Deformable Ionic Polymer Artificial Mechanotransducer with an

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Applications of Polymer, Composite, and Coating Materials

A Deformable Ionic Polymer Artificial Mechanotransducer with an Interpenetrating Nanofibrillar Network So Young Kim, Yongchan Kim, Changhyeon Cho, Hanbin Choi, Han Wool Park, Dayoon Lee, Euna Heo, Sangsik Park, Hojin Lee, and Do Hwan Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10499 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Applied Materials & Interfaces

A Deformable Ionic Polymer Artificial Mechanotransducer with an Interpenetrating Nanofibrillar Network So Young Kim, †, ₸ Yongchan Kim, ‡, ₸ Changhyeon Cho, § Hanbin Choi, † Han Wool Park, † Dayoon Lee, § Euna Heo, ‡ Sangsik Park, † Hojin Lee, ‡, §, * and Do Hwan Kim†, * †

Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea

‡

School of Electronic Engineering, Soongsil University, Seoul 06978, Korea

§

Department of ICMC Convergence Technology, Soongsil University, Seoul 06978, Korea

KEYWORDS: ionic polymer artificial mechanotransducer, ionic interpenetrating nanofibrillar network, blocking force, wide bandwidth, soft actuator, soft haptic interface

ABSTRACT We demonstrate an ionic polymer artificial mechanotransducer (i-PAM) capable of simultaneously yielding an efficient wide bandwidth and a blocking force to maximize human tactile recognition in soft tactile feedback. The unique methodology in the i-PAM relies on an ionic interpenetrating nanofibrillar network (i-INN) that is formed at the interface of polyurethane

(i-TPU)

nanofibrillar

matrix

with

an

ⅰ) an ionic thermoplastic

ionic

liquid

of

1-ethyl-3-

methylimidazolium:bis(trifluoromethylsulfonyl) imide ([EMIM]+[TFSI]−) and ⅱ) ionic poly(3,4ethylenedioxythiophene):poly(styrenesulfonic

acid)

(PEDOT:PSS)

conducting

polymer

electrodes with dimethyl sulfoxide (DMSO) and [EMIM]+[TFSI]− as additives. The i-PAM-based actuator with the ionic PEDOT:PSS exhibits stable operation up to 200 Hz at low voltage as well as blocking force of 0.4 mN, which can be potentially adapted to soft tactile feedback. Furthermore, on the basis of this fast i-PAM, we realized alphabet tactile rendering by using a 3×3 i-PAM array stimulated by a DC input of 2 V. We believe that our proposed approach can provide a rational guide to human-machine soft haptic interface.

ACS Paragon Plus Environment

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1. Introduction With the evolution of human-interactive soft electronic devices, soft haptics such as haptic biological psychology, haptic rendering, and haptic interfaces are attracting attention. In particular, soft haptic feedback,1-5 one of the haptic interface applications, requires soft actuators capable of stimulating human receptors to artificially serve various tactile sensations or feelings. Recently, an ionic electroactive polymer (i-EAP) actuator is spotlighted as a representative candidate for soft actuators because it provides many advantages such as lightweight, flexibility, simple process, and low driving power compared to other types of actuators.6-8 The i-EAP actuator typically consists of a well-defined ionic polymer layer embedded between two electrodes. The main mechanism of the i-EAP actuator is the formation of an electrical double layer (EDL), where the anions and cations in the ionic polymer film migrate to the interface of each electrode under an electric field. Then, the size discrepancy between the cations and anions leads to an effective bending of the actuator.9 Typically, key characteristics in implementing high-performance i-EAP actuators include displacement, blocking force, efficiency, cycle life, and bandwidth. In particular, imparting userdefined optimal characteristics to the i-EAP actuator is a more effective strategy in making it adaptive to target applications. To this end, a high blocking force and large displacement must be achieved for applications in soft robotics and artificial muscles.10 This is because the i-EAP actuator in soft tactile feedback plays a role in serving a variety of stimuli that can be perceived by mechanoreceptors in human skin through different blocking forces and operating frequency ranges. In this regard, among the mechanoreceptors in human skin, Pacinian corpuscles are considered to be the most important in tactile feedback because they play a role in sensing vibration at high frequencies. The vibration, one of the tactile sensations such as pressure, shear, torsion, vibration,


