Article pubs.acs.org/accounts
Cite This: Acc. Chem. Res. 2019, 52, 63−72
Block Copolymer Elastomers for Stretchable Electronics Published as part of the Accounts of Chemical Research special issue “Wearable Bioelectronics: Chemistry, Materials, Devices, and Systems”. Insang You, Minsik Kong, and Unyong Jeong*
Acc. Chem. Res. 2019.52:63-72. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/16/19. For personal use only.
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea
ABSTRACT: As industrial needs for healthcare sensors, electronic skin, and flexible/stretchable displays increase, interest in stretchable materials is increasing as well. In recent years, the studies on stretchable materials have spread to various pivot components, such as electrodes, circuits, substrates, semiconductors, dielectric layers, membranes, and active nanocomposite films. The block copolymer (BC) elastomers have been playing considerable role in the development of stretchable materials. Since BCs are soft elastomers based on physical cross-links, they show differences in physical properties from normal elastomers formed with chemical cross-linking. BC elastomers does not require additional chemical cross-linking procedure, so they can be easily processed after dissolved in various solvents. Their viscoelasticity and thermoplasticity enable the BCs to become moldable and sticky. Although their unique physical properties may serve as disadvantages in some cases, they have been actively applied to create various stretchable electronic materials and their uses are expected to be enlarged more than ever. In this Account, we summarize recent successful applications of BCs for the stretchable electronic devices and discuss the possibility of further uses and the challenges to be addressed for practical uses. Studies on BC-based stretchable materials have focused initially on the fabrication process of stretchable conductors; mixing conductive fillers physically with BCs, infiltrating BCs in a conductive filler layer, and converting metal precursors into metal nanoparticles inside BCs. When conductive fillers with high aspect ratios, such as nanowires or nanosheets are used, the fillers can be infiltrated by the BCs after deposited. Since the contacts between the fillers are maintained during the infiltration process, even thin composite films possess high conductivity and stretchability. The metal precursor solution printing is suggested as a promising approach because it is compatible with traditional printing techniques without clogging the nozzles and allows high filler loading efficiency. When using a BC as a substrate, it is advisable to use a BC/PDMS double layer because of viscoelastic and thermoplastic properties of BCs. If BC/PDMS double layer is used with much thicker PDMS layer instead of viscoelastic BC alone, the double layer substrate can show a perfect elastomeric behavior, and the advantages of the BC substrate are preserved. Additionally, the use of conventional manufacturing techniques is important for commercialization of the stretchable devices. BC substrates having preformed microfibril network on their surfaces facilitate the fabrication of highresolution circuitry by directly depositing metals through a mask on the substrate. Recent successes of fabricating stretchable organic transistors were obtained based on in situ phase separation of polymer semiconductors to form nanofibril bundles on the surface of a BC substrate. They have led to the achievement of high resolution transistor array printed in large area. BCs are expected to expand their applicability, including stretchable batteries, since they make it feasible to fabricate various hybrid nanocomposites, pore size-controlled membranes, and microstructured surfaces. However, it is necessary to secure long-term stability under heat, solvent, and UV; in addition, there is a need for the synthesis of functional BCs for use in stretchable implanted biomedical devices. elastomer substrate4 or with intrinsically stretchable electrodes formed on a stretchable substrate.5 For the on-skin electronics and implanted signal monitoring devices,6,7 the printed device on an ultrathin nonstretchable polymer substrate was
1. INTRODUCTION Development of stretchable electronic materials has been considered as an important factor in the past decade as industrial interest in stretchable electronics has grown for wearable electronics, healthcare devices, electronic skin, and stretchable display.1−3 Stretchable electronics was initiated with the waved metallic interconnections transferred onto an © 2018 American Chemical Society
Received: September 27, 2018 Published: December 26, 2018 63
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 1. (A) Chemical structure of SBS triblock copolymer. The polystyrene (PS) microdomains play role as physical cross-linkers in the polybutadiene (PB) matrix. (B) The unique characteristics of the BC elastomers (good solubility, adhesiveness, viscoelasticity, moldability).
