Written by Annie Duong and Edited by Emily Majorkiewicz
When you feel the cotton of a t-shirt or the keys of a computer keyboard, you may not think twice about your skin’s contact on these objects and may take this sense for granted. Our sense of touch is possibly even more sensitive than previously imagined. Our somatosensory system of neurons helps us interpret the extensive range of tactile stimuli, where cutaneous sensory neurons are the first step leading to our perception of touch [1]. These primary sensory neurons, which have axons from the dorsal root and cranial sensory ganglia, reach the periphery neurons, synapse on spinal cord neurons and send messages to the brain [1]. But, what if these neurons can detect substances on a molecular level?
Researchers from the University of California, San Diego have found that humans have the ability to tell apart surfaces based purely on surface chemistry. Two silicon wafers of different surface textures were used to test whether subjects could determine the difference between the two wafers by touch alone [2]. After the wafers were treated with fluorine, carbon, and oxygen atoms, the discrepancies between the surfaces were found to be miniscule, with only a 0.003 nm difference between them [2]. When the subjects were allowed to freely explore the surfaces and tap them in order to determine if there were any differences between them, it was found that free exploration resulted in subjects discriminating between the surfaces more accurately. To eliminate any confounding variables, moisture levels on the surfaces were kept constant by wiping of the wafers with isopropanol and oxygen plasma treatment [2]. It was found that the subjects were indeed able to discriminate between the surfaces accurately enough that chance had little significance in their responses.
The subjects in the experiments described the difference between either of the wafers as smoother, stickier, or slipperier. It is possible that friction helped them distinguish between the surfaces, but after further experiments, it was clear that they were able to perceive difference on the surfaces on a molecular level [2]. There was a higher accuracy when the subjects were allowed to freely explore rather than tap the surfaces. Another possible reason could be that fingers deposit eccrine secretions and dead skins cells, all of which are composed of amino acids, lipids, and inorganic ions. Stick slip friction, which is the motion that occurs when two objects have sliding contact against each other, is responsible for creating different stick-slip frequency, which is then detected by our skin [3]. These factors may have also influenced how the subjects were able to distinguish between the different surfaces [2].
These findings have potential applications in the biomedical industry. Knowing exactly how touch and its sensitivity works may be useful in developing electronic skin or prosthetics that have the ability to feel in virtual reality settings [3].
References:
[1] Abraira, V. E., and Ginty, D. D. 2013. The Sensory Neurons of Touch. Neuron , 79 (4): 1-41
[2] Carpenter, C., Dhong, C., Root, N., Rodriguez, D., Abdo, E., Skelil, K., Alkhadra, M., Ramirez, J., Ramachandran V., Lipomi, D. 2017. Human ability to discriminate surface chemistry by touch. Mater. Horiz. 5: 70 – 77.
[3] Labios, Liezel. “Less Than Skin Deep: Humans Can Feel Molecular Differences Between Nearly Identical Surfaces.” UC San Diego News Center. 13 Dec. 2017.