Human body’s natural cooling mechanism, also known as sweating, is a messy phenomenon. It leaves you stinking with sweat rings and therefore is not the most desirable experience. It is comical to imagine a world where humans would have also been panting like other animals in order to regulate body heat. At some stage of evolution, we bade goodbye to the fur of our ancestors and welcomed a sweaty skin for the purpose of thermoregulation. In addition to stabilizing the body temperature, sweat glands also secrete antimicrobial substances, which helps in preventing skin diseases.
The human skin has, therefore, been an inspiration for many ‘smart’ materials. Synthesis of chemical surfaces which dynamically alter their composition and physical structure can be achieved by mimicking the skin., While sweating, human skin mainly secretes water. The appeal of mimicking the human skin lies in the fact that secretions by an artificial skin need not be just limited to water. Imagine being able to release antibiotics on demand or initiating chemical reactions with a controlled supply of reactants and substrates. Zhan et. al. designed an artificial skin which does just this.
The above mentioned artificial skin administers fluid by a ‘triggered’ release. On application of some external stimuli, the material releases liquid stored in its pores. The special property here is the ‘triggered’ release and not a passive release of the stored fluid which is usually encountered in most materials. The artificial skins consist of a porous coating with a sub-micrometer pore size. In chemical terms, the material is in fact a ‘highly ordered dielectric liquid crystal polymer network’. A pretty long term, but almost everyone who has a relatively modern television, has made use of this technology and knows it by the commonly used term: Liquid Crystal Display (LCD).
A more interesting aspect of this whole experiment was the external stimuli which is needed to trigger the release. The release is controlled by a Radio frequency (RF) electric field. RF is considered as a low-energy and non-ionizing radiation and is extensively used in a variety of medical applications.
When an RF field is applied, the molecules in the material start oscillating(vibrating) about their respective positions. Although the molecules are tightly embedded in the polymer network, these slight deviations from their original position creates dynamic empty spaces in the network. The polymer network is, therefore, microscopically deformed leading to pressure inside the pores and the liquid stored in these pores is secreted out. An animation showing the phenomenon is depicted below.
Figure 1 Excretion of liquid from the surface when the RF field is switched on. Taken from .
The amount of liquid excreted can be controlled by the strength of the RF field. After the excretion of the fluid, the material can even reabsorb liquid again and conduct the same process for several cycles.
While it is definitely fascinating to make an artificial skin wearing android sweat, the material has a ton of other applications including but not limited to automation of chemical reactions on surfaces, medicine release etc. As a proof of concept, the original paper also demonstrates a triggered release of a chemical dye and Ibuprofen (a common pain killer) using the material.
 Murakami, Masamoto, et al. “Cathelicidin anti-microbial peptide expression in sweat, an innate defense system for the skin.” Journal of Investigative Dermatology 119.5 (2002): 1090-1095. (https://www.sciencedirect.com/science/article/pii/S0022202X15300518)
 Gould, Julie. “Superpowered skin.” Nature 563.7732 (2018): S84-S84 (https://www.nature.com/articles/d41586-018-07429-3)
 Candas, V., J. P. Libert, and J. J. Vogt. “Human skin wettedness and evaporative efficiency of sweating.” Journal of Applied Physiology 46.3 (1979): 522-528 (https://doi.org/10.1152/jappl.1922.214.171.1242)
 El-Domyati, Moetaz, et al. “Radiofrequency facial rejuvenation: evidence-based effect.” Journal of the American Academy of Dermatology 64.3 (2011): 524-535.
Guest post written by Davis Thomas Daniel
After his bachelors in Chemistry from St. Stephen’s College, Delhi, Davis moved to Germany in 2017 to complete his Masters in Chemistry specializing in Magnetic Resonance from University of Bonn. He is currently pursuing his doctoral degree at Forschungzentrum Jülich in Germany where he investigates redox processes in Organic Polymer Batteries using Electron paramagnetic resonance spectroscopy.
He enjoys cooking, surreal humor, casual programming, visiting museums and writing poetry.