stretchable because of the curved surface topology of the human tissues. Recent researches on stretchable electronics have been extended from conductors to semiconductors and dielectrics to fabricate stretchable sensors, transistors, and energy storage devices.8,9 Although the stretchable device components have been mostly integrated on chemically cross-linked elastomers, the use of physical elastomers has been increasing because there are unique characteristics that are hardly achieved in the chemically cross-linked elastomers.10 The physical elastomers include block copolymers,11 thermoplastic homopolymers capable of binding with fillers,12 and hydrogels with hydrogen bonds or strong ionic bonds.13 Among the physical elastomers, BCs are the most-commonly used in stretchable electronics. The typical polymer chain structure of the BC elastomers is a triblock copolymer, which has two short rigid chains at the ends and a long soft chain in the middle (Figure 1A). Polystyrene-based BCs are representative, including polystyrene-block-polybutadiene-block-polystyrene (SBS), polystyreneblock-poly(ethylene butylene)-block-polystyrene (SEBS), polystyrene-block-polyisobutylene-block-polystyrene (SIBS), and polystyrene-block-polyisoprene-block-polystyrene (SIS). Polystyrene (PS) forms rigid microdomains that are linked together
by the soft middle block chains and act as the physical crosslinking points because of its high glass transition temperature (∼100 °C). As summarized in Figure 1B, there are unique characteristics of the BC elastomers. Since they are soluble in common organic solvents, solution processes can be used for coating and printing.14 The BC elastomers are adhesive to most substrate materials (metals, polymers, Si wafer, PDMS elastomer, glass, etc.).15 They are viscoelastic, which means that they can flow and their mechanical stress can be relaxed when the strain is larger than the yield strain.16 It is possible to micromold the surface of a BC substrate by pressing a stamp on it.17 There are a variety of inexpensive commercial BCs to meet the physical properties required for stretchable electronics. High elastic modulus (E) of BC is obtained when the volume fraction (ϕm) of the microdomains is large, therefore the modulus of BC with the spherical microdomains (∼8% < ϕm < ∼13%) is relatively low, higher with the cylindrical microdomains (∼13% < ϕm < ∼ 35%), and highest with the lamellar microdomains (∼35% < ϕm < ∼50%).18 When the polymer chains of the rigid microdomains are entangled each other, the microdomains are stable without chain pull-outs under large mechanical deformation; hence, a 64
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 2. (A) Procedure to fabricate composite between Au NSs and SBS by the infiltration method. The composite is prepared on a PDMS substrate. (B) Cross-sectional TEM image of the Au NSs/SBS composite film. (C) Changes in the sheet resistance and conductivity of the composite film during repeated peeling test. (D) Resistance changes of the composite film during repeated cycle tests at various uniaxial strains (ε). Reproduced with permission from ref 21. Copyright 2017 American Chemical Society.
polyvinylpyrrolidone to hexylamine.14,20 The hydrophilic surfactants used for the synthesis of the fillers should be replaced by hydrophobic ligands to improve miscibility with a BC solution. The Ag NW/SBS composite electrode was used as an implantable heart monitoring sensor and an electric stimulation.20 Infiltrating BC in a conductive filler layer is an effective approach to achieve the high volume fraction of the fillers in the composite. The BC solution penetrates in the void space among the fillers and dries quickly during process. The BC chains make good adhesion with the filler surface because of the tackiness of the soft blocks and then the chains are immobilized after forming the microdomains. Since the volume shrinkage of the BC matrix by thermal annealing can stabilize the percolation contacts, the BC composites do not require a large thickness to achieve high conductance and good electrical stability at large deformation.21 In contrast, the chemical elastomers needs thick composite layers because the precursor liquid with a high viscosity separates the percolation contacts.22 Figure 2A shows an example of the BC infiltrating a conductive filler layer. Au NSs were stacked on a Si wafer through the floating-and-transferring process.23 When an SBS solution was spin-coated and the specimen was thermally annealed at 120 °C, the polybutadiene (PB) matrix covered the Au NS surfaces and the PS microdomains developed parallel to the Au NSs (Figure 2B). Because of the adhesive PB matrix, the conductive fillers were not delaminated from the BC matrix by repeated contacts with a sticky tape (Figure 2C). The composite conductor could maintain stable electrical conductance during repeated stretching cycles over ε = 1.0 because of the affine motion of the fillers with the polymer matrix in deformation (Figure 2D). The infiltration process has been adapted to other thermoplastic polymers, such as polyurethane24 and ethyl cellulose,25 to form Ag NW-based transparent stretchable electrodes.
