Name: fabrication of carbon-based ionic electromechanically active soft actuators
Uploaded: 2020-04-25T13:00:00
Duration: 14 min 42 s
Description: Watch this Scientific Journal Video about Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators at JoVE.com

The authors would like to thank Ron Hovenkamp and Marcel Mulder from Philips Research for helpful discussions. This work was partially supported by institutional research funding IUT (IUT 20-24) of the Estonian Ministry of Education and Research, by the Estonian Research Council grant (PUT1696), by the European Regional Development Fund, by the programme Mobilitas Pluss (Grant No MOBTP47), by the European Union&#8217;s Horizon 2020 research and innovation program under the Marie Sk&#322;odowska-Curie grant agreement No 793377 (BIOACT), and by project IMPACT-MII, an EIT Health innovation project. EIT Health is supported by EIT, a body of the European Union.

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The authors have nothing to disclose.

We presented a simple, fast, repeatable and versatile fabrication method for ionic electromechanically active composite preparation for various actuator applications, and with minor modifications also for energy storage, harvesting33 or sensing34 applications. The current method focuses on membranes with an integral passive and chemically inert component (e.g., a textile-reinforced polymer network or a highly porous Teflon membrane, see also Figure 3) because such membranes significantly simplify the actuator preparation process also in large scale. Moreover, the resulting membranes have a lower risk of swelling and buckling due to solvents (or electrolyte) in the electrode suspension or of short-circuit hotspot formation compared to many other common actuator fabrication methods and materials.
The critical steps in the capacitive actuator laminate preparation are the membrane preparation, electrode fabrication, current collector attachment, cutting, and contacting (Figure 2). Each of these steps leaves room for customization and performance optimization, but also for mistakes. In the following section we will discuss the beneficial modifications and troubleshooting strategies of this fabrication method in further detail. A high-performance composite results from the interplay of several key aspects that need to be kept in mind: sufficient electronic conductivity along the electrode (add gold current collector to carbon electrodes); sufficient ionic conductivity through the membrane (use a thin porous membrane and sufficient amount of low-viscosity electrolyte, reduce the risk on unfavorable interactions between the membrane and the electrolyte by using an inert polymer network); high surface area of the electrode (select a suitable carbon type); tailored electrolytes that result in asymmetrical swelling/contraction of the electrodes (select a suitable electrolyte); mechanical parameters (Young&#8217;s moduli of the components). These main aspects of a high-performance carbon-based actuator are also highlighted on Figure 1B.
A high-performance membrane is the central part of this composite. It has two tasks: prevent electron conductivity (short circuits) between the electrodes while enabling high ionic conductivity. Modifications to the membrane could serve several purposes, for example tool integration as introduced by Must et al.24 or the addition of new properties (e.g., biocompatibility, biodegradability or different mechanical properties). The current fabrication method could be modified to use other polymers and electrolytes in the membrane to introduce new properties to the active laminate. Like the solvent selection strategy introduced here for the textile-reinforced actuators, it is advisable to select poorer solvents for the subsequent electrode fabrication compared to the membrane preparation. This ensures that the membrane remains functional and intact also after the addition of electrodes.
The actuation performance of the final composite is influenced by the selected electrode material (carbon), the electrolyte and possibly their compatibility with each other. This protocol introduces the fabrication of carbon-based capacitive laminates using boron carbide derived carbon and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) ionic liquid. However, the same protocol is adaptable to other high specific surface area carbon materials, such as carbide-derived carbons from other sources (e.g., TiC35, SiC or Mo2C36), carbon nanotubes8,37, carbon aerogel38 or graphene39, and others, as also reviewed recently40. Moreover, also other electrolytes could be used in the actuator preparation. Obtaining a functional composite is not limited to the carbon and ionic liquid types presented in this protocol. The carbon particle size, their possible agglomeration in the electrode suspension and the suspension viscosity are more crucial parameters for the spray-coating process.