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and twisting, is easy to integrate into various systems and is widely used in commercialized products such as mobile phones and alarm bells.11 It is also known that they are more expressive and can provide a wider range of tactile information by altering the amplitude, frequency, rhythm, and envelope of the vibration.12 However, to stimulate the Pacinian corpuscles, driving at a high bandwidth in the range of 70 Hz to 1000 Hz is required.13-14 Recently, numerous attempts were made to produce the i-EAP actuators adaptive to soft tactile feedback. Park et al. demonstrated the i-EAP actuator with a fast switching time of tens of milliseconds and a blocking force of 0.3 mN under a bias voltage of 1.5 V on the basis of singleion conducting block copolymers.6 Oh et al. developed a three-dimensional (3D) graphene/carbon nanotube/nickel heteronanostructure-based actuator with a blocking force of 5.32 mN under a bias voltage of 2 V. Both approaches yielded high blocking forces over a force threshold in the Pacinian corpuscles, but suffered from achieving a high bandwidth.15 In resolving this issue, Chen et al. recently reported on a graphdiyne-based electrochemical actuator that can respond to frequencies from 0.1 to 30 Hz. This actuator showed a high electromechanical transduction efficiency of 6.03% and a blocking force of 3.37 mN.8 Furthermore, through microelectromechanical systems (MEMS), Ali Maziz et al. reported on an ionic microactuator based on a conducting polymer capable of driving up to the kHz range, but this actuator exhibited a very low blocking force of 1 µN.16 Despite these successful attempts at creating high-performance i-EAP actuators, all methodologies developed to date may not be ultimate solutions for realizing soft tactile feedback that needs a reasonable blocking force coupled with a wide bandwidth of over 100 Hz. In this regard, alternative strategy that takes full advantage of ionic actuator systems has been highly sought after. Typically, it can be noted that the bandwidth of the i-EAP actuator is dependent on the film dimensions of the ionic active polymer, electrical conductivity of the electrodes, and


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ionic conductivity of the ionic polymer, as well as the mechanical properties of each component in the i-EAP actuator. Among the versatile factors, the electrical conductivity of the electrodes should be firstly considered as the most important factor in realizing a wide bandwidth of the iEAP actuator. To this end, metal electrodes with superior electrical conductivity might be a great candidate, but they suffer from implementing actuation reliability owing to significant differences in mechanical properties between the metal electrodes and the ionic electroactive polymer layer. Therefore, recently, great attention has been paid to non-metallic electrodes based on carbon materials or conjugated polymers in order to achieve a wide bandwidth, i.e., ultrahigh bending speed. In particular, among conjugated polymers, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) has attracted much interest owing to its high electrical conductivity, excellent mechanical property, and dispersion ability in water as compared to other conjugated polymers or nanocarbons.17 Therefore, the utilization of the PEDOT:PSS electrode for realizing high-performance i-EAP actuator that can be adaptive to soft tactile feedback would have a much stronger impact on highly conductive soft electrodes than any other presently available non-metallic electrodes. Furthermore, a great benefit of the PEDOT:PSS relies on either blending additives such as ethylene glycol (EG), dimethyl sulfoxide (DMSO), and polyethyleneglycol (PEG) with the PEDOT:PSS or hybridizing with highly conductive materials such as metal nanowires, nanosheets, and nanodots. This can serve as an optimal microstructure at the interface of the PEDOT:PSS and the ionic active polymer.7-18 However, despite these successful attempts to improve the properties of the PEDOT:PSS electrodes, the direct utilization of such a PEDOT:PSSbased methodology in the i-EAP actuator is not an optimal solution for developing the i-EAP actuator with high blocking force and wide bandwidth, unless considerable care is taken to select


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the ionic polymers and interfacial properties of the PEDOT:PSS electrodes and the ionic electroactive polymers. Herein, we report on the first ionic polymer artificial mechanotransducer (i-PAM) capable of simultaneously yielding an efficient blocking force and a wide bandwidth for soft tactile sensation. This can be successfully implemented through an ionic interpenetrating nanofibrillar network (iINN) that is formed at the interface of i) an ionic thermoplastic polyurethane (i-TPU) nanofibrillar matrix with an ionic liquid of [EMIM]+[TFSI]− and ii) highly conductive ionic PEDOT:PSS electrodes associated with DMSO and [EMIM]+[TFSI]− as additives. This approach enables the iPAM to exhibit a wide bandwidth of 200 Hz as well as an efficient strain increase of 0.65% and blocking force of 0.4 mN, which is triggered by the enhanced interfacial area at the contact of the i-TPU and the ionic PEDOT:PSS layer via the i-INN topology. As a result, we could demonstrate i-PAM-based soft gripper that can be operated under a low voltage of 2 V. This shows the possibility of fine manipulation, which should be strongly required in the field of surgical robots. In addition, we could eventually develop high-performance i-PAM based actuator that operates stably up to a frequency of 200 Hz, which is strongly required to soft tactile feedback. On the basis of this fast i-PAM, we realized alphabet tactile rendering by using a 3×3 i-PAM array stimulated by a DC input of 2 V. We believe that our proposed methodology can provide a rational guide to human-machine soft haptic interfaces. 2. Results and Discussion In human skin, a high cognition ability to sense external stimuli comes from the phenomenon of ionic mechanotransduction that constitutes various biological processes, particularly the senses of touch and pressure. The main mechanoreceptors initially detect a broad range of tactile stimuli