BC with a large molecular weight can have good elasticity at large strains. Chemical cross-linking of the polymer chains of the rigid microdomains can further improve its elasticity. This Account focuses on the material aspects of BC elastomers as promising components for stretchable electronics, rather than focusing on a few types of stretchable devices. The subsections are divided by the roles of the BCs, which includes nanocomposite electrodes, substrates, transistors, and adhesive layers. The last section deals with the perspectives of the BCs in stretchable electronics and the challenges to be addressed.
2. BC NANOCOMPOSITES FOR STRETCHABLE ELECTRODES Stretchable conductive composites are based on electrical percolation through the conductive fillers.5 A variety of fillers have been used for the conductive BC nanocomposites, including carbon nanotubes (CNTs) and metal nanomaterials (nanowires (NWs), flakes, nanoparticles (NPs), nanosheets (NSs)). 5,14,19−21 Three methods are mainly used to manufacture BC nanocomposites: mixing conductive fillers with BC, infiltrating BC in a conductive filler layer, and chemical reduction of metal precursors embedded in a BC layer. Direct mixing of the conductive fillers in a polymer melt or in a concentrated polymer solution is generally not successful because of the large entropy loss of the polymer chains near the conductive fillers. Mixing a conductive filler suspension with a polymer solution is a better way to obtain a BC/filler mixture. One-dimensional nanostructured materials, such as CNTs and Ag NWs, have been used actively for BC nanocomposites because of the low percolation thresholds.14,19 Ag NWs were successfully mixed in a concentrated SBS solution by exchanging the ligand on the Ag NWs from 65
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 3. (A) Camera image showing immersion of an SBS electrospun fiber mat in a silver precursor solution. The fiber mat is swollen after immersion. (B) Schematic illustration of the swelling of the fiber mat by the precursor solution and chemical reduction of the precursors into Ag NPs. The resulting composite fiber mat is highly stretchable, without losing its conductivity at large strains. (C) SEM image of the composite fiber mat after elongation at ε = 1.0. Cross-sectional TEM image of a SBS/Ag composite fiber is shown as well. (D) Changes in conductivity of the composite fiber mat after the strain is released. Reproduced with permission from ref 26. Copyright 2012 Nature Publishing Group.
Metal NPs are normally not preferred as conductive fillers because they require a large volume fraction for electric percolation (≥30 vol %).5 The precursor approach is effective in terms of the filler loading efficiency. Figure 3A and B show an example.26 When an electrospun SBS fiber mat was immersed in an alcohol solution of a Ag precursor (AgCF3COO), the precursor molecules made complex with the double bonds of SBS. When the precursor was chemically reduced to form Ag NPs, the Ag NPs formed conductive 3D networks in nanoscale inside each fiber (Figure 3C). Even when the Ag coating layer on the fiber surface was completely cracked under elongation at ε = 1.0 (Figure 3C), high electrical conductivity was maintained through the internal NP network over hundreds of stretching cycles (Figure 3D). The same process was applied to other BCs containing double bonds.27 Lee et al. improved the conductive path by incorporating Ag NWs inside the SBS fibers and obtained excellent mechanical durability in repeated uniaxial tests at a large stain (ε = 1.0).28 In printed electronics, it is desirable to develop a conductive ink whose viscosity is adjustable and no aggregation of the fillers takes place. Printing the metal precursor solution meets the requirements for the ink. Figure 4A is an example of the precursor printing.29 The precursor solution penetrates in the thickness direction of the SBS film. The Ag NP network was formed in the film by the chemical reduction. It is notable that the Pt layer observed in Figure 4A was coated during the preparation of transmission electron microscopy (TEM) specimen. Stretchable circuit with a line width of 50 μm was routinely printed by nozzle printing or spray printing through a thin film mask (Figure 4B).26 Repeated precursor printing and chemical reduction resulted in highly stretchable conductive circuits with little resistance change at a large elongation (ε = 0.5), whereas one or two prints led to sensitive resistance change under mechanical strain (Figure 4C). The advantage of this sensor manufacturing process is that the sensing part and
the circuits can be integrated at the same substrate without any physical interface between the components. Figure 4D shows a resistive strain sensor used for sound recognition. When the BC film with circuitry was placed on a 3D elastomeric object and thermally annealed at 120 °C, the BC substrate made conformal contact with the object and provided good adhesion (Figure 4E). This circuit allowed the possibility of stretchable electronics in 3D objects.
