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An integrated design and fabrication strategy for entirely soft, autonomous robots

Abstract

Soft robots possess many attributes that are difficult, if not impossible, to achieve with conventional robots composed of rigid materials1,2. Yet, despite recent advances, soft robots must still be tethered to hard robotic control systems and power sources3,4,5,6,7,8,9,10. New strategies for creating completely soft robots, including soft analogues of these crucial components, are needed to realize their full potential. Here we report the untethered operation of a robot composed solely of soft materials. The robot is controlled with microfluidic logic11 that autonomously regulates fluid flow and, hence, catalytic decomposition of an on-board monopropellant fuel supply. Gas generated from the fuel decomposition inflates fluidic networks downstream of the reaction sites, resulting in actuation12. The body and microfluidic logic of the robot are fabricated using moulding and soft lithography, respectively, and the pneumatic actuator networks, on-board fuel reservoirs and catalytic reaction chambers needed for movement are patterned within the body via a multi-material, embedded 3D printing technique13,14. The fluidic and elastomeric architectures required for function span several orders of magnitude from the microscale to the macroscale. Our integrated design and rapid fabrication approach enables the programmable assembly of multiple materials within this architecture, laying the foundation for completely soft, autonomous robots.

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Figure 1: Fully soft, autonomous robot assembly.
Figure 2: Multi-material, EMB3D printing.
Figure 3: Octobot control logic.
Figure 4: Octobot actuation.

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Acknowledgements

We thank D. Gessel, G. Leyh, M. Pauline, N. W. Bartlett, M. A. Skylar-Scott, T. J. Ober and J. T. Muth for their comments and discussions. We thank L. K. Sanders for assistance with photography and videography and J. C. Weaver for assistance with electron microscopy. We acknowledge support from the National Science Foundation through the Harvard MRSEC (grant no. DMR-1420570) and the Wyss Institute for Biologically Inspired Engineering. This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. R.L.T. acknowledges support from a National Science Foundation Graduate Research Fellowship and J.A.L. acknowledges support from the National Security Science and Engineering Faculty Fellowship.

Author information

Authors and Affiliations

Authors

Contributions

M.W., R.L.T., J.A.L. and R.J.W. conceived the experimental work; M.W. and R.L.T. led the experiments with assistance from D.J.F. and B.M.; M.W., R.L.T., J.A.L. and R.J.W. contributed to data analysis and interpretation and wrote the paper. All authors provided feedback.

Corresponding authors

Correspondence to Jennifer A. Lewis or Robert J. Wood.

Ethics declarations

Competing interests

G.M.W. is the co-founder of Soft Robotics, Inc., a startup company focused on soft robotics, and J.A.L. is the co-founder of Voxel8, Inc., a startup company focused on 3D printing of functional materials.

Additional information

Reviewer Information Nature thanks B. Mazzolai and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 EMB3D printing of an octobot.

a, An EMB3D printing mould is machined from acetal. b, The hyperelastic layers needed for actuation are cast and cross-linked in the actuator regions of the mould. c, A soft controller protected with a polyimide tape mask is loaded onto the pins of the EMB3D printing mould. d, The fuel reservoir matrix material is carefully loaded into the fuel reservoir area of the mould and degassed under vacuum. e, Liquefied fugitive plug material is manually loaded into the soft controller via the inlets and briefly degassed. f, The protective tape is removed after the fugitive plug material physically gels, and the fugitive plug is photo-cross-linked. g, The body matrix material is cast into the mould and degassed. h, Any excess body matrix material is removed with a squeegee step, EMB3D printing begins, and the entire mould and EMB3D-printed materials are placed in a 90 °C oven to cross-link. i, After 2 h, the cross-linked octobot is removed from its mould and kept at 90 °C for a total of 4 days to ensure complete auto-evacuation of the aqueous fugitive inks. j, Before operation, excess body matrix material is removed via laser cutting. k, The final octobot, shown here in a close-up view, is prepared for operation.

Extended Data Figure 2 Rheological properties of the body matrix.

a, Schematic illustration of the behaviour of the body matrix during the EMB3D printing process. (i) When the body matrix is at rest, the fumed silica fillers within the silicone material form a percolated network, giving rise to its equilibrium, at-rest shear yield stress, τy,0. (ii) As the nozzle travels through the matrix during EMB3D printing, the matrix is yielded and the percolated filler network is disrupted, decreasing the apparent yield stress of the matrix material, τy,t. (iii) Sufficient deformation can completely disrupt the fumed silica microstructure and completely eliminate the yield stress of the matrix material (τy,t → 0 Pa). (iv) The fumed silica network does not immediately recover when it returns to a quiescent state. (v) Over time, the network slowly restructures to (vi) its equilibrium microstructure, and τy,tτy,0. b, c, Log–log plots of apparent viscosity (b) and corresponding shear stress (c) versus shear rate for various PDMS matrix formulations, which are prepared by blending Sylgard 184 (10:1 ratio of base to hardener) and SE 1700 (4:1 ratio of base to hardener) at various mass fractions. The formulations are listed by the weight ratio of SE 1700 used (0.0,0.33, 0.5, 0.67 and 1.0). Closed and open circles in c represent measurements taken during the flow sweep and flow ramps of the thixotropic loop studies, respectively. The final body matrix, formulated from the 50 wt% SE 1700 blend, shows clear thixotropic behaviour and a substantial decrease in yield stress upon yielding. Blends with higher concentrations of filler particles show diminished thixotropic behaviour, and the yield stress is not eliminated during nozzle translation. Consequently, crevices or air pockets form during printing, with matrix materials possessing higher concentrations of fumed silica.