This method enables the production of electromechanically active laminate material with reproducible properties in large quantities. Miniaturization of actuators made of this material is mainly carried out using high-precision cutting (e.g., Figure 3C). Alternative methods for preparing fine structures, such as masking, and patterning are possible during spray-coating41. Moreover, millimeter-scale structures can also be patterned in the subsequent gold current collector attachment step. However, in sub-millimeter scale this might become quite difficult. Other types of actuators or carbon-based actuator without gold current collectors might be easier to prepare, if the patternable features must be in the micrometer scale.
Intrinsically soft actuators that respond to electrical stimuli have many advantages thanks to their soft and compliant nature, quiet operation and low required voltage levels. The current protocol shows how to produce such material in larger quantities and with high batch-to-batch and within-batch repeatability without compromising the actuation performance. Modifications to the current method to incorporate more bio-friendly and possibly also bio-degradable components that would enable operation close or inside living organisms in addition to successful total encapsulation approaches, and the integration of the introduced active material into soft robotic or biomedical devices are envisioned for the future.

Ionic electromechanically active polymer or polymeric composites are intrinsically soft and compliant materials that have received increasing interest in different soft robotics and biomimetic applications (e.g., as actuators, grippers, or bioinspired robots1,2). This type of material responds to electrical signals in the range of a few volts, which makes them easy to integrate with conventional electronics and power sources3. Many different types of ionic actuator base materials are available, as described in detail elsewhere4, and again very recently5. Moreover, it has been particularly emphasized recently that the development of soft robotic devices will be very closely related to the development of advanced manufacturing processes for relevant active materials and components6. Furthermore, the importance of an efficient and well-established process flow in the preparation of reproducible actuators that have the potential to move from the laboratory to industry has also been highlighted in previous methods-based studies7.
Over the last decades, many fabrication methods have been developed or adapted for the preparation of actuators (e.g., layer-by-layer casting8 and hot-pressing9,10, impregnation-reduction11, painting12,13, or sputtering and subsequent electrochemical synthesis14,15, inkjet printing16 and spin-coating17); some methods are more universal, and some are more limiting in terms of material selection than others. However, many of the current methods are rather complicated and/or more suitable for laboratory scale fabrication. The current protocol focuses on a fast, repeatable, reliable, automatable and scalable actuator fabrication method to produce active laminates with low batch-to-batch and within-batch variability and a long actuator lifetime18. This method can be used by materials scientists to develop high-performance actuators for the next generation of bioinspired applications. Moreover, following this method without modifications gives soft robotics engineers and teachers an active material for the development and prototyping of new devices, or for teaching soft robotics concepts.
Ionic electromechanically active polymer or polymeric actuators are typically made of two- or three-layer laminar composites and bend in response to electrical stimulation in the range of few volts (Figure 1). This bending motion is caused by the swelling and contraction effects in the electrode layers, and it is typically brought along either by faradaic (redox) reactions on the electrodes (e.g., in case of electromechanically active polymers (EAPs) like the conductive polymers) or by capacitive charging of the double-layer (e.g., in carbon-based polymeric electrodes, where the polymer might only act as a binder). In this protocol (Figure 2), we focus on the latter; we show the fabrication of an electromechanically active composite that consists of two high specific surface area electronically conductive carbon-based electrodes that are separated by an inert ion-conductive membrane that facilitates the movement of cations and anions between the electrodes &#8211; a configuration very similar to the supercapacitors. This type of actuator bends in response to capacitive charging/discharging and the resulting swelling/contraction of the electrodes is typically attributed to the differences in the volume and mobility of cations and anions of the electrolyte8,10,19. Unless surface-functionalized carbon is used as the active material or the capacitive composite is used outside of the electrochemical stability potential window of the electrolyte, no faradaic reactions are expected to take place on this type of electrodes20. The lack of faradaic reactions is the main contributor to the beneficially long lifetimes of this actuator material (i.e., thousands of cycles in air8,18 shown for different capacitive actuators).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61216/61216fig1v4.jpg" /> 