3. BCS AS STRETCHABLE SUBSTRATES BC elastomer substrates not only substitute the chemically cross-linked elastomers30 but also provide novel functions. However, the remaining strain after a large deformation is a significant drawback for use as a stretchable substrate. The formation of a BC/PDMS double layer or chemical crosslinking of BCs has been proposed to attain the complete elasticity under large elongation (ε > 1.0).21 When both the SBS substrate and the PDMS substrate are treated with oxygen plasma and contacted each other, they make a double layer substrate with a strong interface.31 A study showed that when the thickness of the bottom PDMS layer was 10 times thicker than the BC layer, the double layer substrate had the complete elasticity without hysteresis and residual strain (Figure 5A).21 Strain sensors fabricated on the double layer substrate exhibited exactly the same sensing profiles over a large number of cycles (Figure 5B), indicating that the double layer is suitable as a stretchable substrate. Chemical cross-linking of the BC substrates after the required process is another effective way. SEBS grafted with maleic anhydride (SEBS-g-MA) can be cross-linked in the presence of a small amount of polyamide or polyaniline.32 Bridging the conventional device fabrication and the stretchable electronics is important to driving the stretchable electronics to the mainstream. Use of the conventional metal deposition process to obtain stretchable electrodes requires 66
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 4. (A) Schematic showing the Ag precursor printing on an SBS thin film followed by the chemical reduction to create Ag NPs. The crosssectional TEM images correspond to results obtained by printing once and five times. (B) SEM image of a conductive line pattern drawn by sprayprinting through a mask. (C) Changes in the resistance under uniaxial strains. The results are obtained from the patterns by printing once and five times. (D) Recording a piano play with a free-standing strain sensor made by the precursor printing. (E) Formation of conformal contact between the BC composite circuit and a PDMS 3D object by applying thermal annealing. Panels A, C, D, and E Reproduced with permission from ref 29. Copyright 2017 Wiley-VCH. Panel B reproduced with permission from ref 26. Copyright 2012 Nature Publishing Group.
4. BCS FOR STRETCHABLE TRANSISTORS Transistor-operated stretchable device was first demonstrated by connecting the nonstretchable transistors to stretchable interconnects.34 Very recently, intrinsically stretchable transistors have been fabricated by using the BC as a substrate, dielectric, semiconductor composite, and packaging material.24,35 The first intrinsically stretchable polymer transistor was fabricated by depositing electrospun poly(3-hexylthiophene) (P3HT) semiconductor nanofibers on an electrospun SBS fiber mat (Figure 6A).36 The electrodes were made of the Au NS/SBS composite and the ion gel was used as the dielectric layer. Owing to the UV cross-linking of the ion gel through a mask, an array of the stretchable transistors could be fabricated (Figure 6B). The transistor could be stretched without severe performance degradation (Figure 6C). Stretchable transistors using stretchable dielectric layer was fabricated using a composite film of P3HT and SEBS as a uniform stretchable semiconducting layer.37 A solution of stable P3HT nanorods was readily obtained by cooling the
novel structural design on the elastomer substrate. Making microscale curved surface topology may be the possible design. When BC films are subjected to a large strain (ε ≥ 2.0), a network of microfibrils (approximately 1 μm in average) is created on the surface (Figure 5C) during the multiple generation of the crazes.33 This surface microfibril network can be applied to most BC films. Stretchable electrodes could be readily obtained by sputtering metals through a mask (Figure 5D). The thick metal patterns (75 nm) were used as a straininsensitive electrode with a metallic conductivity, whereas the thin metal patterns (35 nm) were used as a strain sensor with excellent reliability (Figure 5E). This approach makes it possible to fabricate simultaneously high performance stretchable metal electrodes and sensors without prestrained substrate, which is compatible with the current industrial manufacturing process. 67
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 5. (A) Stress−strain curves of the BC/PDMS double layer substrate at different strain rates and temperatures. (B) Dynamic tests to monitor the dynamic response of the BC-composite on the double layer substrate at ε = 1.2 and frequencies of 0.002 and 6.0 Hz. (C) Schematic illustration showing the branching process and the formation of the microfibril network. The substrate with the microfibril network is biaxially stretchable. (D) The Au-patterned strain sensors with various line width, exhibiting before (upper) and after (lower) stretching. (E) Relative resistance changes of the Au-sputtered circuits on the microfibril network film during dynamic test at different strains (ε = 0.3, 0.6, and 1.0). The thick (75 nm, red line) Au electrode is highly stable, while the thin (35 nm, black line) Au line is sensitive to stretching. Panels A and B reproduced with permission from ref 21. Copyright 2017 American Chemical Society. Panels C, D, and E reproduced with permission from ref 33. Copyright 2018 Wiley-VCH.
