Design, Fabrication, and Demonstration of Soft Thin-Film Actuators

Summary

Soft actuators can be used in the fabrication of soft robots and wearable devices. Their compliance enables them to maintain functionality even when deformed. Here, a protocol is presented to fabricate soft thin-film actuators and to demonstrate their application in crawling robots, soft robotic grippers, and haptic feedback interfaces.

Abstract

Soft actuators present promising properties regarding applications in soft robotics and human-machine interfaces. Especially when directly interfacing with human skin, soft thin actuators can match the mechanical coupling with human tissue. The exploration of improving the performance and robustness of soft actuators is crucial for the development of flexible devices. The demand for robust, flexible devices poses challenges to the performance and manufacturing methods of the actuators. The article illustrates techniques for the design and fabrication of soft thin-film actuators. These soft actuators are robust, and demonstrate potential in versatile scenarios. This protocol shows the potential of the thin-film actuators in three different applications. First, a crawling robot driven by soft thin-film actuators was fabricated. The robot is extremely robust. It can survive continuous hammering and remain functional even with three metal needles punctured through its body. Second, a soft robotic gripper, which can perform fine manipulation such as grasping a piece of paper without damaging it, was fabricated. Third, a haptic feedback device with a thin-film actuator was fabricated, which can provide touching feedback when interacting with human skin. This protocol demonstrates efficient fabrication processes for robust soft thin-film actuators, with various application potential for driving robust soft robotics.

Introduction

Compared to rigid motors, flexible actuators exhibit superior compliance and adaptability¹. They are widely used in the manufacturing of soft robots and sensors. Current types of soft actuators include pneumatic actuators², chemical fuel actuators³, hydraulic actuators⁴, shape-memory alloy (SMA) actuators⁵, dielectric elastomer actuators⁶, and electroactive polymers actuators⁷. The purpose of this research is to design a flexible actuator with a simple manufacturing process androbust performance. The above characteristics will be described in the following text.

The developed soft thin-film actuator (STFA) operates via the electroadhesion (EA) principle⁸. The structure and operation principle of STFAs are shown in Figure 1. Its structure comprises an active layer and a substrate layer separated by an air gap. When a voltage is applied, electrodes in the active layer generate electric fields, inducing opposite charges in the substrate layer. Electrostatic attraction between these charges causes the active layer to bend toward the substrate. The actuation of the STFA can be controlled by modulating the frequency and amplitude of the voltage signal.

STFAs can achieve fast actuation and maintain functionality under mechanical disturbances. For example, a crawling robot using STFAs maintained functionality under continuous hammer strikes⁹, and even being punctured through with three metal needles (demonstrated in the Supplementary Video 1 and Supplementary Video 2). As grippers, flexible actuators outperform rigid counterparts in adaptability¹⁰. Their compliant surfaces conform closely to object geometries, minimizing damage risks to fragile items during grasping.

The STFAs' operational stability fits the requirements of wearable applications. They directly transduce electrical signals into mechanical stimuli through spatially targeted tactile actuation. A haptic feedback interface constructed with STFAs, demonstrates effective information transfer by delivering multi-intensity signals to the fingertips⁹.

The main contribution of this study is to propose an efficient actuator with a simple fabrication process. The STFA is flexible, robust, and can maintain good performance in terms of power consumption.STFAs' multifunctional potentials are validated through three applications: crawling robots, soft robotic grippers, and haptic feedback interfaces. The crawling robots demonstrate the STFAs' motion-driving capabilities. The haptic feedback interfaces highlight their human-machine interaction utility. These implementations collectively confirm the STFAs' potential in soft robotics and wearable technologies.

Protocol

This protocol describes the fabrication phases of STFA and STFA-based crawling robots, soft robotic grippers, and haptic feedback interfaces. Figure 2, Figure 3, Figure 4, and Figure 5 show the step-by-step device fabrication process. The reagents and the equipment used in this study are listed in the Table of Materials.

1. Fabrication of a soft thin-film actuator

1. Design the interdigitated electrode pattern (Figure 2A).
2. Configure laser engraving parameters (cutting: power = 20, speed = 10, PPI/Hz = 1000).
3. Cut a piece of polyethylene terephthalate (PET) foil with aluminum coating (100 µm in thickness), with dimensions of 30 cm × 20 cm.
4. Position the PET foil, aluminum-side up, into the laser cutter workspace.
5. Perform laser patterning of the PET foil (engraving: power = 55, speed = 100, PPI/Hz = 1000, cutting: power = 20, speed = 10, PPI/Hz = 1000, Figure 2B and Figure 2C).
6. Attach electrical leads: Place two wires at the terminal pads of the patterned PET foil. Secure connections with conductive adhesive.
7. Encapsulate the device: Fully cover the PET foil-lead interface with adhesive tape (Figure 2D).
   NOTE: The completed sample of the STFA is shown in Figure 2E.