Figure 1: The structure of the carbon-based actuator in the neutral (A) and in the actuated state (B). (B) also highlights the key characteristics that determine the performance of an ionic actuator. Note: the figure is not drawn to scale. Ion size has been exaggerated to illustrate the most commonly cited actuation mechanism prevalent in case of an inert membrane that enables the mobility of both anions and cations of the electrolyte (e.g., ionic liquid). <a href="https://www.jove.com/files/ftp_upload/61216/61216fig1v4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Obtaining a functional membrane that remains intact throughout the whole fabrication process is one of the key steps in the successful actuator preparation. A high-performance membrane for an actuator is as thin as possible and enables ionic conductivity between the electrodes while blocking any electronic conductivity. The ionic conductivity in the membrane can result from combining the electrolyte with an inert porous network (e.g., the approach used in this protocol) or by the usage of specific polymers with covalently bonded ionized units or other groups that enable interactions with the electrolyte. The former approach is preferred here for its simplicity, whereas specifically tailored interactions between the electrolyte and the polymer network could also have advantages, if unfavorable interactions (e.g., blocking or slowing down ion movement significantly due to interactions) can be ruled out. The vast selection of ionomeric or otherwise active membranes for electromechanically active actuators and their resulting actuation mechanisms have been reviewed recently21. The membrane selection, in addition to the electrode selection, plays a crucial role in the actuator&#39;s performance, lifetime and actuation mechanism. The current protocol is mainly focusing on inert membranes that provide the porous structure for ion migration (as shown on Figure 1), although parts of the protocol (e.g., membrane option C) could also prove beneficial for active membranes.
In addition to the membrane material selection, its fabrication method also plays an important role in obtaining a functional separator for the composite. Previously used cast membranes tend to melt during the later hot-pressing step and may therefore form short-circuit hotspots22. Moreover, commercial ionomeric membranes (e.g., Nafion) tend to swell and buckle significantly in response to solvents used in the later manufacturing steps12, and some polymers (e.g., cellulose23) are known to dissolve to some extent in some ionic liquids, possibly causing problems with the repeatability of the fabrication process and resulting in poor uniformity of the electrodes. Therefore, this protocol focuses on actuators with an integral passive and chemically inert component in the membrane (e.g., glass fiber or silk with PVDF or PTFE) that stops the composite from swelling and buckling in later fabrication steps or from forming short-circuit hotspots. Moreover, the addition of an inert and passive component simplifies the manufacturing process significantly and enables larger batch sizes compared to more traditional methods.
The inclusion of a passive reinforcement in the membrane was first introduced by Kaasik et al.18 to tackle the above-mentioned problems in the actuator manufacturing process. The inclusion of a woven textile reinforcement (see also Figure 3B and 3D) further introduces the ability to integrate tools into the active composite24 or to develop smart textiles18. Therefore, the membrane option C in the protocol is more suitable for such applications. However, in case of miniaturized actuators (in the sub-millimeter level), the passive-to-active component ratio in the membrane becomes more and more unfavorable and the inclusion of an ordered textile reinforcement might start to negatively influence the actuator&#8217;s performance and the sample-to-sample repeatability. Moreover, the direction of the reinforcement (along or diagonally in respect to the bending direction) might impact the performance of more complexly shaped actuators unexpectedly. Therefore, a less ordered and highly porous inert structure would be more beneficial for miniaturized actuators and more complex actuator shapes.
Polytetrafluoroethylene (PTFE, also know under the trade name Teflon) is one of the most inert polymers know to date. It is typically highly hydrophobic, but surface-treated versions that are rendered hydrophilic exist, which are more easily usable in the actuator fabrication. Figure 3A illustrates the random structure of an inert hydrophilic PTFE filtration membrane that was used in this protocol for actuator preparation. In addition to the uniformity of this material in all directions that is beneficial for cutting out miniaturized actuators or complex shapes, using a commercial filtration membrane with controlled porosity further simplifies the actuator fabrication process by almost eliminating the need for any membrane preparation. Moreover, membrane thicknesses as low as 30 &#181;m are extremely difficult to obtain in the previously described textile-reinforced configuration. Therefore, PTFE-based actuator fabrication methods (options A and B) from this protocol should be preferred in most cases, further considering that option A is faster, but actuators made using option B show larger strains (in the frequency range presented in Figure 4B). The soft gripper introduced in the representative results section was also prepared using the PTFE membrane first soaked in electrolyte.
