Rod-based Fabrication of Customizable Soft Robotic Pneumatic Gripper Devices for Delicate Tissue Manipulation

Summary

This protocol describes a rod-based approach, combining 3D-printing and soft lithography techniques for fabricating the soft gripper devices. This approach eliminates the need for an external air source by incorporating a chamber component and reduces the chance of occlusion during the sealing process, particularly for miniaturized pneumatic channels.

Abstract

Soft compliant gripping is essential in delicate surgical manipulation for minimizing the risk of tissue grip damage caused by high stress concentrations at the point of contact. It can be achieved by complementing traditional rigid grippers with soft robotic pneumatic gripper devices. This manuscript describes a rod-based approach that combined both 3D-printing and a modified soft lithography technique to fabricate the soft pneumatic gripper. In brief, the pneumatic featureless mold with chamber component is 3D-printed and the rods were used to create the pneumatic channels that connect to the chamber. This protocol eliminates the risk of channels occluding during the sealing process and the need for external air source or related control circuit. The soft gripper consists of a chamber filled with air, and one or more gripper arms with a pneumatic channel in each arm connected to the chamber. The pneumatic channel is positioned close to the outer wall to create different stiffness in the gripper arm. Upon compression of the chamber which generates pressure on the pneumatic channel, the gripper arm will bend inward to form a close grip posture because the outer wall area is more compliant. The soft gripper can be inserted into a 3D-printed handling tool with two different control modes for chamber compression: manual gripper mode with a movable piston, and robotic gripper mode with a linear actuator. The double-arm gripper with two actuatable arms was able to pick up objects of sizes up to 2 mm and yet generate lower compressive forces as compared to elastomer-coated and non-coated rigid grippers. The feasibility of having other designs, such as single-arm or hook gripper, was also demonstrated, which further highlighted the customizability of the soft gripper device, and it's potential to be used in delicate surgical manipulation to reduce the risk of tissue grip damage.

Introduction

Soft robots have sparked great research interest within the robotics community and they have been used in different functional tasks such as undulatory locomotion in unstructured environments¹ and gripping². They are mainly composed of soft elastomeric materials and controlled by different actuation techniques through the use of different materials such as electroactive polymer (EAP), shape memory alloy (SMA), or compressed fluid³. EAPs function based on a differential voltage that induces electrostatic forces to produce active strains and thereby generates actuation. The peculiar shape memory effect of the SMAs is deployed to generate the desired actuation based on the force generation during phase transformations upon the change in temperature. Lastly, compressed fluid actuation technique facilitates a simple design strategy to induce stiffness difference in the soft actuators, such that the more compliant regions will inflate upon pressurization. Soft robots are designed to broaden the applications of traditional hard robots, especially in applications where delicate objects are involved. Particularly, in this paper, we present our unique approach in developing soft robotic grippers for delicate surgical manipulation.

Surgical gripping is an important aspect involved in many surgical procedures such as hepatic, gynecological, urological, and nerve repair surgeries^{4, 5}. It is typically performed by rigid, steel tissue gripping tools such as the forceps and laparoscopic graspers for the purpose of facilitating observation, excision, anastomosis procedures, etc. However, extreme caution is required as the conventional gripping tools are made of metal that may cause high stress concentration areas in the soft tissue at the points of contact⁶. Depending on the severity of the tissue damages, various complications, such as pain, pathological scar tissue formation, and even permanent disability, may result. A prior study reported that the complication rate in peripheral nerve surgery was 3%⁷. Therefore, the concept of soft gripping that can provide safe compliant grip can be a promising candidate for delicate surgical manipulation.

