Fabrication of Soft Pneumatic Network Actuators with Oblique Chambers

Summary

Here we present a fabrication method of soft pneumatic network actuators with oblique chambers. The actuators are capable of generating coupled bending and twisting motions, which broadens their application in soft robotics.

Abstract

Soft pneumatic network actuators have become one of the most promising actuation devices in soft robotics which benefits from their large bending deformation and low input. However, their monotonous bending motion form in two-dimensional (2-D) space keeps them away from wide applications. This paper presents a detailed fabrication method of soft pneumatic network actuators with oblique chambers, to explore their motions in three-dimensional (3-D) space. The design of oblique chambers enables actuators with tunable coupled bending and twisting capabilities, which gives them the possibility to move dexterously in flexible manipulators, to become biologically inspired robots and medical devices. The fabrication process is based on the molding method, including the silicone elastomer preparation, chamber and base parts fabrication, actuator assembly, tubing connections, checks for leaks, and actuator repair. The fabrication method guarantees the rapid manufacturing of a series of actuators with only a few modifications in the molds. The test results show the high quality of the actuators and their prominent bending and twisting capabilities. Experiments of the gripper demonstrate the advantages of the development in adapting to objects with different diameters and providing sufficient friction.

Introduction

Soft pneumatic actuators (SPAs) are soft devices that can be actuated by the simple input of air pressure^1,2. They can be fabricated with diverse materials, such as silicone elastomers³, fabrics⁴, shape-memory polymers⁵, and dielectric elastomers⁶. Researchers have benefited from their nature of compliance, dexterous motions, and simple fabrication methods⁷, such that SPAs have become one of the most promising devices for soft robotics applications^8,9. SPAs can realize various sophisticated motions, such as creeping¹⁰, rotation¹¹, and rolling¹² based on various types of deformation, including extending, expanding, bending, and twisting^13,14. To be able to make different types of motions, SPAs are designed in different structures, such as a linear body with parallel channels¹⁵, a monolithic chamber with fiber-reinforcements¹⁶, and networks of repeated sub-chambers¹⁷. Among them, the SPAs with networks of repeated sub-chambers, the soft pneumatic network actuators, are widely employed because they can generate large deformations under a relatively low input pressure. However, in most of the previous designs, this type of actuators can only generate bending motions in 2-D space, which greatly limits their applications.

A soft pneumatic network actuator consists of a linearly arranged group of chambers connected by an internal channel. Each cubic chamber contains a pair of opposite walls which are thinner than the other pair and produces a two-sided inflation in the direction perpendicular to the thinner walls. Originally, the thinner walls of the chambers are perpendicular to the long axis of the actuator body and inflate along with the long axis. These collinear inflations in chambers and the non-extensible base lead to an integral pure bending of the actuator. To explore the actuator's motion in 3-D space, the orientation of the chambers is tuned so that the thinner-side walls are no longer perpendicular to the long axis of the actuator (Figure 1A), which enables the inflation direction of each chamber to offset from the axis and become not collinear. All the parallel but not-collinear inflations change the motion of the actuator into a coupled bending and twisting motion in 3-D space¹⁸. This coupled motion enables the actuators more flexibility and dexterity and makes the actuators a suitable candidate for more practical applications, such as flexible manipulators, biologically inspired robots, and medical devices.

This protocol shows the fabrication method of this kind of soft pneumatic network actuators with oblique chambers. It includes preparing the silicone elastomer, fabricating the chamber and base parts, assembling the actuator, connecting the tubing, checking for leaks, and, if necessary, repairing the actuator. It can also be used to fabricate normal soft pneumatic network actuators and other soft actuators which can be produced with some simple modifications to the molding method. We provide detailed steps to fabricate a soft pneumatic actuator with 30° oblique chambers. For different applications, actuators with different chamber angles can be fabricated according to the same protocol. Apart from that, the actuators can be combined to form a multi-actuator system for various demands.

Protocol

NOTE: The protocol provides the fabrication procedures of a soft pneumatic network actuator. Before the fabrication procedure, a set of molds and several actuator-tubing connectors, which are designed with computer-aided design (CAD) software must be 3-D-printed in advance. The molds are shown in Figure 1B.

1. Silicone Elastomer Preparation

1. Weigh 5 g of silicone elastomer part B and 45 g of part A [9:1 (A:B) parts by weight] in the same mixing container (Figure 2A). Use a syringe to make sure the proportions of each part are accurate.
   NOTE: The mixing ratio varies for different silicone elastomers. The proportion of each part should be adjusted when another silicone elastomer is adopted.
2. Mix the silicone elastomer well with the planetary centrifugal mixer.
   NOTE: The silicone elastomer could be stored at a low temperature to extend its processing time.