After a functional membrane has been prepared, the protocol continues with the electrode preparation and current collector attachment. The carbon-based electrodes are added using spray-coating &#8211; an industrially established procedure that enables high control over the resultant electrode layer thickness. More uniform electrodes are produced with spray coating compared to, for example, the casting method (or possibly also other liquid methods) where sedimentation of carbon particles during the film drying25 are known to occur. Moreover, a further feature of the presented fabrication method relies in the solvent selection strategy that is most important in case of textile-reinforced membranes. More precisely, 4-methyl-2-pentanone (the solvent in the electrode suspension and glue solution) does not dissolve the inert membrane reinforcements or PVDF that is used in the membrane solution of the textile-reinforced membrane. Therefore, the risk of creating short-circuit hotspots in the composite during spray coating is further reduced.
The capacitive laminate is already active after the application of carbon electrodes. However, an order of magnitude faster actuators26 are obtained with the application of gold current collectors. A further important step in the protocol is the attachment of current collectors while the corresponding electrode is in the stretched state (i.e., the composite is bent). Therefore, in the neutral flat state of the actuator, the gold leaf will be buckled in the submillimeter level. This buffering-by-buckling27 approach enables higher deformations without breaking than would otherwise be possible for a fine (~100 nm) metal sheet.
All the actuator manufacturing steps (membrane preparation, electrode spraying, current collector attachment) have also been summarized in Figure 2. For the performance characterization demonstration, we have prepared a gripper that is compliantly grasping, holding and releasing a randomly shaped object with a random surface texture. Simpler geometries, such as rectangular samples with 1:4 or higher aspect ratio (e.g., 4 mm to 20 mm or even 1 mm to 20 mm28) cut out of the active material and clamped in the cantilever position are also very typical for material characterization or other applications utilizing the bending-type behavior.
The article ends with a brief introduction into the typical ionic electromechanically active capacitive material characterization and troubleshooting techniques using the simpler rectangular actuator geometry. We show how to use common electrochemical characterization techniques like cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to characterize and troubleshoot the actuator material in more detail. The visualization of the composite in sub-millimeter level is done using scanning electron microscopy (SEM), for which we use the cryo-fracturing technique to prepare the samples. The polymeric nature of the material makes it difficult to obtain clear cross-sections with just regular cutting. However, breaking frozen samples results in well-defined cross-sections.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61216/61216fig2v4.jpg" /> 
Figure 2: Overview of the fabrication process. Most important steps are highlighted. <a href="https://www.jove.com/files/ftp_upload/61216/61216fig2v4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="0" cellpadding="0" cellspacing="0" style="width:715px;" width="715">
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">~150 &#181;m thick gold plates for custom contacts</td>
			<td style="width:127px;">local jeweler</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">99.9% purity (24K)</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf])</td>
			<td style="width:127px;">Solvionic</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">100 ml Erlenmeyer flask</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">4-methyl-2-pentanone (MP)</td>
			<td style="width:127px;">Sigma Aldrich</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">&#8805;99%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">acetone</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;">technical grade</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">analytical balance</td>
			<td style="width:127px;">Mettler Toledo AB204-S/PH</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="92">
			<td height="92" style="height:92px;width:219px;">carbon powder</td>
			<td style="width:127px;">Y Carbon</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">boron carbide derived carbon, particle size &#60;10 &#181;m, specific surface area 1800 m2/g, pore volume 0.5 cm3/g</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">carbon powder</td>
			<td style="width:127px;">Skeleton Technologies</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">titanium carbide derived carbon</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">circular disk magnets (neodymium) for custom contacts</td>
			<td style="width:127px;">local hardware store</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">d = 2 mm, thickness 1 mm</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">compressed air supply for the airbrush</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:219px;">crocodile clips with jaws insulated from each other (Kelvin clips)</td>
			<td style="width:127px;">local hardware store</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">Optional for making custom contacts. Regular crocodile clips are not suitable because there the jaws are connected to each other at the spring.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">disposable foam cup</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">epoxy glue</td>
			<td style="width:127px;">local hardware store</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">preferaby fast cure epoxy for attaching gold contacts to magnets</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">filter paper for drying</td>
