Stretchable pumps for soft machines

Abstract

Machines made of soft materials bridge life sciences and engineering1. Advances in soft materials have led to skin-like sensors and muscle-like actuators for soft robots and wearable devices1,2,3. Flexible or stretchable counterparts of most key mechatronic components have been developed4,5, principally using fluidically driven systems6,7,8; other reported mechanisms include electrostatic9,10,11,12, stimuli-responsive gels13,14 and thermally responsive materials such as liquid metals15,16,17 and shape-memory polymers18. Despite the widespread use of fluidic actuation, there have been few soft counterparts of pumps or compressors, limiting the portability and autonomy of soft machines4,8. Here we describe a class of soft-matter bidirectional pumps based on charge-injection electrohydrodynamics19. These solid-state pumps are flexible, stretchable, modular, scalable, quiet and rapid. By integrating the pump into a glove, we demonstrate wearable active thermal management. Embedding the pump in an inflatable structure produces a self-contained fluidic ‘muscle’. The stretchable pumps have potential uses in wearable laboratory-on-a-chip and microfluidic sensors, thermally active clothing and autonomous soft robots.

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Fig. 1: Stretchable pumps based on charge-injection EHD.
Fig. 2: Performance of the stretchable EHD pumps.
Fig. 3: Stretchable pump embedded in a textile glove for on-body thermal regulation.
Fig. 4: Fluidic muscles obtained by bonding a stretchable pump to a bending fluidic actuator.

Data availability

All data are available from the corresponding authors upon reasonable request.

Change history

  • 27 August 2019

    Owing to a technical error, this Letter was not published online on 14 August 2019, as originally stated, and was instead first published online on 15 August 2019. The Letter has been corrected online.

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Acknowledgements

We thank H. Shigemune for discussions about EHD, M. Imboden for assistance with the thermal regulation experiments and O. Gudozhnik for developing the 5 kV and 6 kV supplies. We acknowledge financial support from JSPS KAKENHI under grants 16H04306, 18H05473 and 19H05328; MEXT/JSPS under Leading Initiative for Excellent Young Researchers; Swiss National Science Foundation through NCCR Robotics; Japanese TOBITATE! Young Ambassador Program; Hasler Foundation Cyber-Human Systems programme; and the BioRobotics Institute of Scuola Superiore Sant’Anna, Pisa for funding V.C.’s initial stay at EPFL in 2016.

Author information

V.C., J.S., S.M., D.F. and H.S. conceived the project. V.C. and J.S. designed and characterized the devices. V.C., J.S. and Y.K. fabricated the devices. V.C. and H.S. analysed the data. S.M., H.S. and D.F. contributed to data interpretation. V.C., J.S. and H.S. wrote the paper. All authors provided feedback and agree with the final version of the manuscript.

Correspondence to Vito Cacucciolo or Herbert Shea.

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Competing interests

V.C., J.S., S.M., D.F. and H.S declare financial interest in form of a patent application. Y.K. declares no competing interests.

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Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 The two different electrode geometries and corresponding electrohydrodynamic (EHD) mechanisms used in this work.

a, Conduction pumping, with inclined capacitors. Heterocharge layers form near the electrodes. These layers are characterized by a higher concentration of ions of opposite polarity with respect to the nearest electrode. As a consequence, these ions are attracted to the nearest electrode, where they discharge. The inclined geometry of the capacitors allows net flow thanks to the in-flow component of the electric field near to the electrode surface. b, Charge injection, with interdigitated electrodes. When the electric field is high enough to overcome the energy barrier, field emission takes place, with electrons tunnelling from the cathode into the dielectric liquid. The generated ions are accelerated by the electric field until they discharge at the anode, transferring momentum to neutral liquid molecules along the way.

Extended Data Fig. 2 Fabrication process for stretchable pumps using the interdigitated design.

a, Fabrication process for the C pump. i, The fabrication of the electrode layers starts by blade-casting a carbon-based electrode membrane with a thickness of 30 μm on a poly(ethylene terephthalate) (PET) support, which is then cured at 80 °C for 2 h. ii, A 400 μm PDMS membrane is cast on the top of the electrode and cured at 80 °C for 1 h. iii, The sample is turned over to expose the electrode membrane, which is then processed (iv) by laser engraving to define the interdigitated pattern. This process allows the manufacturing of many samples in parallel, up to 24 samples with our equipment at EPFL-LMTS. The central photo shows eight sets of interdigitated electrodes at this stage. v, The channel layer, consisting of a 500-μm-thick laser-cut PDMS membrane, is bonded to the bottom electrode layer by a silicone adhesive film. vi, The top electrode layer, with two laser-cut holes for fluidic connection, is bonded on the top of the channel layer. vii, The PET supports are removed, and the C stretchable pump, shown in the bottom photo, is ready to use. b, Fabrication process for the Ag pump. i, The fabrication starts by blade-casting and curing (80 °C, 1 h) a 400 μm PDMS membrane. ii, On the PDMS, a silver-based stretchable ink is printed through a 23-μm-thick Mylar mask and cured at 80 °C for 3 h. iii, After removal of the mask, the bottom electrode layer is bonded to the 500-μm-thick channel layer with a silicone adhesive film. iv, The top electrode layer, with two laser-cut holes for fluidic connection, is bonded on the top of the channel layer. v, The PET supports are removed, and the Ag stretchable pump is ready to use.