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from the environment. In this regard, a more effective strategy is to impart human touch capability to artificial mechanotransducers, since an understanding of the main factors that can affect the sensory characteristics of human skin is crucial. Biologically, touch sensation in human skin is based on the activation of four mechanoreceptors. When the skin is energized, the skin can be deformed, and then the mechanoreceptors of the skin convert mechanical energy into action potentials, resulting in sensory transmission.13-14 In mechanoreceptors, Ruffini (0–100 Hz) and Merkel (0.4–100 Hz) cells respond to pressure that can produce a continuous signal in response to a sustained stimulus. On the other hand, Meissner (10–200 Hz) and Pacinian (70–1000 Hz) cells respond more efficiently to a derivative of force and vibration.13-14 In particular, since four mechanoreceptors can perceive various stimuli through wide frequency ranges and blocking forces, the proposed i-PAM based actuator should be capable of operating over wide frequency ranges and providing various levels of blocking force for soft tactile feedback. Figure 1a shows a schematic of the i-PAM based actuator with i-INN, which consists of a capacitive-type device with the i-TPU active polymer and the ionic PEDOT:PSS electrodes in order to achieve efficient soft tactile feedback. Further, Figure 1a describes the molecular components of the i-TPU used as an ionic polymer for the i-PAM, which was prepared by the non-covalent association of 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)-imide ([EMIM]+[TFSI]− cation-anion pairs) loaded in TPU. Figure 1b shows free-standing i-TPU film with high transparency (greater than 90% transmittance) in the visible spectrum (@ 550 nm) and high stretchability (Figure S1). In particular, the atomic force microscopy (AFM) image directly reveals that a well-defined mesoporous nanofibrillar network is formed into the i-TPU film. This can potentially induce the interpenetration of the PEDOT:PSS chains into the i-TPU during forming the electrode via solution


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process. In general, the bandwidth of the i-EAP based actuator is greatly influenced by the ionic conductivity of the ionic active layer.19 In this regard, we developed an i-PAM based actuator capable of yielding a wide bandwidth by designing the i-TPU film with ionic liquid [EMIM]+[TFSI]− of 80 wt% without any phase separation, resulting in high ionic conductivity and capacitance (Figures. 1C, S2, and S3). In order to manufacture the i-PAM with the i-INN, soft electrodes were finally formed by spraying PEDOT:PSS solution on top of optimized i-TPU film with an ionic liquid of 80 wt%. This is because the underlying layer of the i-TPU film with mesoporous nanofibrillar topology is sensitive to subsequent solution processing to which the successive conductive layers are subjected for realization of the i-INN with PEDOT:PSS.19 This implies that considerable care should be selectively taken to form a well-defined interpenetrating topology for allowing a wide bandwidth in the i-PAM, although conventional i-EAP actuators have high ionic conductivity and high electrical conductivity of the electrodes. To this end, we formed ionic PEDOT:PSS soft electrodes onto the i-TPU film via spray-coating of PEDOT:PSS solution associated with DMSO and [EMIM]+[TFSI]− as additives (refer this to ionic PEDOT:PSS). At the same time, besides PEDOT:PSS solution with DMSO and ionic liquid, three different PEDOT:PSS solutions were prepared as comparison groups: neat PEDOT:PSS, PEDOT:PSS mixed with DMSO (DMSO-PEDOT:PSS), and PEDOT:PSS mixed with [EMIM]+[TFSI]− (ILPEDOT:PSS). This was done in order to demonstrate the synergistic effects of additives on the mechanical and electrical characteristics of ionic PEDOT:PSS electrodes formed on the i-TPU film, and to find an optimal i-INN topology for high-performance i-PAM based actuators. The resulting ionic PEDOT:PSS electrode (formed from PEDOT:PSS solution blended with DMSO and ionic liquid via spray-coating) shows the lowest sheet resistance among the four types of PEDOT:PSS electrodes (Figure S4). Typically, a decrease in sheet resistance by solvent additives


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such as DMSO and ethylene glycol (EG) results from the enhanced molecular order of the PEDOT segments that can be induced by configuration changes, thereby leading to a highly conductive pathway in the PEDOT:PSS film.21 However, unlike sheet resistance, no noticeable change was observed in AFM images of the corresponding PEDOT:PSS films spray-coated at the surface of the i-TPU regardless of the types of PEDOT:PSS electrodes (Figure S5). Even though the ionic PEDOT:PSS and the corresponding neat PEDOT:PSS did not show any noticeable differences in the plane-view morphology obtained by AFM, an introduction of the ionic liquid and DMSO into the PEDOT:PSS solution resulted in changes in the microstructure and compatibility at the interface of the i-TPU and the PEDOT:PSS. The signal distribution of the elements (C8H7SO3− ion as a marker for identifying the PEDOT:PSS component) as determined by time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis upon etching was different for neat PEDOT:PSS/i-TPU and ionic PEDOT:PSS/i-TPU (Figure 1d).22 We conjecture that the ionically swollen phase of the underlying layer of the i-TPU film during spray-coating using ionic PEDOT:PSS solution (containing DMSO as a solvent to the i-TPU and ionic liquid as a plasticizer to PEDOT:PSS) enabled the PEDOT:PSS chains to effectively interpenetrate into the i-TPU, resulting in the formation of well-defined i-INN. In addition, the ionic PEDOT:PSS solution exhibited a lower contact angle (41 degree) compared to the neat one when each PEDOT:PSS solution was dropped onto the i-TPU film. This indicates better interfacial compatibility of ionic PEDOT:PSS solution with the i-TPU (Figures 1e, S6, and Table S1). This is because the underlying layer of the i-TPU film having ionic liquid of [EMIM]+[TFSI]− might be more friendly to the ionic PEDOT:PSS solution with an identical ionic liquid, which results in a contact angle that is 5 degree smaller than that of the neat PEDOT:PSS solution. Based on the detailed results above, we can observe a more effective i-INN interfacial microstructure between


