The field of soft robotics proposed the development of versatile actuators as one of the first marks on its roadmap to next-generation robotics, due to the integral role of these technologies in the function and performance of robots.1–3 It is known that actuators determine the size, cost, power source, control mechanism, and general design of any robotic system,4 therefore strong emphasis has been placed on the advancement of these components. Although a wide range of actuator designs have been proposed, due to their ease of operation, low cost and ability to achieve a large range of deformations, inflatable actuators have been the primary subject of interest.5 However, these actuators are often considered too slow for most applications as their speed is limited by large inflation volumes that are needed to create deformations and the consequent large viscous forces needed to let this volume pass through narrow tubes. Several design strategies leveraging snapping instabilities,6 material non-linearity,7 stored elastic energy 8, as well as explosive chemical reactions 9 have been proposed to overcome this limitation. However, the functionality of the resulting inflatable actuators is highly limited as very specific considerations have to be taken into account in their design and manufacturing. Combining high actuation speeds without compromising functionality has been a major challenge and a high-speed inflatable actuator concept that can execute an extensive range of application-specific tasks is yet to be demonstrated. In this work, we aim at developing highly-functional and modular actuators that achieve high speeds.
To accomplish this goal, we investigate mechanisms that lay at the basis of nature’s fast-acting structures that rely on the transportation of fluids for actuation. It has been shown that these fluid-driven actuators harness miniaturisation to achieve fast movements.10 For instance, the Aldrovanda can close its leaves in 20ms, which is ~10 times more rapidly than the Venus flytrap, due to its significantly smaller size (approximately 1/10th of Venus flytrap).10 Miniaturised designs influence both the required volume of fluid (ΔVolume) that is needed to be displaced, as well as the distance it needs to travel, enabling rapid movements. Thus, we hypothesise that an analogues miniaturisation strategy can be applied to inflatable actuators (see Figure 1A for the schematic illustration of the concept).
In addition to miniaturisation, it is essential to improve the efficiency of actuators in converting delivered fluids into fast and predictable movement. As the transportation of fluids is a fundamental problem in these systems, available volume influx has to be efficiently converted towards desired deformations by avoiding unnecessary volumetric expansions that do not contribute to the overall desired deformation (Figure 1A). To determine how design can influence this efficiency, we performed Finite Element (FE) simulations (see Supplementary Text for further information) on three common bending actuator designs (eccentric,11 corrugated membrane 12 and fibre-reinforced 13) and measured the inflation volume required to obtain the same degree of bending. To eliminate the effect of size, we normalised the inflation volume by L3, where we have taken the length ‘L’ of the actuator as the characteristic length scale. From this analysis, we verify that the design of actuators has a major influence on bending performance. The same bending angle is reached at lower input volume for those designs that limit parasitic expansion and cross-sectional deformations. As shown in Figure 1B, the actuator with an eccentric design 14 first has to be inflated substantially to start bending. Then, more than 5 times ΔVolume has to be delivered to achieve 100° bending when compared to the fibre-reinforced actuator design. Similarly, the formation of major bulges can be seen in the corrugated membrane actuator design, which is the hallmark of compromised performance. Based on these findings, we conclude that deterministically designed high-modulus fibre architectures are effective in converting fluid volume influx into the desired deformation. In addition to their efficiency, which is largely overlooked and yet to be exploited in the literature, fibre-reinforced actuators are proven to be highly versatile with their ability to achieve tailored motions,1315 making them the ideal choice in our application.
