Design and Fabrication of an Elastomeric Unit for Soft Modular Robots in Minimally Invasive Surgery

Summary

This paper describes the design and fabrication of a soft unit for surgical manipulators. The base module includes three flexible fluidic actuators to achieve omnidirectional bending and elongation, and a granular jamming-based mechanism to enable stiffness control. A complete mechanical characterization is also reported.

Abstract

In recent years, soft robotics technologies have aroused increasing interest in the medical field due to their intrinsically safe interaction in unstructured environments. At the same time, new procedures and techniques have been developed to reduce the invasiveness of surgical operations. Minimally Invasive Surgery (MIS) has been successfully employed for abdominal interventions, however standard MIS procedures are mainly based on rigid or semi-rigid tools that limit the dexterity of the clinician. This paper presents a soft and high dexterous manipulator for MIS. The manipulator was inspired by the biological capabilities of the octopus arm, and is designed with a modular approach. Each module presents the same functional characteristics, thus achieving high dexterity and versatility when more modules are integrated. The paper details the design, fabrication process and the materials necessary for the development of a single unit, which is fabricated by casting silicone inside specific molds. The result consists in an elastomeric cylinder including three flexible pneumatic actuators that enable elongation and omni-directional bending of the unit. An external braided sheath improves the motion of the module. In the center of each module a granular jamming-based mechanism varies the stiffness of the structure during the tasks. Tests demonstrate that the module is able to bend up to 120° and to elongate up to 66% of the initial length. The module generates a maximum force of 47 N, and its stiffness can increase up to 36%.

Introduction

Recent trends in the medical field are pushing for a reduction in the invasiveness of surgical operations. Minimally Invasive Surgery (MIS) has been successfully improved in the last few years for abdominal operations. MIS procedures are based on the use of tools introduced through four or five access points (trocars) placed on the abdominal wall. In order to reduce the number of trocars, the instruments can be inserted by Single Port Laparoscopy (SPL) or Natural Orifice Translumenal Endoscopic surgery (NOTES)¹. These procedures prevent external visible scars, but increase the difficulty for the clinicians in executing the surgery. This limitation is mainly due to the reduced points of access and to the rigid and semi-rigid nature of the instruments, which are not able to avoid or pass around organs^{2, 3}. Dexterity and motility can be improved using articulated and hyper-redundant robots which can cover a wider and more complex workspace, thus enabling a specific target in the body to be reached more easily^{4, 5, 6} and to work as retraction systems when necessary⁷. A flexible manipulator can improve tissue compliance, thus making contact safer than by traditional tools.

However, these manipulators often lack stability when the target is reached and generally they cannot control the contact with the surrounding tissues^{8, 9}. Studies on biological structures, such as the octopus arm¹⁰ and the elephant trunk¹¹, have recently inspired the design of flexible, deformable and compliant manipulators with a redundant number of Degrees of Freedom (DoFs) and controllable stiffness¹². These kinds of devices utilize passive springs, smart materials, pneumatic elements, or tendons^{13, 14, 15}. Generally, manipulators fabricated with soft and flexible materials do not guarantee the generation of high forces.

The STIFF-FLOP (STIFFness controllable Flexible and Learnable manipulator for surgical OPerations) manipulator has been recently presented as a novel surgical device for NOTES and SPL inspired by the octopus’s capabilities. In order to overcome the limitations of previous soft manipulators, it has a soft body as well as high dexterity, high force and controllable stiffness¹⁶.

The architecture of the manipulator is based on a modular approach: multiple units, with the same structure and functionalities, are integrated together. The single unit is shown in Figure 1. It is based on an elastomeric cylinder obtained by a multiphase fabrication. The assembly steps of the mold components and the casting processes enable three empty chambers (for fluidic actuation) and one hollow central channel¹⁷ (for housing a granular jamming-based mechanism¹⁸) to be embedded. The chambers are placed at 120°, so that their combined inflation produces omnidirectional motion and elongation. In addition an external braided sheath is placed externally to limit the outward radial expansion of the fluidic chambers when pressurized, thus optimizing the effect of the chamber actuation in the module motion (bending and elongation).