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the ionic PEDOT:PSS and the i-TPU. Figure 1f depicts well-defined i-INN for high-performance i-PAM, which can be formed at the interface of the i-TPU mesoporous nanofibrillar matrix with an ionic liquid of [EMIM]+[TFSI]− and highly conductive ionic PEDOT:PSS electrodes associated with DMSO and [EMIM]+[TFSI]− as additives. In particular, it can be expected that the i-INN might be attributed to the high capacitance through an effectively larger EDL formation that comes from the enhanced interfacial area, and the corresponding high blocking forces, wide bandwidth as well as high displacement. It is well known that the modulus of soft electrodes plays a very important role in mechanical properties such as the blocking force and displacement of the i-EAP actuator. In general, the lower the modulus value, the larger strain is made, while the larger the modulus value, the greater the blocking force can be induced.19 Figure 2a shows the relative modulus of the ionic PEDOT:PSS electrode and the three comparison groups (neat PEDOT:PSS, DMSO-PEDOT:PSS, and ILPEDOT:PSS) as measured by nano-indentation analysis. The ionic PEDOT:PSS film exhibited a relatively low modulus of 18 GPa compared to the neat PEDOT:PSS with 28 GPa. This might be explained by the plasticizer effect of the ionic liquid despite the enhanced crystallinity that can be induced by DMSO. Accordingly, we can expect that the i-PAM based on ionic PEDOT:PSS might exhibit a larger strain than the neat PEDOT:PSS. Further, by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we investigated the electrochemical responses of ionic PEDOT:PSS electrodes, which can influence the bending actuation of the i-PAM based actuators. In particular, the electrical conductivity and capacitance of the soft electrodes influence the migration and accumulation of mobile ions at the interface, resulting in bending deformations directly related to the expansion and contraction in both the electrode and the ionic polymer.15 Therefore, we conducted CV tests depending on the types of PEDOT:PSS electrodes at a scan rate


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of 100 mVs-1 in potential windows from -1.0 V to +1.0 V. Figure 2b shows that the CV curves for all types of i-PAMs display rectangular-like shapes, indicating ideal electrical double-layer capacitor (EDLC) behavior. In addition, even at increasing scan rates in the range of 25–100 mVs1

, the shape of the CV curves still maintains quasi-rectangular without any remarkable peaks. This

implies an excellent rate capability of the i-PAM based capacitors (Figure S7).23 The speciﬁc capacitances calculated from the integrated areas of the CV curves are plotted as a function of the scan rates in Figure 2c. Surprisingly, the i-PAM with ionic PEDOT:PSS yielded a higher specific capacitance than the other comparison groups irrespective of the scan rate. In particular, at a scan rate of 25 mVs-1, the highest specific capacitance of 246 Fg-1 could be achieved, which was 1.8 times higher than that of neat PEDOT:PSS (136 Fg-1). This is because the well-defined i-INN that was induced by ionic PEDOT:PSS formed on the i-TPU film could serve an enhanced interfacial area, thus improving the specific capacitance through effective EDL formation. Further, Figure 2d shows that Nyquist plots of the i-PAM have semicircles at higher frequencies but straight lines at lower frequencies (Figure S8). The absence of semicircles in the highfrequency region implies that the charge transfer resistance was negligibly small owing to the fast kinetics involved in the ionic PEDOT:PSS electrodes. Furthermore, the ionic conductivities of the i-PAM can be calculated as σ = d/RS, where R and S are the resistance and surface area of the device determined from the Nyquist plot, respectively. From the standard formulation, the i-PAM based on ionic PEDOT:PSS exhibited an ionic conductivity of 8 mS/cm, which was 1.4 times higher than that of neat PEDOT:PSS (5.81 mS/cm). The superior ionic conductivity of ionic PEDOT:PSS can be also ascribed to well-defined i-INN, with effective and rapid ion diffusion through good interfacial compatibility between the electrodes and the i-TPU, as well as high electrical conductivity.


