Getting a grip: Reversible perspiring artificial 'fingertips'

(Nanowerk Spotlight) Sweating fingertips are a key characteristic that has played a critical role in human survival and evolution. Human fingertips are unique in their design and functionality, characterized by distinct patterns and the ability to perspire. These attributes have significantly contributed to our species' survival and success over time. They have also played a fundamental role in shaping the world around us, enabling the development of intricate tools, complex machinery, and the execution of detailed tasks that set humans apart from other species.
The uniqueness of the patterns on our fingertips, or fingerprints, has been leveraged in the field of forensic science for identification purposes. Additionally, the rise of the biometric security industry has further emphasized the importance of these unique patterns. Biometric devices, such as fingerprint scanners, rely on the uniqueness of individual fingerprints to authenticate and grant access to devices and systems, thereby providing a high level of security.
The ability of our fingertips to perspire, or sweat, also plays a crucial role in our interaction with the physical world. When we grasp an object, the sweat from our fingertips improves our grip by increasing friction between our skin and the object. This is especially important when handling smooth or slippery objects. Perspiration also enhances our tactile perception by improving the transmission of mechanical forces, helping us to better sense the textures and contours of different surfaces.

Creating artificial fingertips

Recognizing these unique attributes has provided inspiration for the development of advanced technologies. In a recent study reported in Advanced Materials ("Reversible Perspiring Artificial 'Fingertips'"), researchers managed to mimic the unique fingerprint patterns and sweating ability of humans in artificially created materials.
The unique patterns, or fingerprints, that we see on our fingertips are due to a phenomenon found in a type of liquid crystals (LCs) called cholesteric LCs. Liquid crystals are unique substances that have properties of both liquids and solid crystals. In cholesteric LCs, the molecules spin around an axis, much like a corkscrew. A full spin of the axis, known as the chiral pitch (P), becomes irregular (like a fingerprint) when balanced with forces from a coated glass substrate, causing the molecules to wind in random directions.
To make these patterns permanent, scientists use reactive molecules that, when treated with light, bind together to form a solid network, much like freezing a moment in time. This process creates a kind of coating or 'skin'.
To mimic the ability of human fingertips to sweat, the scientists introduced a way for the material to hold and release liquid by creating a porous structure in the coating. They did this by mixing two types of molecules during the solidifying process. One type of molecule acted as a sort of template, creating pores or tiny holes in the structure. The size of these pores was controlled by how quickly the mixture was exposed to light and turned solid. This resulted in a porous structure with holes ranging from 70 to 120 nanometers in diameter, as shown in images taken with a powerful microscope.
Design of 'perspiring' artificial LCN fingertips
Design of 'perspiring' artificial LCN fingertips. a) 3D visualization of perspiration in human fingertips compared to liquid secretion in artificial LCN fingertip coating. b) Chemical components used for the formation of the LCN coatings. SEM images of the LCN coating after the removal of porogen (5CB). c) Top view of the LCN coating with zoomed-in insert displaying the sub-micron/nanopores. d) Cross-section of the LCN coating. (Reprinted with permission by Wiley-VCH Verlag)

Light-responsive artificial skin

Finally, to make this artificial skin responsive to light, the team included a type of molecule called azobenzene into the structure. This allows the artificial fingertip to "perspire" or change its surface texture when it is exposed to ultraviolet (UV) light. In terms of friction, these artificial materials can exhibit an anti-sliding property that matches natural fingertips, showcasing a higher degree of imitation of biological characteristics in artificial materials.
The researchers were able to mimic biological characteristics in these artificial materials. This advancement opens up exciting possibilities in the field of soft robotics and beyond. Materials that can control their grip through simulated perspiration 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, combining the unique pattern of fingerprints and the ability to mimic human perspiration, represents a significant milestone in creating biomimetic materials that can function and interact with their environment much like human skin.

Applications and future directions

The researchers envision these biomimetic fingertips finding applications in a variety of fields, such as medical instruments and soft robotic devices that can interact in a more human-like manner. The research also involved creating a soft material that mimics the multifunctional capabilities and complementary properties of human fingertips.
These artificial fingertips also included light-responsive qualities. The researchers went a step further, demonstrating a feature not seen in nature – the ability for these artificial fingertips to secrete liquid from the valleys in their patterns.
In conclusion, the researchers' groundbreaking development of an artificial material that closely mimics the unique properties of human fingertips, including distinctive fingerprint patterns and the ability to perspire, represents a leap forward in the field of biomimetic materials. Created using a specialized liquid crystal network, these 'perspiring' artificial fingertips can release and re-absorb liquids when exposed to specific types of light, offering a degree of tactile control that closely resembles that of human skin.
The potential applications for this technology are vast, ranging from improving 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 remarkable capabilities inherent in nature can lead to innovative solutions and advancements in technology.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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