			<td style="width:127px;">Munktell, Filtrak</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">e.g. diameter 150 mm and up if 142 mm PTFE sheet is used.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">flat nose tweezers</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">glass funnel</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">gold leaf on transfer sheets</td>
			<td style="width:127px;">Giusto Manetti Battiloro</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">24K</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">graduated glass cylinder</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">hairdryer or a heat gun</td>
			<td style="width:127px;">e.g. Philips</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">infrared ligth bulb</td>
			<td style="width:127px;">e.g. Philips</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:219px;">liquid nitrogen</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;">CAUTION: Never close the lid of a liquid nitrogen container tightly. The pressure build-up could cause serious injuries.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">magnetic stirrer / hotplate</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">magnetic stirrer bars</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;">about 1 cm long</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">metal pipe</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;">e.g. d = 3 cm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">metal ruler</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">micrometer thickness gauge</td>
			<td style="width:127px;">Mitotuyo</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">range 0-25&#160;mm, precision 0.001&#160;mm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">N,N-dimethylacetamide (DMAc)</td>
			<td style="width:127px;">Sigma Aldrich</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">paintbursh</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">plastic embroidery hoops</td>
			<td style="width:127px;">e.g. Pony</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">select the diameter depending on the desired batch size (e.g. 7.5 cm to 25 cm)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">plastic Pasteur pipettes</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">polyethylene-based laboratory stretch film</td>
			<td style="width:127px;">DuraSeal</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">polyvinylidene difluoride-co-hexafluoropropylene (PVDF-HFP)</td>
			<td style="width:127px;">Sigma Aldrich</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">Mn = 130000, Mw = 400000</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">polyvinylidene fluoride (PVDF)</td>
			<td style="width:127px;">Sigma Aldrich</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">Mw (g/mol) = 534000</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">potentiostat/galvanostat/FRA</td>
			<td style="width:127px;">PARSTAT 2273</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">needed for electrochemical characterization</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">propylene carbonate (PC)</td>
			<td style="width:127px;">Merck</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">99%</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">PTFE filtration membrane</td>
			<td style="width:127px;">Omnipore</td>
			<td style="width:136px;">JVWP14225</td>
			<td style="width:233px;">0.1 &#181;m pore size, hydrophilic , 142&#160;mm diameter, 30&#160;&#181;m thickness, 80% porosity</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:219px;">PTFE filtration membrane</td>
			<td style="width:127px;">Omnipore</td>
			<td style="width:136px;">JGWP14225</td>
			<td style="width:233px;">0.2 &#181;m pore size, hydrophilic , 142&#160;mm diameter, 65&#160;&#181;m thickness, 80% porosity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">scalpel</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">scotch tape</td>
			<td style="width:127px;"></td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">silk (woven textile)</td>
			<td style="width:127px;">Esaki Model Manufacturing</td>
			<td style="width:136px;">#3</td>
			<td style="width:233px;">11.5 g/m2</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">soldering equipment</td>
			<td style="width:127px;">local hardware store</td>
			<td style="width:136px;"></td>
			<td style="width:233px;">For connecting the ~150 &#181;m gold plates to the clips</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">spray gun, airbrush</td>
			<td style="width:127px;">Iwata HP TR-2</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">sputter coater</td>
			<td style="width:127px;">Leica EM ACE600</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:219px;">tabletop scanning electron microscope</td>
			<td style="width:127px;">Hitachi TM3000</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:219px;">ultrasonic processor</td>
			<td style="width:127px;">Hielscher UP200S</td>
			<td style="width:136px;"></td>
			<td style="width:233px;"></td>
		</tr>
	</tbody>
</table>

fabrication of carbon-based ionic electromechanically active soft actuators

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, compliant and biomimetic nature of this deformation, actuators made of the laminate have received increasing interest in soft robotics and (bio)medical applications. However, methods to easily fabricate the active material in large (even industrial) quantities and with a high batch-to-batch and within-batch repeatability are needed to transfer the knowledge from laboratory to industry. This protocol describes a simple, industrially scalable and reproducible method for the fabrication of ionic carbon-based electromechanically active capacitive laminates and the preparation of actuators made thereof. The inclusion of a passive and chemically inert (insoluble) middle layer (e.g., a textile-reinforced polymer network or microporous Teflon) distinguishes the method from others. The protocol is divided into five steps: membrane preparation, electrode preparation, current collector attachment, cutting and shaping, and actuation. Following the protocol results in an active material that can, for example, compliantly grasp and hold a randomly shaped object as demonstrated in the article.