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The actuation performance of the i-PAM was examined by forming electrodes on the i-TPU film with each PEDOT:PSS solution (neat PEDOT:PSS, DMSO-PEDOT:PSS, IL-PEDOT:PSS, and ionic PEDOT:PSS) via a spray-coating method. Figure 3a depicts a schematic of the actuating characteristics of the i-PAM composed of the i-TPU and the ionic PEDOT:PSS electrode. Basically, [EMIM]+ cations in the i-TPU move to the cathode and [TFSI] − anions migrate to the anode when a voltage between the two electrodes is applied. The formation of the EDL at the interface between the i-TPU and the ionic PEDOT:PSS electrode shrinks the anode layer while swelling the cathode, which is owing to the size and diffusivity discrepancy of the ionic liquid of [EMIM]+[TFSI]−.9 In order to demonstrate the possibility of various soft tactile feedback types when our i-PAM based actuator operates at low voltage and high frequency, we monitored the bending motion below 2 V in the frequency range of 0.1–200 Hz (Figure S9). The performance of the i-PAM based actuator was evaluated at a constant input bias in order to confirm its influence on the bending motion of the actuators. Figure 3a displays optical images of the i-PAM based actuators with their corresponding degrees of bending depending on the types of PEDOT:PSS electrodes under a DC voltage of ± 2 V. All of the i-PAM based actuators yielded large and symmetrical bending deformation according to the positive/negative bias conditions. However, as expected, the i-PAM based actuator with ionic PEDOT:PSS electrodes exhibited a best bending displacement of 5.1 mm, which was 2 times better than that of the neat PEDOT:PSS-based actuator (2.5 mm), 1.6 times higher than that of the DMSO-PEDOT:PSS-based actuator (3.1 mm), and 1.2 times higher than that of the IL-PEDOT:PSS-based actuator (4.2 mm). We conjecture that this is because a well-defined i-INN that can be induced by ionic PEDOT:PSS formed on i-TPU film can provide an enhanced specific capacitance through an effectively larger EDL formation. This results from an increase in the interfacial area of the i-TPU and the ionic PEDOT:PSS.


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Figure 3b shows the time-domain bending responses of the i-PAM based actuators when an alternating square-wave voltage of ± 2 V is applied at a frequency of 20 mHz. The measurement results reveal that the i-PAM based actuator using the ionic PEDOT:PSS electrode shows enhancement in the displacement and response time of bending motions in comparison to other types of i-PAM based actuators using neat, DMSO-, and IL-PEDOT:PSS electrodes. Further, it can be noted that back-relaxation phenomenon was not observed in all electrode conditions. This was caused by the asymmetric diffusion characteristics of the cations and anions, and was therefore a major obstacle to realizing actuator stability.24 Generally, when an actuator displays fast restoration from alternating positive/negative potentials, it suggests the occurrence of fast ion migration under the given stimuli. Rapid ion diffusion along well-connected ionic channels can originate from a well-defined i-INN, which enables a rapid actuation response with an extended actuation time of the i-PAM based actuator using the ionic PEDOT:PSS electrode. Figure 3c shows the strain-frequency dependence of the i-PAM with different types of PEDOT:PSS electrodes as examined at an input bias of ± 2 V. The strain of the actuators tended to decrease with increasing frequency for all types of electrodes, which might be owing to the reduced available time for ion diffusion between the i-TPU and the PEDOT:PSS electrodes. The i-PAM based actuator with ionic PEDOT:PSS yielded the maximum strain at all frequency ranges. This is because of the synergistic effect between superior ionic conductivity and enhanced interfacial area by the formation of a well-defined i-INN. Although the strain became smaller as the frequency increased, the i-PAM based actuator with ionic PEDOT:PSS electrode still showed a change in the strain (~ 0.01%) even at a frequency of 200 Hz. (More detailed information is provided in Figures S10 and S11.) Further, as shown in Figure 3d, we confirmed that the i-PAM based actuator with ionic PEDOT:PSS electrode could maintain durable actuation performance over 30,000 cycles


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without any apparent decreases in the bending displacement. In addition to its large bending strain and long-term durability, the i-PAM based actuator with ionic PEDOT:PSS electrode can also provide an effective blocking force adaptive to efficient soft tactile feedback. This is because the ionic mechanotransducer in soft tactile feedback can play an important role in serving a variety of stimuli that can be perceived by mechanoreceptors in human skin through various operating frequency ranges and blocking forces. Figure 3e shows that the i-PAM based actuator with ionic PEDOT:PSS can yield a blocking force of 0.4 mN when a DC bias of ± 2 V is applied. This is 1.5 times that of neat PEDOT:PSS (0.25 mN) under the same bias condition (Figure S12). While the blocking forces of previous i-EAP actuators were implemented in the range of 0.05–5 Hz, we observed a reasonable blocking force for the case of the i-PAM based actuator even at high operating frequencies (>10 Hz). This might be derived from the i-INN topology in our work. As a result, successful operation of the i-PAM based actuator with a wide bandwidth at low voltage can enable its use as a soft tactile device. Figure 3f clearly shows a comparison of the iPAM based actuator using ionic PEDOT:PSS against other types of actuators that have been demonstrated by many research groups6, 15, 18, 25-29 in terms of the operating frequency and bending strain (Table S2). In particular, note that the i-PAM based actuator that is operated even in a highfrequency range (10–200 Hz) can yield a significant progress as compared with previous ones. We conjecture that this prominent capability in the i-PAM based actuator is originally owing to a welldefined i-INN between the i-TPU and the ionic PEDOT:PSS, which can potentially pave the way for more advanced soft surgical robots, artificial muscles, and soft tactile feedback. Based on high-performance of the i-PAM based actuators with ionic PEDOT:PSS electrodes, we could demonstrate the first soft gripper capable of controlling a minute force effectively, which should be of importance for intelligent surgical robots.30 As shown in Figure 4a, the soft gripper