Here, we present a combination of 3D-printing and modified soft lithography techniques, which adopted a rod-based approach, to fabricate customizable soft robotic pneumatic grippers. Traditional fabrication technique of soft robots based on compressed fluid actuation requires a mold with pneumatic channels printed on it and a sealing process to seal the channels⁸. However, it is not feasible for miniaturized soft robots which need small pneumatic channels where occlusion of channels can easily happen in the sealing process. The traditional technique requires the sealing of the pneumatic channels to be done by bonding a coated sealing layer to it. Hence, the layer of elastomeric material which initially serves as a bonding layer may spill into the tiny channels and occlude those channels. It is also not possible to position the pneumatic channels at the middle of the structure and connect to a chamber component using conventional techniques. The proposed approach allows the creation of miniaturized pneumatic channels connected to an air-filled chamber using rods and does not require sealing of the tiny channels. In addition, the chamber connected to the pneumatic channels serve as an air source which does not require external air sources for compressed fluid actuation. It allows both the manual and robotic control modes by facilitating the chamber compression to actuate the gripping component, thereby providing users the option of controlling the amount of force that they are applying through the gripper. This approach is highly customizable and can be used to fabricate various types of soft gripper designs such as grippers with single or multiple actuatable arms.

Protocol

Note: All the soft pneumatic grippers were fabricated by casting silicone-based elastomeric mixtures into customized 3D-printed molds, which followed a fabrication process comprising three steps: molding gripper-arm components with embedded pneumatic channels, molding chamber component connected to the pneumatic channels, and sealing the chamber component filled with air.

1. Preparation of Elastomers

1. Place a container for mixer on a weighing scale and tare it. Pour parts A and B of the silicone-based elastomer in the container with a 1:1 weight ratio.
2. Cover the container and measure the total weight.
3. Place the container and material into a centrifugal mixer. Adjust the weight balance on the mixer to the weight measured in step 1.2.
4. Set the mixing and de-aeration modes to 2,000 rpm and 2,200 rpm respectively for 30 sec. Mix the elastomer components thoroughly to achieve uniform curing.

2. Mold Design and Production

Note: The geometry of the mold will vary depending on the specific requirements for different applications. The following steps illustrate general key steps in CAD software that are required to create the chamber and gripper component of the mold.

1. Design the molds and sealing mold using computer-aided design (CAD) software. See Figure 1 for the geometry and specific dimensions of the molds used in this manuscript.
   1. Design of the outer boundary box
      1. Right-click on the top plane and click on "Normal to" button to normalize to the top plane.
      2. Click on "Sketch" on the top left corner to open a "Sketch" window. Then, click on the "Sketch" button on the top left corner of the toolbar to draw a rectangular base of chamber component.
      3. Click on the "Smart Dimension" feature, which is located beside the "Sketch" button, to define sketch dimensions. Ensure that the sketch is fully defined (i.e., all drawing lines become black) and exit the sketch when done.
      4. Click on the "Features" window. Then, click on "Extruded Boss/Base" feature to extrude selected contours in the Y-direction.
      5. Click on the top surface of the model to preselect the sketch plane. Sketch a rectangle and define the dimensions as described in 2.1.1.2 and 2.1.1.3.
      6. Click on the "Features" window. Then, click on "Extruded Cut" feature to extrude cut a cavity for casting elastomers (Figure 2A). Ensure that the wall thickness is 2.5 mm.
   2. Design of the inner chamber
      1. Right-click on the surface on the Y-direction of the opening area. Then, click on "Normal to" to normalize to that surface.
      2. Next, click on the "Sketch" window to draw a rectangle for chamber component as described in steps 2.1.1.2 and 2.1.1.3.
      3. Click on the "Features" window. Then, click on "Extruded Boss/Base" feature to extrude the chamber component in the Y-direction (Figure 2B).
         Note: The depth of the cut in step 2.1.1.6 is 2.5 mm larger than this extruded base.
   3. Design of the gripper component
      1. Click on the surface of the model in negative X-direction to preselect the sketch plane for gripper component. Create a rectangle in the "Sketch" window as described in steps 2.1.1.2 and 2.1.1.3.
      2. Click on the "Features" window. Then, click on "Extruded Boss/Base" feature to extrude the selected contour in the negative X-direction.
      3. Click on the top surface of the gripper component to preselect the sketch plane. Create a shape of gripper in the "Sketch" window (Figure 2C) and exit the sketch when the dimensions is fully defined as described in steps 2.1.1.2 and 2.1.1.3.
      4. Click on the "Features" window. Then, click on "Extruded Cut" to cut a cavity for casting elastomers in the gripper component. Ensure that the wall thickness is 2.5 mm.
   4. Design of the connection between chamber and gripper
      1. Create a rectangle in the "Sketch" window on the top surface of the chamber piece as described in 2.1.1.2 and 2.1.1.3.
      2. Click on the "Features" window. Then, click on "Extruded Cut" to create a connection between chamber and gripper components (Figure 2D).
   5. Design of the pneumatic channels
      1. Create 1.5 mm diameter circles on the surface of the chamber piece in the positive X-direction as described in steps 2.1.1.2 and 2.1.1.3.
      2. Click on the "Features" window. Then, click on "Extruded Cut" to create channels for wire rods insertion (Figure 2E). Ensure the holes are not cut through the gripper component.
2. In a separate CAD file, draw a sealing mold with a cavity of length and width that are 1 mm larger than the outer dimensions of the chamber component of the gripper. Note: The wall thickness is 2.5 mm.
   1. Click on the "Sketch" window to create a rectangle on the top plane as described steps 2.1.1.2 and 2.1.1.3.
   2. Click on the "Features" window. Then, click on "Extruded Boss/Base" feature to extrude selected contours in the Y-direction.
   3. Click on the top face of the model to preselect the sketch plane. Sketch a rectangle and define the dimensions as described in steps 2.1.1.2 and 2.1.1.3).
   4. Click on the "Features" window. Then, click on "Extruded Cut" feature to extrude cut a cavity for casting elastomers. Ensure that the wall thickness is 2.5 mm.
3. Save each mold piece as a .STL file for 3D printing.
4. Load the .STL file into the 3D printer with resolution of 30 µm and print the mold pieces⁹.
5. Remove any support material on the mold pieces and wash the mold pieces with water.

