A smart robotic adhesive skin using shaped-memory polymers

• Main Author: Li, Yan
• Other Authors: K Jimmy Hsia
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/168534
• Institution: Nanyang Technological University

In recent years, there has been a significant increase in the popularity of robotic hands leading to a wide range of applications that span across industry, agriculture, and our daily lives. To expand the capabilities of robotic hands, researchers are actively developing smart skin technology. The smart skin, also known as e-skin or intelligent skin, is a thin, flexible, and stretchable structure that can sense and respond to changes in its environment. It endows robotic hands many advanced functionalities such as the sensing capabilities of temperature, pressure, and sliding. One category of smart skin is the adhesive skin, which can provide strong/weak adhesion and has been found to be effective in handling soft, heavy, fragile, or hard objects, thus enhancing both the adaptability and grasping abilities of robotic hands. Electro-adhesion and Gecko-adhesion are two commonly used adhesion methods for adhesive skins. However, their application is limited to permanent adhesion, which means that adhesion occurs whenever contact is formed. In certain scenarios, such as robotic detection, sensing, or gripping and releasing of lightweight objects, adhesion may not be desirable. To overcome this limitation, this report proposes a smart adhesive skin that provides on-demand attachment and detachment for robotic hands. This smart adhesive skin utilizes Shape Memory Polymer (SMP), a type of smart material that owns dramatically tunable modulus and can be deformed and fixed into a temporary shape upon stimulation and recover its original shape upon re-stimulation. The SMP adhesive skin is featured by four distinct states: rigid contact state with little adhesion, conformal contact state facilitating adhesion, DMT-regime strong adhesion for attachment, and JKR-regime weak adhesion for detachment. Furthermore, the report emphasizes the desirable properties of SMP, including a low glass transition temperature, a significant change in modulus through the glass transition temperature, shape-locking ability upon cooling, and permanent shape-memory property upon heating. An epoxy-based SMP material composed of epoxy monomer E44 and the curing agent JEFFAMINE D-230 with a mass ratio of 81:46 was firstly selected followed by mass ratio optimization through the DMA tests. Normal and shear adhesion tests were conducted on SMP pillars in both the rubber-to-glass (R2G) and rubbery states, showing the strong and weak adhesion strength, respectively. Adhesion tests were further conducted to optimize the aspect ratio the SMP pillar. Considering ease of fabrication and values of compliance and adhesion strength, an SMP pillar with an aspect ratio of 1 was chosen for subsequent application. SMP skin molds were then created by arranging the SMP pillars in a matrix and designing them in various shapes based on the geometries of the robotic hand. The SMP adhesive skins were integrated with flexible heating elements in the molds and then attached to the robotic hand for demonstration. With the adhesive skin, each finger of the robotic hand can simultaneously pick up a quail egg, which is beyond the capabilities of a human hand. Plus, this adhesive skin can facilitate the robotic hand to grip objects made of various materials (e.g., wood to plastic and glass) with different shapes. Finally, a demonstration of gripping and easy release of a very light cloth showed the capability of the SMP adhesive skins in handling light-weight objects under the glassy state with little adhesion. The ability to handle diverse objects with on-demand adhesion and detachment underscores the potential of SMP adhesive skins to empower the robotic hands in a wide range of applications, including manufacturing, medicine, and space exploration.