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was fabricated using three i-PAMs as the fingers that can discreetly and quickly grab and release objects when voltage is applied. In particular, the i-PAM based soft gripper can control the blocking force depending on the voltage level (0–2 V). A maximum value of 0.32 mN was obtained at DC + 2 V (Figure 4b). It is well-known that lifting waste such as tumor or cancer tissues without damaging the human body during surgery is one of the main concerns for surgical robots. Accordingly, for this sensitive and sophisticated application, we introduced a ring-shaped tissue model as an object to grip with an elasticity similar to human tissue. In general, the elasticity of a human tissue matrix is very small, and it can be easily deformed by a simple tweezer grip, as shown in Figure 4c. Unlike a conventional tweezer, we observed that by the i-PAM based soft gripper, the ring-shaped object could be gripped and released without any external deformation or damage. This is because the i-PAM gripper yields a minute blocking force that prevents artificial tissue from being deformed as compared to a conventional tweezer showing a minimum blocking force of 8 mN. Thus, the soft gripper proposed by us might be applied to a surgical robot that needs to move very small objects or lift very sensitive objects such as blood vessels in the narrow gaps of the human body. In addition, to demonstrate capability of the i-PAM adaptive to soft haptic feedback, we eventually developed an i-PAM tactile array as shown in Figure 5a. Typically, numerous textures and stimuli can be expressed by simultaneous change of displacement and blocking force according to operating frequency ranges in soft haptic feedback. In particular, note that highfrequency rendering with small amplitude can express a soft feeling to human, whereas lowfrequency rendering with relatively large amplitude can express a rough feeling.31 To this end, we investigated specific actuation characteristics including the change of displacement and the blocking force of the i-PAM tactile array according to applied various frequency conditions from


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0 to 100 Hz, which is crucial for soft haptic feedback. Firstly, we demonstrated tactile rendering of the alphabet with a 3×3 i-PAM array by Arduino device that can be utilized for position mapping (Figure S13). When voltage is applied to the alphabetic mapping, human hand can be stimulated by changes of displacement at the specific position in the manner of braille. Figure 5b shows the alphabet representation (“A” to “D”), which could be visually expressed by the displacement variation of this i-PAM array under the same frequency conditions. Also, inset graph presents the mapping results associated with the blocking force corresponding to each alphabetic character in the i-PAM array system. (The experimental results for more alphabet representations are shown in Figure S14.) Furthermore, to manipulate the i-PAM array more suitable for soft haptic feedback, we designed the 4×5 i-PAM array with four different input signal conditions. This is because blocking force and displacement in each i-PAM pixel can be controlled by changing the driving frequencies of the i-PAM array (Figure 5C). According to applied frequency conditions (0.1, 5, 50, and 100 Hz), the i-PAM array exhibited an effective blocking force mapping of 0.46, 0.31, 0.09, and 0.04 mN, respectively. Moreover, the corresponding displacement was in the range of 4.81, 1.49, 0.5, and 0.03 mm, respectively. (more detailed information on various conditions is shown in Figure S15.) This means that the i-PAM array proposed by us can effectively display the blocking force and displacement that are generated over a wide frequency range from 0 to 100 Hz, which potentially shows novel soft haptic feedback that expresses various textures and stimuli. 3. Conclusion In summary, we successfully developed the first ionic polymer artificial mechanotransducer (iPAM) capable of simultaneously yielding an efficient blocking force and wide bandwidth for efficient soft tactile sensation. The methodology relies on inducing a well-defined ionic interpenetrating nanofibrillar network (i-INN) topology of i-PAM, which is formed at the interface


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of the ionic polymer matrix and the ionic PEDOT:PSS electrode layer with hybrid additives. As a result, our i-PAM based actuator exhibited a wide bandwidth of 200 Hz as well as a blocking force of 0.4 mN. Also, we demonstrated successful applications of a soft gripper for surgical robots and an alphabet tactile rendering system for soft tactile feedback with the i-PAM based actuator. Ultimately, we believe that this wide-bandwidth and efficient blocking force that are implemented in the i-PAM will be an effective way to create human-interactive soft devices capable of directly recognizing the human sensation interface and even monitoring visible changes in real time. 4. Experimental Section Fabrication of the i-TPU Film: TPU chips (KA-480) were purchased from Kolon Industries, Inc. [EMIM]+[TFSI]− and N,N-Dimethylformamide (DMF) solvent were supplied by C-TRI and Sigma-Aldrich, respectively. Firstly, the TPU chips were dissolved into DMF (mass ratio 1:3), and the solution was stirred at 80 ºC for 1 hr. Then, 80 wt% [EMIM]+[TFSI]− on the weight of the TPU was added and the mixed ionic TPU (i-TPU) solution was stirred at 80 ºC for 14 hr. The i-TPU films were fabricated using spin-coating of well-mixed i-TPU solution on top of PDMS film as a substrate. The i-TPU films were heat treated at 120 ºC for 48 hr under optimized conditions (starting from 40 °C with an increase of 10 °C/hr up to 100 °C) to fully remove a residual solvent. Fabrication of the i-PAM based actuator using ionic PEDOT:PSS electrode: The i-PAM based actuators were fabricated using spray coating of ionic PEDOT:PSS solution on both sides of the iTPU film (~ 80 µm). Firstly, the ionic PEDOT:PSS solution was prepared by dissolving PEDOT:PSS of 5 mg (PH 1000, Ossila Ltd.) into distilled water of 15 mg with 5 wt% dimethyl sulfoxide (DMSO) (Junsei Chemical Co., Ltd.) and 1.5 wt% 1-Ethyl-3-methyl-imidazolium:bis (trifluoromethylsulfonyl) imide ([EMIM]+[TFSI]−) (C-TRI) as additives. The mixed solution was stirred for 3 hr and spray-coated onto both sides of the i-TPU film at 120 ºC, followed by air-