3. Soft Single/Double-actuatable Arm Pneumatic Grippers

1. Molding gripper-arm components with embedded pneumatic channels
   1. Insert two 3D-printed chamber-blocks on the left and right side of the chamber component (Figure 3A) in order to generate a sealed chamber with pneumatic channels connected to it.
   2. Insert two 1.5 mm-diameter titanium wire rods through the chamber, keeping a 2 mm distance from the gripper tips to create the pneumatic channels (Figure 3A). Note: Use one wire rod for the single-actuatable-arm gripper.
   3. Pour the elastomeric mixture into the mold to fully fill the gripper component.
   4. Ensure there is no visible air bubbles present.
   5. Place the mold into oven for curing at 60 °C for 10 min. Once elastomer is cured, remove the mold from the oven.
2. Molding chamber component connected to the pneumatic channels
   1. Pull the wire rods and the two chamber-blocks out from the mold.
   2. Place a 3D-printed gripper-block on top of the gripper component in order to create the chamber (Figure 3B). Insert the wire rods to block the holes in the wall of the mold.
   3. Pour the elastomeric mixture into the mold to fill the remaining part of chamber component and ensure there is no visible air bubbles trapped in the mold.
   4. Cure the part at a temperature of 60 °C for 10 min. Remove the mold from the oven once elastomer is cured.
   5. Remove the gripper-block and demold the fully cured gripper with chamber structure.
3. Sealing the chamber component filled with air
   1. Pour the elastomeric mixture into the sealing mold and cure it at 60 °C for 10 min.
   2. Brush a layer of elastomeric material on the cured 2.5 mm sealing layer. Place the cured gripper with chamber structure on top of the coated sealing layer and bond the two pieces together (Figure 3C).
   3. Subsequently, cure the entire structure fully at 60 °C for 15 min.
   4. Demold the fully cured soft robotic gripper device.

4. Insertion of Soft Robotic Pneumatic Gripper Device into Handling Tool

1. Design the handling tools as described in the Supplemental file 1 using CAD software and save it in .STL file. See Figures 4 and 5 for the dimensions of the tools.
2. Load the .STL file in the 3D printer and print the mold pieces⁹.
   Note: All printing steps for manual control handling tool, rectangular cap, and movable piston (Figure 4) can be completed within 3 hr 48 min. The printing time for fabricating the robotic control handling tool and rectangular cap (Figure 5) is 1 hr 56 min. See Supplemental file 2 for 3D printer operation instructions.
3. Peel off any support material on the tools after the printing is completed. Then, wash the tools with water.
4. Insert the gripper into the manual control handling tool (Figure 4A) and cover the opening area with a movable rectangular cap (Figure 4B).
5. Insert a movable piston (Figure 4C) to facilitate chamber compression.
6. Insert the gripper and linear actuator into the robotic control handling tool (Figure 5A). Note: The linear actuator replaces the movable piston in manual control mode for chamber compression.
7. Cover the opening area with a movable rectangular cap (Figure 5B).