The primary endpoint to distinguish between a successful and a failed experiment is the material&#39;s response to electrical signals after being contacted to a power supply. In electrical engineering, copper is a well-known material for contact making. However, copper is also electrochemically active and therefore not suitable for making contact with an ionic system introduced here. Using copper contacts could cause short circuits due to dendrite formation through the composite. Moreover, in case of material characterization, it is impossible to distinguish between currents (and actuation) stemming from the electroactive material and that stemming from the electrochemical activity of copper29. We have previously shown that actuation &#8211; although unreliable &#8211; without any added active material (i.e., without the carbon-based or conductive polymer electrodes) is possible in case of wet ionomer membranes (e.g., Nafion) and just copper terminals29. Therefore, all experiments with the active material here have been performed using inert gold contacts only.
Electrochemical impedance spectroscopy (EIS) is a nondestructive method for the characterization and troubleshooting of the capacitive actuator material before usage. The impedance spectra in Figure 4C and 4D were captured using a potentiostat/galvanostat/FRA in two-electrode configuration. The sample (20 mm x 4 mm x 150 &#181;m) was placed between gold contacts, the input signal amplitude during the impedance measurement was set to 5 mVRMS and frequencies from 200 kHz to 0.01 Hz were scanned. Figure 4C and 4D show the typical impedance spectra from actuators with high (~300 &#8486; cm2) or with low (~5 &#8486; cm2) internal resistance, respectively. The spectra were recorded using a sample with the dry PTFE membrane and another sample with the soaked membrane, respectively. Higher ionic conductivity through the material tends to correspond to faster actuators and possibly also more displacement at the same actuation frequency (see Figure 4B), if all other parameters (e.g., mechanical parameters) are kept unchanged and the material in general is active.
The nondestructive nature of EIS is especially beneficial for the detection of short circuits in the composite. In case of actuators prepared following the current protocol, short circuits are most often caused by current collector debris on the actuator&#8217;s sides (see cutting instructions in Step 4.1.1) or more rarely by a faulty membrane (e.g., when not covering all pinholes in the textile-reinforced membrane as instructed in Section 1.5). A resistor (in this case a short-circuit) would be presented as a dot on the Nyquist plot of an EIS experiment. Observing such response is a certain indicator of a faulty sample (see Figures 4C and 4D for reference spectra of functional capacitive actuators). Short-circuited samples would typically not actuate. Furthermore, these would most often be rendered permanently useless due to resistive heating and the resulting melting of the composite when tried to actuate.
In its functional form, this material is a double-layer capacitor that shows bending motion in response to charging and discharging of the double layer thanks to specifically tailored electrolytes used in its fabrication. Cyclic voltammetry (CV) is a widely used technique in electrochemistry to study different systems. During a CV experiment, the potential of the working electrode (in this case one of the actuator&#39;s electrodes) is varied in respect to a counter electrode (here the other electrode of the actuator) with constant speed (e.g., 800 mV/s between &#177;2 V) and the current response from the system is recorded using a potentiostat. A typical current response from the capacitive laminate is presented in Figure 4E. The current response of the sample with the soaked PTFE membrane (in dark gray in 4E) resembles that of an ideal capacitor: the current does not depend on the electrode potential and upon reversing the potential, the current direction (and therefore its sign) is changed (almost) immediately, resulting in a (nearly) rectangular voltammogram. The current response of the sample with an initially dry membrane (in pink in 4E) shows less ideal capacitor behavior at this scan rate, probably due to high internal resistance of the material (as also evidenced by EIS in Figure 4C). Still, both samples show the capacitive nature of the composite. On the other hand, light gray lines in Figure 4E show possible behavior from faulty samples (e.g., short-circuited ones) that would closely follow the Ohm&#8217;s law.
