Nanotechnology

Gripping: Reversible sweaty artificial ‘fingertips’

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May 11, 2023

(Nanowerk Highlights) Sweaty fingertips are a key characteristic that plays an important role in human survival and evolution. The human fingertip is unique in design and function, characterized by a distinct pattern and ability to sweat. These attributes have contributed significantly to the survival and success of our species over time. They also play a fundamental role in shaping the world around us, enabling the development of complex tools, intricate machines and the execution of detailed tasks that set humans apart from other species.

The unique patterns on our fingertips, or fingerprints, have been used in the field of forensic science for identification purposes. Moreover, the rise of the biometric security industry has further emphasized the importance of this unique pattern. Biometric devices, such as fingerprint scanners, rely on the uniqueness of an individual’s fingerprint to authenticate and grant access to devices and systems, thereby providing a high level of security.

The ability of our fingertips to sweat also plays an important role in our interactions with the physical world. When we grip an object, the sweat from our fingertips improves our grip by increasing the friction between our skin and the object. This is especially important when handling smooth or slippery objects. Sweat also improves our tactile perception by increasing the transmission of mechanical forces, helping us to better feel the textures and contours of different surfaces.

Making artificial fingertips

Recognizing these unique attributes has provided inspiration for the development of advanced technologies. In a recent study reported in Advanced Materials (“Reversible Sweating Artificial Fingertips”), researchers managed to replicate the unique fingerprint patterns and sweating abilities of humans in artificially created materials.

The unique patterns, or fingerprints, we see on our fingertips are caused by a phenomenon found in a type of liquid crystal (LC) called cholesteric LC. Liquid crystals are unique substances that have the properties of both liquid and solid crystals. In cholesteric LC, the molecules rotate around an axis, like a corkscrew. The full rotation of the axis, known as the chiral pitch (P), becomes irregular (like a fingerprint) when counterbalanced by forces from the glass substrate on which it is coated, causing the molecules to spin in random directions.

To make these patterns permanent, scientists use reactive molecules that, when treated with light, bind together to form a dense network, like a momentary freeze. This process creates a kind of coating or ‘skin’.

To mimic the ability of human fingertips to sweat, scientists introduced a way for materials to retain and release fluids by creating porous structures in the coating. They do this by mixing the two types of molecules during the solidification process. Either type of molecule acts as a kind of template, creating tiny pores or holes in the structure. The size of these pores is controlled by how quickly the mixture is exposed to light and solidifies. This results in a porous structure with holes ranging in diameter from 70 to 120 nanometers, as shown in images taken with a powerful microscope. Artificial LCN fingertip design that 'sweats' Artificial LCN fingertip design that ‘sweats’. a) 3D visualization of sweat on human fingertips compared to fluid secretion in LCN artificial fingertip coatings. b) Chemical components used for the formation of the LCN layer. SEM image of the LCN layer after porogen (5CB) removal. c) Top view of LCN layer with enlarged insert showing sub-micron/nanopores. d) Cross section of LCN layer. (Reprinted with permission by Wiley-VCH Verlag)

Artificial leather that is responsive to light

Finally, to make the artificial skin responsive to light, the team incorporated a molecule called azobenzene into its structure. This allows the artificial fingertip to “sweat” or change the texture of its surface when exposed to ultraviolet (UV) light. In terms of friction, these artificial materials can exhibit anti-slide properties that match those of natural fingertips, indicating a higher degree of imitation of biological characteristics than artificial materials.

The researchers were able to mimic the biological characteristics in the artificial material. These advances open up exciting possibilities in the field of soft robotics and beyond. Materials that can control their grip through simulated sweat could lead to the creation of more human-like prosthetics or robotic systems that can interact with their environment in more sophisticated ways.

This innovation in material design, incorporating a unique fingerprint pattern and the ability to mimic human sweat, is a significant milestone in creating biomimetic materials that can function and interact with their environment much like human skin.

Future applications and directions

The researchers envision these biomimetic fingertips finding applications in areas such as medical instruments and soft robotic devices that can interact in more human-like ways. The research also involved creating a soft material that mimics the multifunctional capabilities and appendages of the human fingertip.

This artificial fingertip also includes responsive qualities to light. The researchers went a step further, demonstrating a feature not seen in nature – the ability of these artificial fingertips to eject fluid from the valleys in its patterns.

In conclusion, the researchers’ development of a breakthrough of an artificial material that closely resembles the unique properties of the human fingertip, including a distinctive fingerprint pattern and the ability to sweat, represents a leap forward in the field of biomimetic materials. Created using a special network of liquid crystals, these ‘sweaty’ artificial fingertips can release and reabsorb fluids when exposed to certain types of light, offering a level of tactile control very close to that of human skin.

The potential applications for this technology are wide-ranging, from increasing the dexterity and sensitivity of robotic devices to new possibilities in drug delivery and even advanced information transfer between machines. Ultimately, this research underscores the exciting future of biomimicry, where harnessing the extraordinary abilities inherent in nature can lead to innovative solutions and technological advances. By
Michael is the author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: A Small FutureAnd
Nanoengineering: Skills and Tools for Making Technology Invisible
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