solution below 0 °C and reheating the solution to the room temperature.38 When the solution containing the P3HT nanorods and SEBS was spin-coated on a plasma-treated PDMS substrate, the P3HT nanorods grew into nanofibril bundles and moved to the surface of the SEBS layer (Figure 6D). The P3HT nanofibril bundles buried on the surface of the SEBS layer were electrically stable under severe mechanical deformation (ε = 0.5). The carrier mobility of the transistor decreased as the uniaxial strain increased but the transistor showed excellent stability in repeated stretching cycles (Figure 6E). The SEBS thin film was also used as a dielectric layer and a packaging material. Because of the hydrophobicity of SEBS, the transistors showed no performance change at high moisture levels (90%) over two months. Bao and co-workers improved the concept (Figure 6F).39 They found that the nanoconfined nanofibrils had improved ductility, so that their stretchability was enhanced considerably without noticeable cracks at large uniaxial elongation (ε = 1.0) (Figure 6G). Recently, they fabricated a high-resolution transistor array by using an SEBS dielectric layer cross-linked with azide crosslinkers to prevent the damage during additional coating processes.40
5. BCS AS A STRETCHABLE ADHESIVE LAYER BC thin films can be used as an adhesive layer holding other materials or bonding two stretchable surfaces. Figure 7A shows an example. When a dry powder of microparticles (MPs) was rubbed in between two elastomer substrates, the MPs assembled to form a close-packed hexagonal monolayer.41,42 Rubbing the MPs on an elastomer template with a line-andspace pattern created a single crystal MPs monolayer over the whole surface.43 The assembled MPs were transferred to a BCcoated PDMS substrate (Figure 7B).43 Because of the sufficient adhesion of the BC layer, the MPs monolayer could be stretched without damage (Figure 7C). Figure 7D exhibits the MPs assembly under uniaxial stretching at ε = 0.3, in which the hexagonal MP array changed to a monoclinic lattice. With the BC thin layers, it easy to create electrode/MP monolayer/electrode that can be used as a pressure sensor depending on the conductance through the MPs.44,45 Figure 7E shows the MPs that are position-registered in each hole of a polyurethane stencil. 45 The stencil film including the conductive MPs could be cut-and-pasted on a variety of electrodes and used as a pressure sensor. The MP-based pressure sensors could accurately follow the fast dynamics of external stimuli (Figure 7F). 68
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 6. (A) Schematic showing the structure of the transistor consisting of the P3HT nanofibers embedded in an ion gel. (B) Digital image exhibiting the dimensional change of the transistor array at elongational strain of ε = 0.7. (C) Transfer curves obtained from (B) at different strains. (D) Schematic showing the formation of P3HT nanofibril bundles and their phase separation on the SEBS substrate surface. (E) Transfer curves of the P3HT nanofibril transistor obtained at different strains. (F) Schematic of the nanoconfined semiconductor network embedded in the SEBS matrix. The chemical structures of the semiconducting polymer DPPT-TT and SEBS are shown. (G) Transfer curves obtained from the transistor made of (F) in the parallel direction to the strain. The results at ε = 0 and ε = 1.0 are compared. Panels A−C reproduced with permission from ref 36. Copyright 2014 Wiley-VCH. Panels D and E reproduced with permission from ref 37. Copyright 2015 Wiley-VCH. Panels F and G reproduced with permission from ref 39. Copyright 2017 American Association for the Advancement of Science.