While the previous analysis pointed that fibre reinforced design has the highest potential to achieve high speeds, it did not provide any information regarding the influence of the materials and/or geometry on the performance of the actuators. To investigate the dependency of inflation volume on these two factors, we conducted a series of FE analyses. We designed and simulated a fibre reinforced bending actuator consisting of double-helical fibres (both clockwise and counterclockwise) with a pitch of 390µm as well as 3 longitudinally placed fibres on one side that create bending, surrounding a hollow cylinder with an internal diameter of 1mm, a wall thickness of 0.2mm and length of 10mm. In our simulations (which were conducted using the commercial code Abaqus/Standard), we discretised the cylinder using second-order hybrid tetrahedral elements (element type: C3D10H and the fibres with 3-node quadratic beam elements (element type: B32) and actuated the models supplying incompressible fluid to the internal cavity via a fluid-cavity interaction. We modelled the material of the cylinder as incompressible neo-Hookean with the initial shear modulus m0, while the fibres are assumed to be linear elastic with Young’s modulus E and Poisson’s ratio of 0.3. To begin with, we considered m0=15kPa and E=160MPa and found that both the bending angle and pressure vary more or less linear with inflation volume ( see Figure 2A). From the graphs of Figure 2A, we can identify three actuator characteristics: the inflation volume that is needed to achieve a bending angle of 90° (ΔV90), the corresponding pressure (p90) and the needed energy (ΔE90), which can be calculated as the area under the pressure-volume curve. For this particular actuator, ΔV90=2.35µl, p90=5.4kpa and ΔE90=0.0064mJ. Next, we tested different initial shear moduli values ranging between 7.5kPa<m0<120kPa for the elastomeric matrix and Young’s moduli values ranging between 80Mpa<E<1.28Gpa for the fibres. As these two parameters directly influence the bending stiffness of either the cylinder (EI=3m0· π/4·(ro4-ri4), with ro and ri being the inner and outer radius of the cylinder) and the fibres (EI=E·π/4·r4, with r being the fibre radius), we can use them to assess the influence of both geometrical and material parameters on the performance of the actuators. In Figure 2B-D, we display these influences by plotting respectively ΔV90, p90 and ΔE90 for different bending stiffnesses of either the cylinder or the surrounding fibres, where stars indicate the previously reported values. Regarding the pressure that is needed to achieve a 90° bending angle (Figure 2B), we can conclude that a higher pressure is needed when the bending stiffness of the cylinder increases. Surprisingly, the bending stiffness of the fibre does not have a large influence on the required input pressure levels. Regarding the inflation volume (Figure 2C), we found that a combination of a stiff tube with compliant fibres and a combination of stiff fibres with a compliant tube lead to higher inflation volumes than when both are stiff or compliant. We can thus conclude that there is an optimal ratio of bending stiffnesses, where fibres need to be ~15 times stiffer than the elastomeric matrix, which has been indicated by a dotted line on the figure. Lastly, The energy that is needed to bend 90° (Figure 2D) is dominated by and follows the same trends as the pressure dependency, which is logical since the relative pressure variation is larger than the relative volume variation while varying stiffnesses.
To create these highly dynamic bending actuators with an optimal fibre composition, we cannot rely on production processes described in the literature, as they either are incompatible with small scale production processes, 13,15–17 or lack control over fibre placements 18–20. Therefore we developed a new manufacturing strategy that facilitates the fabrication of miniaturised composite soft actuators with precision at small length scales. We use melt electrowriting (MEW) technology,21,22 a class of additive manufacturing system, which combines the capability of electrospinning systems to produce ultra-fine fibres (fibre diameters between 1-50µm) with the design freedom of 3D printing. In this automated process, we apply a thin layer of uncured soft silicone-based elastomer on the rotational collector of our MEW system. By using a rod integrated into our MEW device, we move the stage in x-direction back and forwards with an elastomer and create a thin, uniform layer of the silicone-based tubular structure. We then start melt electrowriting of fibrous network designs on this partially-cured silicone tube to achieve enhanced bonding between the fibres and matrix material. (see Figure S1 and Movie S1 for the schematic illustration and video of the fabrication process, respectively). After the completion of the 3D printing process, we allow the silicone to fully-cure and connect the actuators to a pressure source after sealing their tip.
As demonstrated in Figure 3, we successfully fabricated a miniaturised bending actuator with an internal void diameter of 1mm, length of 10mm and a wall thickness of 0.2mm using our manufacturing technique (see Figure S2 for the technical drawing). The scanning electron micrograph (Figure 3A) shows the accurate placement of the fibres as well as their good continuity and consistency (see Table S1 for the detailed characterisation of the dimensions of the fabricated actuator). We selected the constituent materials of this actuator in accordance with the established principles depicted in Figure 2. As our findings indicate that the use of soft matrix materials reduces the required actuation energy, we applied the softest grade silicone within the product family of a widely used elastomer (Ecoflex with a shore hardness of 00-10). For the fabrication of the fibre phase, we preferred polycaprolactone (PCL) due to its excellent rheological properties and processability via MEW process as well favourable mechanical properties (elastic modulus of 320MPa), leading to an actuator with a fibre-to-matrix bending stiffness ratio of ~7.5. Although this ratio is smaller than the identified ideal ratio, our simulations suggest that this actuator (internal diameter of 1mm, and length of 10mm) require ΔVolume of 2.3559µl to achieve 90° bending, which is marginally higher than that of an actuator built with materials having a bending stiffness ratio of 15 (ΔVolume of 2.3143µl). Overall, this material combination yielded high performant bending actuators that are also easy to manufacture, handle and characterise as demonstrated in Figure 3.
