This section presents the design and fabrication of a fully integrated module according to the scheme in Fig. 4.
The soft actuator is a 3DOF cylindrical FFA, which incorporates three expandable chambers equally arranged around the main central axis (\(N_{A}\)= 3). They are essentially three empty chambers created in a cylindrical bulk of elastomeric material, which can be pressurized either alternately to bend the cylinder or simultaneously to elongate it [26–28]. Cylindric 3DOF FFAs need to limit the severe lateral expansion (i.e. ballooning effect), which inevitably happens at the external side of the chambers when inflated, due to the thin wall of the elastomeric material, as reported in most of the works employing FFAs. Limiting this expansion would not only make the actuation safer, thus avoiding the risk of explosion, but would also enhance the FFA’s performance, in terms of maximum bending angle and regularity of the curvature radius.
Various strategies have been proposed to tackle this problem and mainly regard the addition of reinforcement elements around or embedded in the elastomeric material [29, 30]. The approach here is to use an external bellow-shaped sheath which, as demonstrated in [31], limits the lateral expansion of the elastomer underneath and enhances the response of the actuator to the input pressure. The pictures in Fig. 5 highlights the difference between an FFA without any elastomer expansion retain mechanism (Fig. 5a), and one with an external sheath (Fig. 5c).
With the retain mechanism, the effect of the chamber insufflation is entirely transformed into the motion of the module, while in the pure elastomeric structure, part of the pressurization is spent and lost inside the external expansion chamber. A vascular graft was selected to act as a retain sheath, since it is bellow-shaped and with the appropriate size and flexibility. Vascular grafts are bellow-warped knitted polyester prostheses which are conventionally used to replace a piece of vascular tract. Their proprieties match perfectly with the requirements for a retaining sheath to contain the elastomer ballooning effect of FFAs, as they are dilatant-resistant but, at the same time, they can be bent, elongated and squeezed with minimal stress. A vascular graft is also a certified and biocompatible medical device, thus adequate for potential contacts with internal organs.
The fabrication of the FFA, as shown by the flow diagram in Fig. 5b, starts with the production of the elastomeric body (9 mm in diameter and 30 mm in length), and then the sheath is added (10 mm is the actual module external diameter). First, the inner body is fabricated by casting uncured silicone (EcoFlex 0050, Smooth-On, Inc., USA) [32] into a cylindrical mold containing the positive shape of the chambers and channels in order to create the empty spaces in the silicone. The mold is an assembly of different components (Fig. 5d): an external cylindrical shell in Delrin and laser cut (Universal Laser VLS 2.30) plexiglass plates for aligning the 3D printed (Invision Si2, 3D Systems, Inc., UK) chamber molds at the base and top of the shell mold during the silicone curing. Once poured into this mold, the silicone is stabilized at room temperature for 2 h and then cured at \(60^\circ \)C for 30 min. When removed from the mold, the silicone body is used as a mold itself to pour a higher tensile strength silicone (Dragonskin 30, Smooth- On, Inc., USA) [33], which acts as a spine of the module and as a pipe to carry the fluidic power towards the modules top side, where it is available for another module. For the characterization of the material, see [34], where tests according to ISO37:2005(E) are reported in detail for all the silicones used in the FFA. Finally, the retaining sheath (a knitted polyester vascular graft produced by Vaskutek Inc., Terumo Company, UK [35]) is slid around the silicone body and fixed at its top and bottom extremities (Fig. 5c).
As previously mentioned, a two-way operation latching solenoid valve is connected at each chamber, as reported in Fig. 6. The chosen valves (LF solenoid valves, series 120, by Lee Products Ltd, UK) are the most miniaturized (3 mm in diameter) and light weight (300 mg) solenoid valves available on the market. Thanks to their magnetically bi-stable (latching) mechanism, they need power just for switching their state (from closed to open, and vice versa) with a power consumption of about 1.8 mW s/switch (the switching requires a 1ms pulse of about 5 VDC). To provide the control signals to the valves and to communicate wirelessly with the central system, a double-sided PCB electronic board was developed. The PCB board (Fig. 6) is 10 mm in diameter and incorporates a wireless microcontroller (CC2430, Texas Instruments, USA) embedding a telemetry communication system (wireless controllability over 2.4 GHz, ZigBee compliant IEEE 802.15.4 transceiver), a voltage regulator, and drivers for the valves (A3901, Allegro Microsystems LLC, USA). The PCB board communicates wirelessly with the dongle connected to the master control system (Fig. 4), which incorporates another CC2430-based board. This module communicates with the control PC through an off-the-shelf USB/serial universal asynchronous receiver-transmitter (UART) and a converter development module for the FT232R IC device (UM232R, Future Technology Devices International Ltd, UK). Through this dongle board, the master system also controls the external proportional pressure regulator (proportional pressure micro-regulator, series K8P, Camozzi Group, IT).