Figure 7G shows another example of the adhesive use.46 Polyurethane MPs coated with PEDOT:PSS was positionregistered on a PEGDA gel. SIS was spin-coated on a Aupatterned ultrathin PET substrate and used as the adhesive layer for transferring the MPs on itself. The PET/Au/SIS/MPs layer was placed on a ZnO/PS composite thin film deposited on a Ti/Au layer. The interface between the MPs and the ZnO/PS layer acted as a diode allowing the current only from the ZnO/PS layer to the MPs. This device structure was used to produce a flexible high-resolution pressure sensor matrix with no electrical crosstalks between the pixels. Figure 7H represents the use of the sensor matrix as an electronic balance. The same sensor was wrapped around a finger and used as an electronic skin for Braille reading (Figure 7I).
nanocomposites can be investigated as various stretchable components such as dielectrics, semiconductors, oxygen/ humidity barriers, and functional substrates. Also, the preparation of porous BC membranes has been well developed,48 which can be a good candidate for a separator to be used in a stretchable battery. Moldability is another unique feature of BC elastomers. It is possible to create various stretchable 3D microstructures by simple stamping on a viscoelastic polymer49 and the structure can be permanently fixed by chemical cross-linking. Printing the BC composite circuit directly to the metal pad may prevent the interface failure between the stretchable circuit and a nonstretchable metal pad. BC electrolytes can serve as a stretchable actuator for soft robots and electronic skins.50 There are many challenges that need to be addressed for use in real electronics. The chemical species of BCs has so far been limited mainly to PS-based elastomers. A variety of BCs suitable for stretchable electronics should be tested or newly synthesized. The solubility of BCs is good for the solution process, but problematic when additional printing is necessary. To solve this problem, BCs that allow ex-situ chemical crosslinking should be synthesized and the related manufacturing processes for high-resolution patterning should be developed together. Because of the viscoelastic properties of BC, the
6. PERSPECTIVES AND CHALLENGES There are other unique physical characteristics of BCs that are not highlighted in this account. In addition to the conductive nanocomposites, other nanocomposites are readily produced to be stretchable. A composite with zinc−silver oxide (Zn− Ag2O) NPs was used as an anode of a stretchable Li-ion battery.47 The adhesive BC stabilized the active NPs and maintained the battery performance unchanged during repeated stretching cycles at large deformation (ε = 1.0). BC 69
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
Figure 7. (A) Schematic showing the transfer of microparticles (MPs) monolayer to the BC/PDMS double layer substrate. (B) SEM image of the transferred MP monolayer. (C, D) Digital photograph of the transferred MP monolayer uniaxially stretched at ε = 0.2 and the corresponding SEM image of the MPs with the diffraction pattern at the stretched state. (E) Schematic illustration showing the cut-and-paste of the cartridge film. (F) The electrical response of the cut-and-paste pressure sensor sandwiched between two ITOs and between two Au electrodes. (G) Schematic description of the highly flexible pressure sensor using the BC thin film as an adhesive. (H) Pressure distribution on the electronic scale when a Pshaped plastic object was pressed on the flexible sensor matrix. (I) Braille reading e-skin wrapped on a finger reading the sign “Male handicapped restroom” and the pressure map of the red box. Panels A−D reproduced with permission from ref 43. Copyright 2016 American Chemical Society. Panels E and F reproduced with permission from ref 45. Copyright 2018 American Chemical Society. Panels G−I reproduced with permission from ref 46. Copyright 2018 Wiley-VCH.
increases a lot. Decomposition of BCs at high temperatures (∼200 °C) and UV should be improved for long-term stable device operation. For uses in stretchable implantable devices, technological challenges such as mechanical toughness, antioxidation, and long-term prevention of moisture penetration are remained to be tackled. In addition, biocompatibility should be checked for more diverse BCs. PS-based BCs or PMMA-based BCs with saturated aliphatic matrices are currently used for implantable devices.51 For the use as a stretchable encapsulation material, BCs with further hydrophobic surface treatment or BC multilayer structures made of
electrical performance of a device formed on a BC substrate often varies with the number of stretching cycles. Rapid performance changes usually occur at localized defects or necks formed during stretching cycles, hence uniform strain profile and complete elasticity are essential. As mentioned in the substrate section, bonding a BC substrate on a chemically cross-linked elastomer or cross-linking a BC substrate can be a good approach to eliminate the issue. Another possible approach is to fully extend the BC film and then use it as a substrate. Once the BC film is fully extended, the strain profile becomes uniform and its strain range of complete elasticity 70
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research composite thin films containing 2D nanomaterials can be good choices.