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condition annealing equipment (47900 Furnace, Barnstead Thermolyne Corp.) at 160 ºC. Then, 1µm-thick ionic PEDOT:PSS electrodes were formed. Finally, after careful cutting of the i-PAM film, the ionic mechanotransducer shape was examined using an Olympus microscope equipped with a charge-coupled-device (CCD) camera (IMT i-solution). Morphological features and mechanical properties of the i-PAM based actuator: Atomic force microscope (AFM) images were taken with a Digital Instruments Multimode-N3-AM under ambient conditions. The AFM was operated in tapping mode by using silicon probes with a force constant of 40 Nm-1. To investigate the interfacial characteristics between the ionic PEDOT:PSS electrode and the i-TPU, we conducted time-of-flight secondary ion mass spectrometry (TOFSIMS) depth-profiling studies. TOF-SIMS is used to provide in-depth chemical information of diverse materials by sputtering away the samples using an ion source. The analysis was performed upon etching at the National Center for Inter-University Research Facilities (NCIRF) in Korea. Negative ion spectra were obtained from the basement membrane side of the samples using a ToF SIMS-5 (ION-ToF, Germany) instrument with a 30-keV Bi3+ analysis beam, Ar+ 1500 10-keV sputter gun mounted at 45 º to the sample. The area of analysis was 500 × 500 μm2. The contact angle was measured with a Phoenix 150 (SEO). The contact angles in this study were derived through the average values for five samples according to types of electrodes (Table S1). Nano indentation tester (Micro Material), with a load resolution of 1 nN and a displacement resolution of 0.0003 nm, is used in these experiments. Electrochemical and electrical characteristics of the i-PAM based actuator: Cyclic voltammetry analyses were conducted with a multichannel potentiostat (VSP, BioLogic) upon varying the scan rates (25 mV, 50 mV, and 100 mV with a potential window of - 1 V to + 1 V). Electrochemical impedance spectroscopy (EIS) was performed at room temperature using an


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electrochemical analyzer PGSTAT302N (Metrohm Autolab) in a frequency range of 0.1 Hz–100 kHz with a 25-mV AC signal. All of the impedance spectra were fitted using appropriate equivalent circuit models built in NOVA software (Metrohm Autolab) to evaluate the series resistance (Rs) of the devices. An Agilent E4980A precision LCR meter was used for capacitance measurements at room temperature. Electromechanical characteristics of the i-PAM based actuator: The measurement system used to analyze the i-PAM based actuator performance was operated at voltage with a sinusoidal and square waveform that is produced using a function generator (33500B, Agilent Technologies, Inc.). The i-PAM was fixed to a measuring holder with two separate copper electrodes, and was analyzed for actual motion, displacement, and blocking force through a connection to a charge-coupleddevice camera (CCD) (IMT i-solution), laser sensor (optoNCDT1700-10, Micro-Epsilon), force sensor (programmable xy and z axis stage at 0.1 µm resolution, FS1M-0.1N, NanoControl), DC power supply, function generator, oscilloscope (DPO3052, Tektronix), and PC. The detailed measurement system information is described in Figure S9. In addition, the measured displacement was calculated with the bending strain according to the following equation:

ε=

2hδ L + δ2 2

(1)

where ε is the bending strain, δ is the actuator displacement, h is the thickness of the actuator, and L is the length of the actuator. In this study, the capacitance-type displacement sensor is used as a force sensor (FS1M-0.1N, NanoControl). The force sensor is calibrated using a ‘weight’, and the output of the sensor amplifier and the voltage of the capacitance sensor are measured using a 3-point reference ‘weight’, and the sensitivity is determined by linear approximation. Since the


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force sensor used with an electrical i-PAM based actuator is an important thing where 0 V is important at no load, the positions and conditions of the mechanical instrument reset the 0 V offset to an estimate before measurement. This 0V offset calibration can be adjusted with an "offset" trimmer located on the force sensor. The blocking force results in the manuscript are the offset measured with a minimum of three-actuator devices and are the mean value measured within a 1% error range. Design and measurement of the i-PAM gripper and alphabet-rendering tactile array for soft tactile feedback: For the i-PAM based soft gripper experiments, a three-fingered gripper was fabricated by attaching three i-PAM elements to a copper plate using silver paste. The copper plate was used as a common electrode of the three i-PAM arms, and the opposite side of the i-PAM was connected to a function generator (33500B, Agilent Technologies, Inc.) using 0.05-mm copper wire. As shown in Figure S9b, the force measurement probe tip is set at the center position of the i-PAM sample. The i-PAM based actuator, which is constrained by the force sensor, generates the blocking force by the DC voltage conditions and determines the grip-performance. Figure S13 shows the i-PAM based tactile array driving system, which represents a way of rendering the alphabet by each i-PAM pixel. In the array system, we manipulated the i-PAM using an Arduino with a matrix code that can apply an input information at the desired position. Using the output voltage of the Arduino, the exact voltage regulated by the voltage-controlled amplifier was used to represent possible operations in the actuator array. The controlled voltage was obtained from Equation (S2).