The central channel houses a cylindrical device composed of an external membrane filled with granular material. When a vacuum pressure is applied, it changes its elastic properties causing a stiffening which affects the entire module’s properties.

Motion and stiffness performances are controlled by an external setup including an air compressor and three pressure valves for actuating the chambers and one vacuum pump for activating the vacuum in the stiffening channel. An intuitive user interface allows control of actuation and vacuum pressures inside the module.

This paper details the fabrication process of the single module of this manipulator and reports the most significant results on basic motion capabilities. Considering the modular nature of the device, the assessment of the fabrication and performance of just one single module also enables the results to be extended and to predict the basic behavior of a multi-module manipulator integrating two or more modules.

Protocol

Note: This protocol describes the fabrication phases of a single module, which includes the fluidic chambers, stiffening channel, actuation pipelines and external sheath. The following procedure has to be executed under a fume hood and wearing lab coat and gloves for safety reasons. As previously mentioned, the fabrication process of the elastomeric unit is based on the sequential use of molds designed with CAD software. They are composed of the 13 pieces shown in Figure 2 and listed in Table 1.

1. Preparation of the Silicone

1. Weigh 12 g of part A and 12 g of part B in the same plastic glass or Petri dish and mix them together, stirring.
   Note: Material proportions can vary depending on the specific silicone used, in this case it consists of two parts: part A (the base) and part B (the catalyst). They are used in proportion 1A:1B in weight.
2. Place the glass containing the mixed silicone materials in a degasser machine at 1 bar vacuum pressure. Keep the glass under vacuum until all the bubbles are removed from the silicone material. For the employed silicone the degassing process takes about 10 min. Once the materials are completely free from the presence of bubbles, restore the atmospheric pressure into the machine and use the silicone.

2. Fabrication of the Siliconic Module

1. Assembly of the mold.
   1. Insert the stiffening cylinder and the top of the chambers into cap_A (Figure 3a).
   2. Close the shells around the second layer of cap_A.
2. First silicone casting.
   1. Pour the silicone inside the assembled mold up to the edge of the shells (Figure 3b).
   2. Place the mold in an oven at 60 °C for about 30 min.
3. Rearrangement of the mold.
   1. Remove the external shells and cap_A (Figure 3c).
   2. Insert the cylinders from the bases of the chambers and the stiffening cylinder inside cap_B (Figure 3d).
   3. Close the shells again around the module, sliding them of 10 mm upward in order to have a gap of 10 mm between the top surface of the module and the edges of the shells (Figure 3e).
4. Second silicone casting.
   1. Pour the silicone inside the rearranged mold up to the edge of the shells on the top side (i.e. also up to the stiffening cylinder) (Figure 3f).
   2. Put the mold into an oven at 60 °C for about 30 min.
   3. Remove the external shells, cap_B and the chambers (except the stiffening cylinder) (Figure 3g).

3. Insertion of the Tubes

1. Cut 3 tubes to the same desired length (300 mm for example).
2. Put siliconic glue around one end of each tube for 10 mm, without obstructing the tubes.
3. Insert the tubes inside the 2 mm dedicated channels in the siliconic unit (Figure 3h).
4. Allow a curing time of 12 min at room temperature or put the module inside an oven at a higher temperature (50° - 60°) to speed up the drying process.

4. Fabrication of the Crimped Braided Sheath

1. Cut 700 mm of an expandable braided sheath (about 15 times the height of the module).
2. Insert a metallic cylinder of 30 mm in diameter and 250 mm in length inside the sheath.
3. Push down and force the sheath by sliding over the cylinder, in order to create crimps.
4. Mechanically fix the sheath in place with a clamp and heat with a heating gun at 350 °C for 2-3 min until a permanent deformation is obtained.
5. Let the sheath cool down and remove the internal cylinder.