■
(6) Sekitani, T.; Yokota, T.; Kuribara, K.; Kaltenbrunner, M.; Fukushima, T.; Inoue, Y.; Sekino, M.; Isoyama, T.; Abe, Y.; Onodera, H.; Someya, T. Ultraflexible organic amplifier with biocompatible gel electrodes. Nat. Commun. 2016, 7, 11425. (7) Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwödiauer, R.; Graz, I.; BauerGogonea, S.; Bauer, S.; Someya, T. An ultra-lightweight design for imperceptible plastic electronics. Nature 2013, 499, 458−463. (8) Trung, T. Q.; Lee, N. E. Recent progress on stretchable electronic devices with intrinsically stretchable components. Adv. Mater. 2017, 29, 1603167. (9) Song, W. J.; Park, J.; Kim, D. H.; Bae, S.; Kwak, M. J.; Shin, M.; Kim, S.; Choi, S.; Jang, J.-H.; Shin, T. J.; Kim, S. Y.; Seo, K.; Park, S. Jabuticaba-Inspired Hybrid Carbon Filler/Polymer Electrode for Use in Highly Stretchable Aqueous Li-Ion Batteries. Adv. Energy Mater. 2018, 8, 1702478. (10) Li, Z.; Zhang, R.; Moon, K. S.; Liu, Y.; Hansen, K.; Le, T.; Wong, C. Highly Conductive, Flexible, Polyurethane-Based Adhesives for Flexible and Printed Electronics. Adv. Funct. Mater. 2013, 23, 1459−1465. (11) Spontak, R. J.; Patel, N. P. Thermoplastic elastomers: fundamentals and applications. Curr. Opin. Colloid Interface Sci. 2000, 5, 333−340. (12) Perez, L. D.; Zuluaga, M. A.; Kyu, T.; Mark, J. E.; Lopez, B. L. Preparation, characterization, and physical properties of multiwall carbon nanotube/elastomer composites. Polym. Eng. Sci. 2009, 49, 866−874. (13) Chen, Y.; Kushner, A. M.; Williams, G. A.; Guan, Z. Multiphase design of autonomic self-healing thermoplastic elastomers. Nat. Chem. 2012, 4, 467−472. (14) Choi, S.; Park, J.; Hyun, W.; Kim, J.; Kim, J.; Lee, Y. B.; Song, C.; Hwang, H. J.; Kim, J. H.; Hyeon, T.; Kim, D.-H. Stretchable heater using ligand-exchanged silver nanowire nanocomposite for wearable articular thermotherapy. ACS Nano 2015, 9, 6626−6633. (15) Brown, K.; Hooker, J. C.; Creton, C. Micromechanisms of tack of soft adhesives based on styrenic block copolymers. Macromol. Mater. Eng. 2002, 287, 163−179. (16) Han, C. D.; Baek, D. M.; Kim, J.; Kimishima, K.; Hashimoto, T. Viscoelastic behavior, phase equilibria, and microdomain morphology in mixtures of a block copolymer and a homopolymer. Macromolecules 1992, 25, 3052−3067. (17) Larsson, P.-L.; Carlsson, S. On microindentation of viscoelastic polymers. Polym. Test. 1998, 17, 49−75. (18) Hamley, I. W. Developments in Block Copolymer Science and Technology; John Wiley & Sons: New Jersey, 2004; Chapter 1. (19) Costa, P.; Carvalho, M. F.; Correia, V.; Viana, J. C.; LancerosMéndez, S. Polymer nanocomposite-based strain sensors with tailored processability and improved device integration. ACS Appl. Nano Mater. 2018, 1, 3015−3025. (20) Choi, S.; Han, S. I.; Jung, D.; Hwang, H. J.; Lim, C.; Bae, S.; Park, O. K.; Tschabrunn, C. M.; Lee, M.; Bae, S. Y.; Yu, J. W.; Ryu, J. H.; Lee, S.-W.; Park, K.; Kang, P. M.; Lee, W. B.; Nezafat, R.; Hyeon, T.; Kim, D.-H. Highly conductive, stretchable and biocompatible Ag− Au core−sheath nanowire composite for wearable and implantable bioelectronics. Nat. Nanotechnol. 2018, 13, 1048−1056. (21) Cho, S.; Song, J. H.; Kong, M.; Shin, S.; Kim, Y.-T.; Park, G.; Park, C.-G.; Shin, T. J.; Myoung, J.; Jeong, U. Fully Elastic Conductive Films from Viscoelastic Composites. ACS Appl. Mater. Interfaces 2017, 9, 44096−44105. (22) Xu, F.; Zhu, Y. Highly conductive and stretchable silver nanowire conductors. Adv. Mater. 