Thermoregulatory Integration in Hand Prostheses and Humanoid Robots Through Blood Vessel Simulation

The increasing prevalence of humanoid robots in service roles highlights the need for more natural and comfortable human-robot interaction. While previous research has focused on replicating skin texture and firmness, the thermal aspect of tactile sensation has been largely overlooked. The temperature of a robot's skin significantly influences human perception and interaction, impacting feelings of trust and comfort. This study addresses this gap by developing artificial skin capable of replicating the human body's temperature regulation mechanism. This involves understanding and mimicking the circulatory system's role in thermoregulation, which involves processes like vasodilation and vasoconstriction to maintain a constant core body temperature. The goal is to create artificial skin with a lifelike temperature distribution to improve human-robot interaction, particularly in applications such as prosthetic limbs.

Existing research on artificial skin has focused on replicating the texture and firmness of real skin using materials like silicone rubber and foam. Temperature control has been attempted using Peltier devices, resistive heaters, and thermoelectric modules, but these methods face challenges in application to large surfaces and in creating diverse temperature distributions like those found in the human body. This paper builds upon previous work by focusing specifically on the thermoregulatory aspect and proposing a system inspired by the human circulatory system.

The study involved several steps: (i) Fabrication of blood vessel-mimicked fibers from stretchable silicone, creating hollow fibers with a diameter of ~500 µm; (ii) Fabrication of artificial skin in planar, facial, and hand shapes using silicone rubber, incorporating the blood vessel-mimicked fibers; (iii) Development of a thermoregulation system using peristaltic pumps to control water flow (temperature, flow rate, and frequency) through the fibers; (iv) Application of the thermoregulatory artificial skin to mannequin faces and robot hands; and (v) Measurement of the properties of the fibers and the artificial skin using FE-SEM, UTM, and a thermal infrared imaging camera. The fabrication process involved meticulous control of silicone curing, fiber arrangement, and water parameters to mimic human skin's thermal infrared temperature and distribution patterns. Experiments systematically varied water temperature, flow rate, frequency, skin thickness, and fiber configuration to observe their impact on heat dissipation.

The researchers successfully fabricated flexible, elastic fibers mimicking blood vessels. These fibers, when integrated into artificial skin and coupled with a water circulation system, effectively replicated the temperature variations observed in human skin. The study demonstrated a direct correlation between water temperature and flow rate and the temperature of the fibers. Higher water temperatures and flow rates led to higher fiber temperatures, but the increase in fiber temperature was less pronounced than the increase in water temperature due to increased heat loss at higher temperatures (consistent with Fourier's Law). The artificial skin, applied to mannequin faces and robot hands, exhibited realistic thermal infrared patterns, closely resembling those of human faces and hands. The mechanical properties of the fibers showed high strain (~700%) and stress (1.2 MPa) before rupture. The system demonstrated precise control over the temperature distribution, offering a solution to the challenges of creating lifelike temperature distributions in prosthetics and humanoid robots.

The findings address the critical need for more human-like tactile interactions with robots and prosthetics. The successful replication of human skin's thermoregulatory function significantly improves the comfort and natural feel of artificial limbs and robotic faces. The system's adaptability, as demonstrated by its application to both faces and hands, suggests its potential for widespread use in diverse robotic applications. The use of water as a heat transfer medium offers advantages over other fluids due to its high thermal conductivity. The ability to control the temperature distribution precisely by adjusting water parameters offers significant advantages over previous methods.

This study demonstrates a significant advancement in the field of biomimetic robotics. The developed thermoregulatory artificial skin offers a promising solution for creating more natural and comfortable interactions between humans and robots. Future research could focus on further miniaturization of the system, integration with more sophisticated control algorithms, and exploration of different materials for improved biocompatibility and durability.

The current system utilizes a relatively simple water circulation system. Future iterations could incorporate more sophisticated control mechanisms for even finer temperature regulation. The long-term durability and biocompatibility of the materials used in the artificial skin require further investigation. Further research is needed to assess the system's performance in dynamic environments and under various environmental conditions.