The performance of different functional actuators is presented in Figure 4A and Figure 4B. Figure 4A shows snapshots from the video where a 5-finger thermoformed actuator grips, holds and releases a randomly shaped object in response to voltage steps. Simpler geometries are typically used for the material characterization purposes. For example, Figure 4B highlights the dry and soaked PTFE membrane actuators&#8217; maximum bending angle28,30 in response to triangular voltage signals between &#177;2 V. In order to characterize different actuator materials, samples (4 mm x 20 mm x 150 &#181;m) were placed between gold clamps in the cantilever position (leaving 18 mm free length for actuation) and the bending angle was recorded using a video camera. Alternatively, the movement of a single point along the actuator (e.g., 5 mm from the contacts) has been typically monitored in time and used in strain difference calculations31,32. Video processing, although more complicated, gives more information on the whole bending profile of the sample and also enables to reanalyze the performance later, if such a need should arise. The 0.1 Hz point in Figure 4B corresponds to the exact same signal as used in the cyclic voltammetry experiments of Figure 4E, both in terms of actuation voltage as well as actuation frequency. Using the same signal for characterization and actuation allows us, for example, to make conclusions about the capacitive nature of the material and about the stability and lack of electrochemical reactions during actuation.
Electrochemical methods (EIS, CV), visualization of the actuator structure in the (typically) micrometer level (SEM) and displacement characterization are the most common methods for characterizing ionic actuators and evaluating the success of the fabrication process. However, custom experiments to evaluate the actuator&#39;s performance in a more specific application are often developed to evaluate application-specific performance (e.g., the ability to carry a load).
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61216/61216fig3.jpg" /> 
Figure 3: Imaging. Scanning electron micrographs showing the highly porous PTFE membrane (A) and a cross-section of an actuator made using the same membrane showing no delamination (C). SEM micrograph showing a cross-section of a textile-reinforced actuator (D) and an optical photograph of the corresponding silk reinforcement (B). Samples for SEM cross-sections were first cryo-fractured using liquid nitrogen, mounted to a metal sample holder and then sputtered with 5 nm of gold for better definition using a sputter coater. A tabletop scanning electron microscope was used for imaging at 15 keV acceleration voltage. <a href="https://www.jove.com/files/ftp_upload/61216/61216fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61216/61216fig4.jpg" /> 
Figure 4: Representative results of the actuator. (A) Voltage steps and corresponding images of the five-arm gripper compliantly grasping an object with random shape (actuator without contacts 21 mg; polystyrene foam load 17.8 mg); (B) total bending angle of 4 mm x 20 mm x 150 &#181;m PTFE-based actuators clamped between gold contacts (18 mm free length) in response to a triangular actuation signal (&#177;2 V) at different actuation frequencies (n=3, error bars represent one standard deviation of the mean); (C and D) typical electrochemical impedance spectra of the electromechanically active capacitive laminates (5 mVRMS signal amplitude); (E) typical cyclic voltammetry of the capacitive laminates (triangular actuation signal using 800 mV/s scan speed that corresponds to the 0.1 Hz points in B). Grey lines on the cyclic voltammograms are for comparison and show response from a potential faulty actuator (essentially a resistor) that would closely follow the Ohm&#8217;s law. <a href="https://www.jove.com/files/ftp_upload/61216/61216fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61216/61216fig5.jpg" /> 
Figure 5: Spin-drying during membrane preparation. (A) schematics of the setup (B) image of the setup with a frame with reinforcement attached. During spin drying, the centrifugal force directs the residual solvent in the membrane layer towards the edge of the frame. This can be beneficial for accelerating the drying process. However, in case of completely wet membranes, this could result in the loss of active material (polymer and ionic liquid) and should therefore be avoided. <a href="https://www.jove.com/files/ftp_upload/61216/61216fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators at JoVE.com

Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators

This article describes a fast and simple manufacturing process of ionic electromechanically active composite materials for actuators in biomedical, biomimetic and soft robotics applications. The key fabrication steps, their importance for the actuators&#39; final properties, and some of the main characterization techniques are described in detail.

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, ...

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