5. Evaluations and Grip Compressive Test

1. Evaluate the functionality of the soft gripper by performing gripping tests with a jumper wire.
   1. Place a jumper wire on the table.
   2. Adjust the gripper so that the wire is in between of the two gripper arms.
   3. Move the movable piston to compress the chamber in order to actuate the gripper arms to hold the wire.
      Note: Only the manual control handling tool is used in the gripping demonstration.
   4. Hold and move the wire to a box located at 20 cm away from wire's original location.
2. Place a calibrated force sensing resistor between the two jaws of the gripper. Ensure the gripper jaws grip on the sensing area. Note: The diameter of the sensing area is 14.7 mm.
3. Compress the chamber to actuate the gripper arms to grip on the force sensing resistor.
4. Measure the maximum grip compressive forces that the soft single-actuatable-arm and double-actuatable-arm pneumatic grippers could generate as described in ¹⁰.
   Note: The readout values will be displayed on a laptop. The maximum grip compressive forces are measured at the point of maximum pressure that the pneumatic channels can withstand.
5. Cut out the individual elastomeric gripping jaws from a soft double-actuatable-arm pneumatic gripper.
6. Insert the forceps tips into the pneumatic channels of the elastomeric gripping jaws.
7. Place a calibrated force sensing resistor between the two jaws of the forceps.
8. Measure the compressive forces¹⁰ generated by elastomer-coated forceps and forceps during a simulated nerve surgery conducted by a neurosurgeon.
   Note: The neurosurgeon applies a force that is similar to what he normally applies during actual surgery on the force sensing resistor.
9. Average the data obtained from five trials in each test.

Results

The soft robotic pneumatic gripper devices were capable of picking up objects with dimensions of up to 1.2 mm in diameter (Figure 6). The maximum grip compressive force generated by the single- actuatable-arm, and double- actuatable-arm soft gripper devices were 0.27 ± 0.07 N and 0.79 ± 0.14 N respectively, as compared to 1.71 ± 0.16 N and 2.61 ± 0.22 N compressive forces in simulated surgery by the elastomer-coated forceps and by uncoated forceps (Figure 7

Discussion

We have successfully demonstrated that the soft robotic pneumatic gripper devices allowed compliant gripping of objects, which exerted much lower compressive forces on the gripped object than the elastomer-coated forceps tips and forceps exerted. Forceps is an essential tool for nerves manipulation during peripheral nerve repair surgeries^{11, 12}. However, its metallic structure required extreme caution in usage from the surgeons in order to prevent nerve damage caused by excessive gripping forces and the incide...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Weighing Scale	Severin	KW3667	(Step: Preparation of elastomers)
Ecoflex Supersoft 0030 Elastomer	Smooth-On	EF0030	(Step: Preparation of elastomers)
Planetary Centrifugal Mixer and Containers	THINKY USA Inc.	ARE-310	(Step: Preparation of elastomers)
Solidworks CAD	Dassault Systèmes	Solidworks Research Subscription	(Step: Soft single/double-actuatable arm pneumatic grippers)
Objet 3D Printer	Stratasys	260 Connex2	(Step: Soft single/double-actuatable arm pneumatic grippers)
Titanium Wire Rods	Titan Engineering	N/A	(Step: Soft single/double-actuatable arm pneumatic grippers)
Natural Convection Oven with Timer	Thermo Fisher Scientific	BIN#ED53	(Step: Soft single/double-actuatable arm pneumatic grippers)
Linear Actuator	Firgelli Technologies	L12	(Step: Insertion of soft robotic pneumatic gripper device into handling tool)
Jumper Wire	sgbotic	CAB-01146	(Step: Evaluations and grip compressive test)
Force Sensing Resistor	Interlink Electronics	FSR402	(Step: Evaluations and grip compressive test)