2. Fabrication of the crawling robot

1. Draw the top view pattern of the front and back leg for the crawling robot using CAD software (Figure 3A).
2. Cut the leg patterns using the laser cutter from a 100 µm-thick polyethylene terephthalate (PET) foil (cutting: power = 20, speed = 10, PPI/Hz = 1000, Figure 3B).
3. Prepare two 0.1 mm-thick double-sided adhesive tapes (11 mm × 3 mm each). Attach them to the front and rear ends of the STFA (Figure 3C).
4. Bend the tips of the limbs at approximately 45° with tweezers and ensure tips form a consistent angle (Figure 3D).
5. Fabricate an STFA with overall dimensions of 35 mm × 11 mm, featuring 1 mm-wide electrodes and 0.5 mm interelectrode spacing.
6. Assemble the crawling robot by adhering the front and back legs to the STFA (Figure 3E).

3. Fabrication of the soft robotic gripper

1. Fabricate STFA with Polydimethylsiloxane (PDMS) as the substrate:
   1. Add 20 g of PDMS Sylgard part A and 0.4 g part B to a mixing pot and mix the silicone with a Vacuum mixer (phase 1: pressure = 100 kPa, operating time = 30 s.; phase 2: pressure = 1 kPa, operating time = 30 s).
   2. Use the micro automatic coating machine (running speed = 2, scraper height = 0.7 mm) to apply the mixed silicone onto an A4-sized PET substrate (Figure 4A).
   3. Cut the mask with interdigitated electrode pattern (cutting: power = 20, speed = 10, PPI/Hz = 1000, Figure 4B).
   4. Place the mask on silicone-coated PET and use an airbrush (with nozzle distance of 10 cm, pressure of 15 psi, angle of 45-60 degrees, and dosage of 5 mL) to spray carbon powder to form the electrodes (Figure 4C).
   5. Remove the mask and verify that there is no carbon powder smear between adjacent electrodes after mask removal (Figure 4D).
   6. Cut out the STFA and dissolve P(VDF-TrFE-CTFE) powder in Dimethyl Formamide (4.76 wt%)¹¹. Use the airbrush (with nozzle distance of 10 cm, pressure of 15 psi, angle of 45-60 degrees, and dosage of 10 mL) to spray the mixed solution on the electrode (Figure 4E).
   7. Attach electrical leads to the STFA (Figure 4F).
   8. Remove the A4-sized PET substrate
2. Integrate the STFAs into the robotic arm.
3. Route the electrical connections.

4. Fabrication of the haptic feedback interface

1. Fabricate an STFA with overall dimensions of 35 mm × 11 mm, featuring 1 mm-wide electrodes and 0.5 mm interelectrode spacing.
2. Design the cover layer and substrate layer patterns for the STFA (Figure 5A).
3. Design the external contour pattern for the substrate layer (Figure 5B).
4. Laser-cut of the cover and substrate layers following step 1 (cutting: power = 20, speed = 10, PPI/Hz = 1000, Figure 5C).
5. Laser-cut a 0.8 mm-thick adhesive tape matching the contour pattern from step 4.3 and bond it to the substrate layer (cutting: power = 20, speed = 10, PPI/Hz = 1000, Figure 5D).
6. Paste the STFA onto the substrate layer (Figure 5E).
7. Paste the cover layer onto the substrate layer (Figure 5F).
8. Integrate STFA with connectors to assemble the haptic feedback interface (Figure 5G).
   NOTE: The structure and the completed sample of the haptic feedback interface are shown in Figure 5H,I.

Results

To demonstrate the proposed method, the protocol outlines the fabrication of prototype devices and the performance of functional validation. First, the manufacturing process of the STFA is detailed. Figure 2A shows the designed electrode pattern, comprising interdigitated electrodes terminating in connection pads for lead attachment. Figure 2E shows the completed STFA. During operation, applying a voltage across adjacent interdig...

Discussion

Using this methodology, successful fabrication of STFAs was achieved, followed by a demonstration of their potential in three distinct applications. The STFA fabrication involves three critical steps: (1) appropriate 2D design: the electrode pattern geometry dictates the actuator's dimensions, while the quantity and dimensions of interdigitated electrodes critically determine the actuation performance; (2) reliable electrical interfacing: proper connection of the aluminum electrode layer to external leads is achieved...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.1 mm Double-sided tape	3M
0.8 mm Double-sided tape	3M	VHB
3D printer	Bambu	X1-Carbon
Airbrush	Clench	LP-131	The nozzle distance is 10 cm, the pressure is 15 psi, the angle is 45 to 60 degrees
AutoCAD	Autodesk
conductive adhesive	Jinshida	K-818
Dimethyl Formamide	Macklin	CAS 68-12-2
Laser cutter	Trotec	Q400	Engraving: power=55, speed=100, pulses per inch (PPI)/Hz = 1000 Cutting: power = 20, speed = 10, PPI/Hz = 1000
Micro automatic coating machine	BEVS	1818H	running speed = 2, scraper height = 0.7 mm
P(VDF-TrFE-CTFE)	Piezotech-Arkema
Polyethylene terephthalate (PET) foil with aluminum	Pinhao Technology
Robotic arm	Yeahbot	Am1
Silicone Elastomer Base	Sylgard	184
Single-sided tape	Deli