2. Chamber Part Fabrication

1. Spray the mold release agent for silicone elastomer products evenly on the surfaces of the mold part A and part B.
2. Assemble part A and part B of the mold for the fabrication of a chamber. Hold both ends of the mold with clips to prevent the leakage of silicone elastomer.
3. Take 5 mL of silicone elastomer with a syringe and inject it slowly into the hole of the mold for fabricating the connection end (the cylindrical structure at one end of the actuator for connecting the tubing). Then, fill the whole mold with the silicone elastomer (Figure 2B).
   NOTE: Keep a low flow rate and move back and forth slowly, to let the silicone elastomer enter the tiny structures of the mold.
4. Pierce the bubbles that form on the surface with the tip of a needle until there are no more bubbles visible (Figure 2C).
5. Scrape off any excess silicone elastomer with a blade along the upper surface of the mold.
6. Place the mold in the oven at 70 °C until the silicone elastomer is cured.
7. Use a syringe to inject silicone elastomer into the bubbles and holes which appear on the surface of the actuator.
8. Scrape off any excess silicone elastomer on the surface.
9. Place the mold in the oven at 70 °C until the silicone elastomer is cured.

3. Base Part Fabrication

1. Spray the mold release agent for silicone elastomer products evenly on the surface of the mold part C.
2. Pour the silicone elastomer into part C of the mold.
3. Pierce the bubbles that form on the surface with the tip of a needle until there are no more bubbles visible.
4. Scrape off any excess silicone elastomer with a blade along the upper surface of the mold.
5. Place the mold in the oven at 70 °C until the silicone elastomer is cured.

4. Actuator Assembly

1. Evenly pour a layer of silicone elastomer, 1 mm in thickness, on one face of the base part.
2. Place the chamber part on the base part. Use a syringe to inject the silicone elastomer into the space between the chamber part and the base part (Figure 2D).
3. Place the actuator in the oven at 70 °C until the silicone elastomer is cured.

5. Tubing Connection

1. Tap the 3-D-printed actuator-tubing connector to accept the screw of a male stud push-in fit pneumatic fitting.
2. Use a needle to pierce the connection end of the actuator along the centerline of the cylinder. Increase the diameter of the hole with a steel rod, to about 2 mm.
3. Screw the actuator-tubing connector into the actuator (Figure 2E).
4. Push a section of tubing into the male stud push-in fit pneumatic fitting.

6. Leak Check and Repair

1. Connect the actuator to an air source.
2. Place the whole actuator in the water and pressurize the actuator (Figure 2F). Observe whether bubbles are formed due to a leak.
3. Use a syringe to inject the silicone elastomer into leak points. Place the actuator in the oven at 70 °C until the silicone elastomer is cured.
4. Repeat steps 6.1 - 6.3 if needed.

Results

Single Actuator:
To verify the fabrication method and demonstrate the function of the actuator, 30°, 45°, and 60° actuators were fabricated and tested. For the experiment set-up, an air pump was employed to activate the valve. The valve was connected to the actuator to control the internal pressure. The single actuator was fixed at its connection end and placed vertically. While the actuator was being pressurized, two digital cameras were used to capt...

Discussion

The paper presents a method protocol to guide the fabrication of soft pneumatic network actuators with oblique chambers. Following the protocol, one actuator can be fabricated independently within 3 h. The key steps in the protocol can be summarized as follows. (i) The silicone elastomer is prepared in proportion and mixed well. (ii) The silicone elastomer is poured into the mold for the fabrication of the chamber part and the base part. (iii) The bubbles on the exposed surface are pierced and any excess silicone elastom...

Materials

Name	Company	Catalog Number	Comments
Silicone elastomer	Wacker	ELASTOSIL M4601 A/B	Material of the actuators
Syringe	Shanghai Kindly Medical Instruments	10 ml	Used to inject silicone rubber into the hole of the mold for fabricating the connection end
Precision scale	Shanghai Hochoice	UTP-313	Used to weigh the silicone rubber
Planetary centrifugal vacuum mixer	THINKY	ARE-310	Used to mix the silicone rubber and defoam after mixing process
Release agent	Smooth-on	Release 200	Used for ease of demolding
Needle	Shanghai Kindly Medical Instruments		Used for Piercing the bubbles form on the surface
Utility blade	M&G Chenguang Stationery	ASS91325	Used for Scraping off excess silicone rubber along the upper surface of the mold
Vacuum oven	Ningbo SI Instrument	DZF-6050	Used to reduce the cure time of the silicone rubber
Male stud push in fit pneumatic fitting	Zhe Jiang BLCH Pneumatic Science & Technology	PC4-01	Used to connect the tubing and the 3D-printed actuator tubing connector
Tubing	SMC	TU0425	Used for actuating the actuators
Vacuum pump	Zhe Jiang BLCH Pneumatic Science & Technology		Used as the air source
Pressure valve	Zhe Jiang BLCH Pneumatic Science & Technology	IR1000-01BG	Used for adjusting the input air pressure