Extended Data Fig. 3 Four generations of stretchable pumps plotting generated pressure vs. applied voltage.

The ‘inclined 1’ and ‘inclined 2’ generations have inclined capacitors as the electrode configuration. Inclined 2 is a scaled version of inclined 1, with half the channel size, half the gap between opposite electrodes and many more electrode pairs (43 rather than 5). The interdigitated generations have the same channel size and gap between opposite electrodes as inclined 2 but use interdigitated electrodes rather than inclined capacitors. The C version has laser-engraved carbon-silicone composite electrodes and uses 3M Novec 7100 as the dielectric fluid. The Ag version has mask-printed silver-based electrodes and uses 3M Fluorinert FC-40 as the dielectric fluid. The Ag devices can sustain higher fields than the C devices, thanks to the different dielectric liquid and to the different electrode fabrication method.

Extended Data Fig. 4 A 5 kV programmable power supply weighing 16 g, including Li-ion battery.

This custom-made power supply is based on an EMCO d.c.–d.c. converter from XP-power (https://www.xppower.com/Product/A-Series) and includes a microcontroller to programme the output. Dimensions are 5 cm × 4 cm × 0.8 cm. In this work, we also used a 6 kV, 20 g version. For a given voltage, pump performance is the same when driven by these low-mass power supplies or by larger laboratory power supplies, because the current draw from the pumps is far less than the maximum current that the d.c.–d.c. converter can supply.

Extended Data Fig. 5 Three stretchable pumps connected in series.

The data shown in Fig. 2f were taken using these three pumps connected in series to increase pressure. Alternatively, pumps could be connected in parallel for higher flow rate. Each pump is 7.5 cm long.

Extended Data Table 1 Comparison of pump performance

Supplementary information

Video 1

Stretchable pump stretched and twisted. The stretchable pump is made of a transparent elastomer (Dow Corning Sylgard 184 PDMS) and stretchable electrodes (carbon- or silver-based). The video shows a pump being extensively and repeatedly deformed without damage.

Video 2

Untethered operation of a stretchable pump inflating a heart-shaped balloon. The video shows a stretchable pump pushing liquid between two ventricles of a heart-shaped balloon (see final states in Fig. 1d). Each side of the pump is connected to one ventricle of the balloon, which is filled with Fluorinert FC-40 dielectric liquid. When a voltage of +6 kV is applied, the pump starts displacing liquid from the left ventricle of the balloon towards the right one. The microphone is on to show that the stretchable pump operates silently. 31 seconds after starting with equal volumes on both sides, the volume of the right-hand side ventricle is double that of the left-hand one. The pump is powered by a 20 g untethered power supply (including rechargeable battery, microcontroller, USB connector, and dc-dc converter, see Extended Data Fig. 4), showing the portability of this technology despite the high voltages used.

Video 3

Active wearable for thermal regulation using a stretchable pump embedded in a glove. We developed a wearable device illustrating active thermal regulation on the human body by integrating a stretchable pump and a flexible fluidic circuit in a textile glove. The scope of the device is to transport heat to a different part of the body (in this case, from forearm to hand) by circulating the liquid through a serpentine. The heat in our experiment is generated by a flexible-foil heater mounted on the forearm. This heater simulates the generation of heat by the human body (e.g., during intense physical activity). Thanks to its low mass (1g) and compliance, the pump can be sewn on the glove and does not interfere with the wrist movements. We used an infrared camera to map the temperature. The video shows that when the heater is on and the pump is off, the heater reaches an average steady state temperature of 45 °C. Once the pump is activated, it pushes cold liquid into the serpentine on the heater from one side (top side in the movie) and extracts hot liquid from the other side (bottom side in the movie). The hot liquid cools down when circulating through the cooling serpentine on the glove and is pumped back to the heater. As a consequence, the pump cools down the heater to an average temperature of 42 °C. Thanks to its low power consumption, the pump does not heat-up the fluid circulating through it.

Video 4

Self-contained fluidic muscles. This demonstration shows a fluidic muscle composed of a bending fluidic actuator with a stretchable pump integrated in its bottom layer. The inlet of the pump is connected to a fluidic reservoir on the back of the actuator, while its outlet is connected to the bellows-shaped active chamber. The actuator is filled with dielectric liquid before its activation and then the inlet tube is sealed. When a voltage is applied, the pump pushes the fluid from the reservoir to the bellows-shaped chamber, whose inflation causes bending of the actuator. Higher values of the voltage lead to higher bending angles. The values of the voltage required to activate the stretchable pump are higher in this configuration compared to the characterization experiments. The reason for this is that the pre-pressurization deforms slightly the pump, increasing the gap between each electrode pair. The experiment is conducted on a lubricated horizontal surface to minimize the effects of gravity and friction.

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