A fabrication strategy for millimeter-scale, self-sensing soft-rigid hybrid robots

A fabrication strategy for millimeter-scale, self-sensing soft-rigid hybrid robots
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Soft robots offer distinct advantages compared to their rigid counterparts. Their compliance, flexibility, and robustness enable a wide range of tasks, including in-pipe inspection, drug delivery, search and rescue, and minimally invasive surgery. To meet the requirements of these applications, researchers have developed soft robots with various actuation, sensing, and control strategies. However, these soft robots typically involve manual assembly of core hardware components such as actuators, sensors, and controllers. This process not only increases fabrication time but also reduces consistency, especially in small-scale soft robots. Additionally, due to their flexible and deformable nature, soft robots exhibit nonlinear, hysteretic, and viscoelastic behavior, making them inherently more difficult to model and control than rigid systems.

Figure 1. SHY robots overview. (a) A schematic illustration of a SHY robotic module. A SHY robotic module contains a mechanical controller, a PTFE soft-foldable actuator, and tinned copper electrodes to operate in an encoded manner and self-sense its motion. (b) SHY robotic modules are fabricated by laminating films with various properties (i.e., rigid, flexible, soft, and conductive) (c) A PTFE soft-foldable actuator is made of soft and compliant materials and produces motion upon pressurization. (d) Tinned copper films are installed within the robot to embed an ionic resistive sensor. (e) 2-D (flat) and 3-D (expanded) configurations of SHY robotic modules. Three distinct types of mechanisms (translational, bending, and rotation with translational motion) encode the motion of SHY robotic modules. (f) The SHY continuum robot can sense its shape in real-time using the embedded ionic resistive sensors.

Addressing these challenges, a scalable monolithic fabrication method for millimeter-scale soft-rigid hybrid (SHY) robots has been developed in this paper, simplifying the integration of core robotic components (Figure 1a). This method employs heat-sensitive acrylic adhesive films to selectively bond the constituent materials (Figure 1b). While heat-sensitive adhesive films have been researched for bonding rigid and flexible materials, there have been challenges in bonding soft and flexible films (i.e. thermoplastic films), which typically cannot withstand the required temperatures.

To overcome this limitation, polytetrafluoroethylene (PTFE) films, which have a high melting temperature (i.e., 327°C), are introduced for creating soft-foldable actuators for the first time. PTFE films provide high thermal and chemical durability and biocompatibility. Pristine PTFE films have high chemical resistance and cannot bond with adhesive films. Thus, promoting adhesion and permanent bonding through lamination via heat and pressure, permanent chemical surface modification of the PTFE films with hydrogen gas (H2) plasma is performed. Unlike previous attempts in bonding soft and flexible films, such as silicone films (e.g., PDMS bonding via oxygen plasma) and thermoplastic films, where the success of bonding is influenced by factors such as time, temperature, humidity, and cleanliness, the PTFE film bonding is not sensitive to these ambient conditions and time constraints.

Through selective bonding of films, the soft-foldable actuator is created with a bellow shape upon actuation (Figure 1c). When used alone, the soft-foldable actuator can only create linear motion. However, this motion can be mechanically encoded by integrating a mechanical controller composed of rigid-flexible materials. Furthermore, the robot's motion can be self-sensed by integrating an ionic resistive sensor and using an ionic solution as both the working fluid and sensing medium (Figure 1d). By detecting the changes in electrical resistance across its body, the current expansion state of the robot can be estimated.

Figure 2. SHY continuum robots with various end effectors (a grasper, a needle, and an optical fiber). (a) The SHY continuum robot performs object pick and place tasks using the attached grasper. (b) The SHY continuum robot guides the needle toward the target locations, punctures the tissue simulator, and injects red dye. This in-vitro experiment demonstrates robot needle steering and puncturing capabilities. (c) The SHY continuum robot steers an optical fiber toward a brown ring target. Upon reaching the target, the robot successfully delivers focused light, demonstrating its ability to precisely deliver light for medical applications, such as laser ablation.

In this work, SHY robotic modules with three distinct motions (Figure 1e)—translational, bending, and roto-translational—are designed and fabricated in two scales: 5 mm and 11.5mm. These modules can be connected in series to form a continuum robot with real-time shape-sensing capabilities (Figure 1f). Each module can be separately actuated via its own fluidic line, which can be integrated without hindering the robot's motion.

This innovative approach not only simplifies the fabrication process but also enhances the functionality and consistency of small-scale soft robots, making them suitable for complex applications in fields like food handling, medical procedures, and manufacturing. To showcase these potential applications, this paper demonstrates the versatility of SHY robots by attaching various end effectors (Figure 2), including a grasper, needle, and optical fiber, to perform tasks such as object grasping (Figure 2a), in-vitro tissue biopsies (Figure 2b), and light steering for medical applications (Figure 2c). By attaching different end effectors and modules, SHY robots can be tailored to specific applications.

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Robotic Engineering
Technology and Engineering > Electrical and Electronic Engineering > Control, Robotics, Automation > Robotic Engineering
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