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. More detailed information about additional characterizations of Figures and tables containing representative transmittance, electrochemical impedance spectroscopy (EIS), and C-f characteristic of free-standing i-TPU film with ionic concentration; electrical, physical, mechanical, and electrochemical properties of the i-PAM based actuators; device measurement system; photographs of array designs. (PDF) The movement of i-PAM tactile rendering of the alphabet ("A to "J") by stimulating signal input of 0.5 Hz, 2VPP (AVI)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (D.H.K) *Email: [email protected] (H.L) Author Contributions ₸

S.Y.K and Y.K contributed equally to this work.

ACKNOWLEDGMENT This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (10051514)) and KDRC (Korea Display Research Corporation) support program and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF2017R1A5A1015596).

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Figure 1. Development and characterization of the i-PAM based on i-TPU and ionic PEDOT:PSS. (a) Schematic of human mechanoreceptors including frequency response ranges for touch sensation and the i-PAM based actuator, which consists of a capacitive-type device with the i-TPU active polymer and the ionic PEDOT:PSS electrodes. Inset shows molecular formula of TPU, PEDOT:PSS, DMSO, [EMIM]+ (red), and [TFSI]− (blue). (b) (Top) Illustration of free-standing iTPU film with high transparency and high stretchability, and (bottom) AFM image of nanofibrillar network in the ionic TPU. The inserted photo was taken directly by us. (c) Ionic conductivity of the i-TPU film in term of ionic liquid content. (d) Representative TOF-SIMS depth profile through both neat PEDOT:PSS/i-TPU and ionic PEDOT:PSS/i-TPU against sputter time. (e) Picture of contact angle showing interfacial compatibility effect (top: neat PEDOT:PSS, bottom: ionic PEDOT:PSS). (f) Illustration of well-defined i-INN for high-performance i-PAM with ionic PEDOT:PSS compared to neat PEDOT:PSS.


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Figure 2. Mechanical and electrochemical properties of the i-PAM film. (a) Modulus of the ionic PEDOT:PSS electrode and other three comparison groups. (b) Cyclic voltammetry (CV) curves for the i-PAM with all types of different electrodes at 100 mVs-1 scan rate. (c) Calculated specific capacitance values of electrodes at various scan rates (25, 50, and 100 mVs-1). (d) Nyquist plots of the i-PAM according to types of electrodes under 100 mV.


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Figure 3. Actuation performances of the i-PAM for soft tactile feedback. (a) Schematic diagram of the i-PAM with the i-TPU film and the ionic PEDOT:PSS electrodes during actuation, and optical images of the i-PAM based actuators with their corresponding degree of bending depending on the types of PEDOT:PSS electrodes under a DC voltage of ± 2 V. (b) Time-dependent bending response of the i-PAM based actuator when an alternating square-wave voltage of ± 2 V is applied at a frequency of 20 mHz. (c) Strain-frequency dependency of the i-PAM based actuators at ± 2 V AC sinusoidal-wave input between 0.1–200 Hz. (d) Reliability of the i-PAM under continuous operation in air condition at ± 2 V and 1 Hz. (e) Blocking force responses of the i-PAM based actuator in terms of types of PEDOT:PSS electrodes when applying ± 2 V. (f) Strain-frequency curve of the i-PAM based actuator with ionic PEDOT:PSS in comparison with that of actuators reported in literature.


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Figure 4. Application of the i-PAM to soft gripper. (a) Schematic picture of the i-PAM based soft gripper. (b) Generation of blocking force of the i-PAM gripper arms by applying DC voltage from 0 to 2 V. (c) Illustration of elasticity map in human tissue matrix and photographs showing highly sensitive force control of the i-PAM soft gripper compared to conventional tweezer during gripping and releasing the ring-shaped artificial tissue with low elasticity.


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Figure 5. Demonstration of the i-PAM based soft actuator array for soft tactile feedback. (a) (Left) Schematic picture of the i-PAM (4×5) tactile array showing selective actuation feedback in each iPAM pixel and (right) schematic diagram of the human tactile roughness felt by frequency control. (b) Photographs showing effective i-PAM (3×3) tactile rendering of the alphabet by stimulating DC input of 2 V. Inset graph shows blocking force mapping according to the i-PAM conditions. (c) Rendering map of blocking force and displacement of the i-PAM (4×5) tactile array according to various frequency conditions.


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Table of Contents


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Deformable Ionic Polymer Artificial Mechanotransducer with an

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