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

Humanoid and service robots are increasingly present in daily life, yet their touch often feels hard and cold, evoking discomfort, especially in personal contexts like healthcare. Similarly, prosthetic users and their interlocutors may experience discomfort due to unnatural thermal sensations. Prior work largely focused on materials and interaction dynamics, successfully emulating the texture and firmness of skin, but often overlooked thermal feel. Temperature is crucial for tactile experience, influencing trust, companionship, and comfort. The research question is how to endow artificial skin on humanoid robots and prosthetic hands with human-like thermoregulation and temperature distribution for both infrared recognition and natural touch. The study proposes an artificial skin that mimics human vascular thermoregulation by circulating heated water through flexible, vessel-mimicking silicone fibers embedded under a silicone skin, aiming to reproduce human-like thermal patterns on faces and hands and reduce discomfort during contact.

Existing electronic skin approaches commonly use silicone rubbers and foams to replicate the smooth, elastic surface of human skin. Temperature control in e-skin has employed Peltier devices, resistive heaters, and thermoelectric modules. However, scaling these solutions to broad surfaces on humanoids or prostheses introduces wiring complexity and makes it difficult to generate the heterogeneous temperature distributions characteristic of human skin. Human thermoregulation relies on cardiovascular mechanisms (heart rate modulation, vasodilation/vasoconstriction, sweating, non-shivering thermogenesis), indicating that circulation-based heat transport is central. This motivates a bioinspired approach that uses a circulating fluid and compliant conduits to emulate the distribution and modulation of heat seen in real skin.

Study design and fabrication steps: (i) fabricate blood vessel-mimicked fibers and control their heat dissipation; (ii) fabricate artificial skin in plane, face, and hand forms; (iii) integrate thermoregulation on a mannequin face and a robotic hand; (iv) characterize fiber and skin properties. Blood vessel-mimicked fiber fabrication: Liquid silicone (Ecoflex 00-35, Smooth-On) mixed 1:1 (Part A:Part B) was applied to a 0.5 mm diameter carbon rod (RC Lab). The rod, mounted on an electric drill at ~35 rpm, was coated uniformly while rotating; each coat cured ~5 min while rotating to prevent sagging. The sequence was repeated for five coats to reach ~2 mm outer diameter. The cured silicone was removed from the rod, yielding a hollow fiber with an internal diameter ~500 µm. Circulation and thermal control system: A 2 L beaker with 1.8 L water on a heating plate (MSH-20D) with an external probe (DH.WMH03021) set inflow water temperatures from 30–70 °C. A peristaltic pump (BT300-2J, Longer) with 1250 mm, 2.4 mm-diameter tubing (#19) circulated water through the fibers in a closed loop. Three 400 mm fiber segments were connected via a three-way connector (QN659250), returning to the beaker via an 800 mm #19 tube. Micro silicone tubes (2 mm) bridged connections. Flow rate was controlled from 1.10–11.0 cc/min by pump speed (10–120 rpm). An Arduino-controlled pump enabled inflow frequency modulation from 1–8 Hz to emulate pulsatile flow; the system was tested for leakage under varying conditions. Artificial skin fabrication—planar: Silicone rubber (Mold Max 14 NV, Smooth-On) mixed 10:1, pigmented with 0.6 ml apricot pigment per 300 g silicone, was cast in a 599 × 402 × 30 mm³ tray to thicknesses of 0.5, 1.0, 2.0, and 3.0 mm, curing 4 h ambient. Artificial skin fabrication—facial: A 1 mm plaster bandage captured a mannequin face and dried 12 h. A mold (Mold Max 60, 100:3 mix) was made from the impression, cured 12 h, and a 1 mm facial skin was cast (release agent ER-200), cured 4 h. Artificial skin fabrication—hand: Dense silicone rubber was coated over a skeletal robotic hand and cured 4 h to form a base layer. Vessel-mimicked fibers were laid atop, then a 1.0 mm planar skin was sectioned into six pieces to cover five fingers and the dorsum. Thermoregulation integration—face: Two peristaltic pumps (BT100-2J; A and B) each drove three fibers (six total) arranged on a mannequin face following human thermal distribution, covered with face-shaped skin. Thermoregulation integration—hand: Nine fibers were integrated. Pump A drove six fibers via one three-way and three Y connectors (QN830160); pump B drove three fibers via one three-way connector. Fibers were placed according to human hand thermal distribution over the silicone base and covered with planar skin sections. Characterization and testing: Morphology was imaged by FE-SEM (JSM-7610F PLUS). Mechanical properties (resilience, endurance) were measured via UTM (TD-U01). Thermal behavior was assessed using a hot plate set to 30, 50, and 70 °C and ice water (cooling) beneath the skin to probe thermal mapping and interference. For face-shaped skin circulation: Pump A ran 54 °C water at 5.39 cc/min, 2 Hz for 15 min; Pump B ran 52 °C at 4.47 cc/min, 2 Hz for 15 min. For hand-shaped skin: Pump A ran 52 °C at 6.26 cc/min, 2 Hz for 15 min; Pump B ran 50.5 °C at 5.39 cc/min, 2 Hz for 15 min. Thermal infrared images were captured with a FLIR T420 and analyzed using FLIR Tools to quantify temperature distributions and oscillations.