5. Integration of the External Sheath

1. Pass the tubes through the holes of cap_C.
2. Pour 3 g of silicone into cap_C.
3. Clamp cap_C to a support that is higher than the work plane.
4. Insert the bottom side of the module previously fabricated into cap_C.
5. Slide the crimped sheath around the module.
6. Push the first crimps of the sheath inside cap_C and dip them into the freshly poured silicone (Figure 3i).
7. Put the mold into an oven at 60 °C for about 20 min.
8. Repeat the same procedure from point 5.1-5.6 to fix the sheath at the top side, using cap_D (Figure 3j).
9. Remove cap_C and cap_D.
10. Remove the central cylinder (Figure 3k).

6. Fabrication of the Granular Jamming Membrane

1. Pour 5 g of liquid latex into a plastic glass.
2. Immerse the cylinder for the membrane (last piece shown in Figure 2) inside the liquid latex until the surface is completely covered.
3. Let it dry under a hood for 20 min.
4. Repeat points 6.2 and 6.3.
5. Remove the membrane from the mold.

7. Insertion of the Granular Jamming Membrane

1. Cut a tube (2 mm in diameter) to the desired length (300 mm for example).
2. Cut a squared piece of about 100 mm² of nylon tissue and close one end of the tube with this tissue using a plastic paraffin film or superglue.
3. Weigh 4 g of coffee powder and fill the membrane.
4. Insert the tube (the end with the filter) inside the filled membrane and fix it around the tube using a plastic paraffin film.
5. Apply a vacuum on the other side of the tube (the membrane becomes stiffer).
6. Insert the membrane inside the empty central channel of the siliconic module (Figure 3l).
7. Glue the ends of the stiffening membrane to the silicone module.
8. Close the rings around the top side of the module (Figure 3m).
9. Pour 2 g of silicone into the rings in order to level the surface.
10. Let the silicone dry under hood or in an oven at 60°.
11. Remove the rings.
12. Repeat from points 7.8 to 7.11 for the bottom side (Figure 3n).

Results

The various phases of the fabrication, described in the Protocol, are illustrated in Figure 3.

In order to evaluate the effectiveness of the technique and the outcomes of the final prototype, the module was tested in different working conditions. An external setup allows control of both the actuation and stiffness of the module. It includes an air compressor that activates three valves. They are connected to the siliconic tubes integrated in the chambers and allow their pressu...

Discussion

The technique described in this protocol enables the fabrication of a pneumatically actuated soft unit usable for modular compliant structures. Thanks to the design of the molds and their simple assembly, it is possible to fabricate one complete module in about 4 hours with 7 main steps. The process of fabrication involves specific materials, which are easily available, and work should be carried out under a fume hood. An external set up including air valves, air compressor and vacuum pump is necessary to activate the mo...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ecoflex 00-50 Trial Kit	SmoothOn		Used for the fabrication of the soft unit, combining equal amounts of liquid parts A (the base) and B (the catalyst)
Latex	Antichità Belsito		Used for the fabrication of the granular jamming membrane
Peroxide-Cured Silicone Tubing	Cole Parmer	T-06411-59	Used for actuating the chambers and applying vacuum
PET expandable braided sleeving	RS	408-249	Used for the fabrication of the external braided sheath
Silicone Rubber	Momentive	127374	Used to fix the actuation tubes to the module
Parafilm	Cole Parmer	EW-06720-40	Used to fix the latex membrane to the vacuum tube
Fume hood Secuflow	Groupe Waldner		Working space
Precision scale	KERN EW		Used to weight silicone, latex and coffee powder
Oven/degasser	Heraeus		Used to degass the silicone and reduce its cure time
Vacuum pump	DVP Vacuum Technology		Used to apply vacuum to the latex membrane