After pressurizing the actuator, we indeed observed the intended large bending movements with minimum parasitic deformations (see Figure 3A) (see Figure S6 for a bending actuator without helical fibres exhibiting large parasitic deformations). Figure 3B shows the magnitude of the deformations achieved by the actuator at given air pressure both experimentally and as computed by means of FE modelling (see Movie S2). By downscaling the dimensions to diameter 1mm we were able to fabricate bending actuators that reach full stroke (270°) when inputting only volumes of less than 7.5μl. Furthermore, by modelling the volumetric expansion of our actuator using FEM, we see radially restricted actuators are characterized by a linear displacement-volume relationship. This means that the input volume is efficiently redirected towards only one spatial dimension, giving a leveraging effect for fast actuation. To confirm this, we have tested our bending actuators under a high-frequency pneumatic input (on-off), where the input air pressure was adjusted such that a full stroke was reached at the end of the cycle (see Figure 3C and Figure S8 for details). We were able to achieve an actuation frequency reaching 30Hz, where complete bending and recovery to initial state takes place within ~30ms (see Movie S7). Further, we see that the dynamics are limited by the deflation part of the cycle. In contrast to inflation where we can adjust the input pressure to reach full stroke quicker, the deflation of the actuator is limited by an atmospheric back-pressure, resulting in a maximum actuation frequency of 30 Hz.
The presented methodology of combining miniaturisation with a fibre reinforcement design showed to be a highly successful pathway of creating highly dynamic actuators. However, this methodology is not limited to only bending deformations. Using the fabrication freedom of additive manufacturing, we can deposit fibres at arbitrary positions and orientations, as displayed in Figure 4. The fibre architectures that give rise to the unique deformation of these actuators are displayed using SEM images, where for twining we combined helical fibres with three grouped eccentric fibres, for extending we remove the eccentric fibres, and for contracting we only use eccentric fibres that are evenly spaced. Further, these actuators were dynamically tested, resulting in a maximum actuation frequency of 20Hz for twining, 30Hz for contraction and 30Hz for elongation actuators (see Movie SI8-10 for high-speed actuation videos, Figure SI3-5 for technical drawings of the actuators and Figure SI7 for detailed characterisation results).
Finally, to demonstrate the performance of our actuators, we applied them in a setting where speed, small scale and compliance are of the utmost importance: catching of a fly without killing it. Towards this goal, we developed a soft robotic flycatcher (Figure 5B). The flycatcher consists of three bending actuators that are placed in a triangular pattern around a 3D-printed base with a central cylindrical target area (see Supplementary Text for further information and FigureS10 for the technical drawing). When a fly is detected, we apply air pressure to the three bending actuators using a syringe, rapidly closing the trap, successfully catching the fly in the process (see Figure 5B and Movie S11). Furthermore, by combining various actuator designs in a linear or parallel manner, a wide range of miniaturized compliant devices such as actuators that transform into very complex shapes (Movie S12) and endoscopic systems that are able to navigate through complex and constrained environments (Movie S13) can be developed.
In conclusion, by enabling the seamless implementation of the concepts of fibre-reinforcement to control volumetric expansion and miniaturisation, we were able to create a wide variety of actuator deformations with the application of only a few microliters of actuation volume. As volume flux is typically the limiting factor for speed, we were able to create high dynamic motions (up to ~30Hz) using standard pressure regulators. Our additive manufacturing-based automated manufacturing platform allowed us to down-scaling the dimensions of fibre-reinforced actuators without compromising their functionality. Such actuators that operate with low-volume and -pressure fluids and exhibit minor volumetric changes are also highly advantageous in applications where space of operation is limited. In the present study, we focused our investigation on inflatable elastomers. Yet, in future studies, alternative actuation methods can be explored by incorporating different matrix materials that respond to alterations in osmotic conditions, pH, magnetic fields or temperatures. The deformation of this type of soft matter can be guided via our 3D-printed fibrous network, which may unlock new research directions towards the development of a new generation of soft smart materials, actuators and robots.