• Vessel-mimicking fibers: Hollow silicone fibers (~2 mm OD, ~500 µm ID) exhibited high compliance with ~700% strain at rupture and ~1.2 MPa ultimate stress.
• Thermal control via water temperature: At fixed 4.47 cc/min and 2 Hz, increasing water temperature from 30 to 70 °C raised fiber temperature from 25.4 to 43.1 °C.
• Thermal control via flow rate: At 60 °C and 2 Hz, increasing flow from 1.10 to 5.39 cc/min increased fiber temperature from 23.8 to 43.3 °C.
• Combined mapping: Across 30–70 °C water and 1.10–11.0 cc/min (2 Hz), fiber temperature rose with both water temperature and flow rate. At 11.0 cc/min and 2 Hz, water temperature increases from 30 to 70 °C yielded fiber temperatures from 27.3 to 51.1 °C (Δfiber ≈ 24 °C).
• Heat-transfer interpretation: Due to Fourier’s law, higher internal water temperatures increase heat loss, producing larger water–fiber temperature gaps: 30/27.3, 40/33.7, 50/39.5, 60/48.3, 70/52 °C pairs correspond to differences of 2.7, 6.3, 10.5, 11.7, and 17.9 °C.
• System-level demonstrations: Thermoregulatory artificial skin applied to a mannequin face and a robotic hand reproduced human-like thermal infrared distributions. Face demo used two pumps at 54/52 °C and 5.39/4.47 cc/min (2 Hz). Hand demo used 52/50.5 °C and 6.26/5.39 cc/min (2 Hz). Infrared imaging confirmed human-like thermal patterns.
• Human factors relevance: Given human sensitivity to ~0.5 °C differences, achieving a human-like thermal range suggests reduced discomfort during touch with prosthetic/robotic hands and improved IR camera recognition of humanoid faces.

The study demonstrates that embedding compliant, vessel-like silicone fibers carrying heated water beneath silicone skin can mimic human thermoregulation on robotic faces and prosthetic hands. By tuning inflow water temperature, flow rate, and pulsation frequency, the system modulates local heat emission analogous to vasodilation/vasoconstriction, producing heterogeneous, human-like thermal distributions. Infrared imaging verified that these distributions resemble those of real faces and hands, addressing the need for naturalistic thermal signatures for both machine perception (IR recognition) and human tactile comfort. The findings directly address the research question by providing a practical, scalable alternative to resistive/Peltier-based heating that struggles with wiring and uniformity over large, contoured surfaces. The approach leverages superior thermal transport of water, the compliance of silicone conduits, and adjustable pumping to achieve fine-grained thermal control that aligns with human perceptual thresholds, promising more natural human-robot interactions and prosthesis acceptance.

This work introduces a bioinspired thermoregulatory artificial skin that uses water-circulating, compliant silicone fibers to emulate vascular heat distribution, achieving human-like thermal signatures on robotic faces and prosthetic hands. The system provides controllable, heterogeneous temperature patterns validated via infrared imaging and offers a route to improved tactile comfort and enhanced IR camera recognition. Future research could integrate closed-loop thermal sensing for autonomous temperature regulation, optimize fiber routing for specific anatomical regions, miniaturize and embed pumping/heating hardware, and combine the thermal system with multimodal e-skins for comprehensive lifelike touch.