Extended Data Figure 3 Storage modulus recovery of the body matrix after yielding.

a, A plot of storage modulus (G′) as a function of time illustrates how the modulus of the body matrix recovers during three-phase thixotropy tests. After a probe phase, a shear stress of 100 Pa is applied for varying times during a deformation phase, resulting in temporary fluidization of the matrix material. During the recovery phase, the modulus increases over time. b, The ratio of the loss modulus to the storage modulus, G″/G′ = tan(δ), is plotted as a function time for each of the recovery phases measured in a. The onset of recovery of the yield stress of the body matrix material—and the onset of fumed silica filler percolation in a recovering matrix material—is assumed to be the moment G′ = G″ or tan(δ) = 1 (horizontal dashed line). Therefore, the ‘recovery time’ of the body matrix material (indicated by the vertical dashed lines) is approximately the time at which tan(δ) = 1 after deformation. Because the momentary deformation incurred by nozzle translation through a discrete volume of matrix material during EMB3D printing happens within a time period shorter than 1 s and with a magnitude of less than 100 Pa, the thixotropic recovery time of the body matrix material is less than 200 s—the approximate time it takes the body matrix to recover after being sheared by a 100-Pa stress for 1 s.

Extended Data Figure 4 Rheological and printing behaviour of inks and matrix materials.

a, A log–log plot of apparent viscosity as a function of shear rate for the fugitive ink (red), catalytic ink (black), body matrix material (blue) and fuel reservoir matrix material (green). b, An octobot with fluorescently dyed fugitive inks (red, not auto-evacuated) and hyperelastic actuator layers (blue) fabricated by moulding and EMB3D printing.

Extended Data Figure 5 Auto-evacuation of the fugitive and catalytic inks.

Photographs of the reaction chambers of an octobot showing the upstream portions of the actuator networks (top) and a one-pad actuator (bottom) at various times, t. These photographs reveal the auto-evacuation of the fugitive and catalytic inks, which leaves behind open channels that serve as mesofluidic features.

Extended Data Figure 6 Infilling the soft controller from the fuel inlets.

Water (with red or blue dye) is introduced into the fuel reservoir via the fuel inlets. Continuity between the fuel reservoirs, soft controller and downstream EMB3D-printed components is possible because of the fugitive plugs, which auto-evacuate along with the EMB3D-printed inks. Scale bar, 5 mm.

Extended Data Figure 7 Characterization of EMB3D-printed actuators.

a, CAD model of a four-bladder actuator design. Other than bladder number, the design is similar to the actuators illustrated in Fig. 4a. b, EMB3D-printed actuator is shown before inflation. c, Actuator inflated to working pressure. Scale bars in b and c, 5 mm. d, Pressure versus displacement curves for four-bladder actuators with varying hyperelastic-layer thickness, h (see legend; values in micrometres). The data points reported are the mean inflation displacement angles for two representative actuators over five inflation cycles and the shading indicates 95% confidence intervals. e, f, Blocked force versus pressure curves for two-bladder (e) and four-bladder (f) actuators of varying hyperelastic-layer thickness, h (see legends; values in micrometres). The lines shown are third-degree polynomial fits of data collected from five inflation–deflation cycles of representative actuators. The shaded regions indicate 95% confidence intervals. Detailed procedures for characterizing actuator performance are provided in Methods.

Extended Data Figure 8 Autonomous switching between actuating states during octobot operation.

a, Switching in the soft controller was tracked with time according to the octobot operation recorded in Supplementary Video 5. b, The corresponding inflation of actuators associated with the blue and red and actuation states are also reported with time.

Extended Data Table 1 Break-in and working gauge pressures for printed actuators

Supplementary information

EMB3D printing of an octobot

The video shows the multi-material EMB3D printing step during octobot fabrication. Fluorescent dyes are added to the fugitive ink (yellow) and hyperelastic layers in the actuators (blue) to assist in visualization of the internal features. The video is at 12x playback speed. (MP4 9264 kb)

Decomposition of 50 wt% aqueous hydrogen peroxide monopropellant fuel in the presence of catalyst

This demonstration shows the reaction of 50 wt% hydrogen peroxide catalyzed by the neat Pt Black powder used in the catalytic ink (at left) and Pt-laden plugs formed from EMB3D printing and subsequent auto-evacuation of catalytic ink (at right). The video is at 1x playback speed. (MP4 9555 kb)

Demonstration of printed catalyst integrity with different fuel concentrations

Catalytic ink was EMB3D printed to form Pt laden plugs in the shapes of squares. These features were dropped into hydrogen peroxide at 50% (left) and 90% (right). This demonstration shows that high concentration hydrogen peroxide fuels resulted in combustion of the Pt-laden plugs formed with the catalytic ink. The video is at 1x playback speed. (MP4 9531 kb)

Thermal camera version of Supplementary Video 3

The video is at 1x playback speed. (MP4 9363 kb)

Octobot operation from a top-down perspective

A representative octobot autonomously actuating. The video, acquired using a backlight, is at 10x playback speed. (MP4 9275 kb)

Octobot operation from a front-view perspective

A representative octobot autonomously actuating. The video, acquired using a blue background, is at 15x playback speed. (MP4 9530 kb)

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Wehner, M., Truby, R., Fitzgerald, D. et al. An integrated design and fabrication strategy for entirely soft, autonomous robots. Nature 536, 451–455 (2016). https://doi.org/10.1038/nature19100

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