from 54 percent to 69 percent of the apparent contact area. “Both root-mean-square and average roughness values were found to decrease with increasing contact area,” Maiti said. “These findings have implications for the design of gripping systems and robotic fingers, where roughness, thickness, and contact pressure collectively affect grip performance.”
Additional work will refine measurement techniques by the optics and tribology teams and expand the study to a wider range of surfaces, conditions, and participants. A key focus will be developing new systems with more resolutions to capture the skin cells present at the epidermis layer, enabling even deeper insight into skin mechanics. “In the coming years, this research will move toward improving predictive models of skin-surface interaction and applying these findings in areas such as biomedical devices, wearable technology, and robotic touch systems,” Maiti concluded.
Mark Crawford is a technology writer in Corrales, N.M.
INTERESTED IN LEARNING MORE?
“Quantification of the Real Contact Area of a Finger-Pad During Sliding Using a Novel Optical Coherence Tomography System and the Influence of Skin Thickness” was published in the May 2026 issue of the Journal of Tribology.
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