To respect the modularity specification, all modules should have uniform interfaces. As each module contains three fluidic actuators, a fluidic power distribution point is required inside the module itself. This distribution point can be designed as a fluidic chamber with five branches: one branch for the single inlet (the main fluidic line) and 1+\(N_{A}\) branches for the outlets (the \(N_{A}\) valves inlets and the continuing fluidic line for the next module). Although commercial solutions offer several options for such a multi-way connector, but none of them fitted in the current module design, because of their shape/size or their material stiffness. For this reason, a specifically customized multi-way fluidic distributor was designed, optimized and then integrated into the module.
The fluidic distributor can take different shapes depending on the position of the inlet and the outlets. In this work the geometry of the fluidic distributor (five-way) is represented by a Y-like empty space inside the elastomeric material, as depicted in Fig. 7c, which can be reached by the inlet pipe at one side, and by the four outlets (three of them equally organized around the center) on the other side.
Unlike the FFA’s chambers, the fluid pressure must not cause a minimal expansion in the elastomeric bulk of material. Therefore the fluidic distributor needs to be designed to keep its initial shape even when it has the pressurized fluid inside. A preliminary (finite elements method (FEM) analysis, operated in SIMULIA ABAQUS (Dessault Systems, USA), was performed before the actual fabrication of the elastomeric connector. The aim of the FEM study was to understand if the material of the distributor was able to sustain the range of expected working pressures, without producing considerable deformation in the elastomer. The simulation was performed on a distributor made up of a silicone with a higher tensile strength than the one in the FFA, i.e the DragonSkin 30 (by Smooth-On Inc., USA), using a dynamic implicit analysis and applying a tetrahedral mesh with quadratic order elements to the elastomeric structure. The simulated pressures supplied to the branched chamber inside the silicone fluidic connector ranged from 0 up to 70 kPa (0–10 psi), which is twice the maximum working pressure range of the FFA. A peak of 180 kPa was also imposed to detect if considerable deformations of the distributor would happen in case of possible over-pressurization due to a failure of the control system. This FEM analysis, (Fig. 7a, b, d) confirmed that the distributor would experience negligible deformations (less than 6 % in radius expansion)within the FFA working pressure range. However, for higher pressures (more than 70 kPa) the deformation of the structure begins to be noticeable (over 10 %) and is mainly located at the lateral border of the distributing chamber, where the silicone wall is thinner (as highlighted in Fig. 7d). According to these preliminary simulation results, the design of the actual fluidic distributor incorporates a reinforcement layer in this area. This reinforcement is provided by the addition of an even higher tensile strength silicone, the Smooth Sil 950 (by Smooth-On Inc, USA).
The fabrication of our elastomeric fluidic distributor involves the production of the elastomeric body, again, by casting silicones (DragonSkin 30 for the main part and Smooth Sil 950 for the reinforcing layer). In this case the molds are assembled from laser cut plexiglass sheets (2 mm in thickness) (Fig. 8). The fabrication is depicted in Fig. 8, which shows the steps to build an empty branched chamber inside the silicone bulk and make it accessible for the inlet/outlet pipes and valves. The two sides of the fluidic distributor, depicted in green and blue in Fig. 8, are first cast separately using the respective mold (step 1). Then they are hermetically sealed by pouring another layer of uncured silicone around the two merged sides (step 2). This strategy facilitates the integration of the reinforced layer, which can be attached to the outlet side (see the picture in the inset in Fig. 8), by adding a silicone casting Smooth Sil 950 to the outlet side, using the outlet side mold, before casting and filling up the same mold with DragonSkin 30.