2012, 24, 5117−5122. (23) Moon, G. D.; Lim, G. H.; Song, J. H.; Shin, M.; Yu, T.; Lim, B.; Jeong, U. Highly stretchable patterned gold electrodes made of Au nanosheets. Adv. Mater. 2013, 25, 2707−2712. (24) Chortos, A.; Koleilat, G. I.; Pfattner, R.; Kong, D.; Lin, P.; Nur, R.; Lei, T.; Wang, H.; Liu, N.; Lai, Y.-C.; Kim, M.-G.; Chung, J. W.; Lee, S.; Bao, Z. Mechanically Durable and Highly Stretchable Transistors Employing Carbon Nanotube Semiconductor and Electrodes. Adv. Mater. 2016, 28, 4441−4448.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Insang You: 0000-0002-7218-1790 Minsik Kong: 0000-0002-0865-7635 Unyong Jeong: 0000-0002-7519-7595 Notes
The authors declare no competing financial interest. Biographies Insang You received a B.S. degree from Yonsei University in 2014. He is pursuing his Ph.D. degree under the supervision of Prof. Unyong Jeong in the Dept. of Materials Science and Engineering from Pohang University of Science and Technology (POSTECH). His research interests include the material development for versatile Eskin sensors. Minsik Kong received a B.S. degree in the Dept. of Materials Science and Engineering from Pohang University of Science and Technology (POSTECH) in 2016. He is pursuing his Ph.D. degree under the supervision of Prof. Unyong Jeong in the same department. His research interest includes stretchable conductor and the stretchable electrochemical system. Unyong Jeong received his Ph.D. in Chemical Engineering from POSTECH in Korea. After he spent two years as a postdoctoral research associate at University of Washington, he joined to Yonsei University in 2006, and he moved to Materials Science and Engineering at POSTECH. His research interest includes organic/ inorganic hybrid materials for electronic devices, synthesis of nanostructured materials, and assembly of nano- and microstructured materials.
■
ACKNOWLEDGMENTS This work was supported by the Center for Advanced Soft Electronics funded by the Ministry of Education, Science and Technology as a “Global Frontier Project” (CASE2015M3A6A5072945) and the Korea Research Institute of Chemical Technology (KRICT) as a Project No. SI1803 (Development of One-patch device for HMI based on 3D Device Printing).
■
REFERENCES
(1) Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D.-H.; Brooks, G. A.; Davis, R. W.; Javey, A. Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis. Nature 2016, 529, 509−514. (2) Stoppa, M.; Chiolerio, A. Wearable electronics and smart textiles: a critical review. Sensors 2014, 14, 11957−11992. (3) Koo, J. H.; Kim, D. C.; Shim, H. J.; Kim, T. H.; Kim, D. H. Flexible and Stretchable Smart Display: Materials, Fabrication, Device Design, and System Integration. Adv. Funct. Mater. 2018, 28, 1801834. (4) Kim, D.-H.; Ghaffari, R.; Lu, N.; Rogers, J. Flexible and stretchable electronics for biointegrated devices. Annu. Rev. Biomed. Eng. 2012, 14, 113−128. (5) Park, M.; Park, J.; Jeong, U. Design of conductive composite elastomers for stretchable electronics. Nano Today 2014, 9, 244−260. 71
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
Article
Accounts of Chemical Research
fabrication of an intrinsically stretchable transistor array. Nature 2018, 555, 83−88. (41) Park, C. J.; Lee, T. I.; Xia, Y.; Shin, T. J.; Myoung, J. M.; Jeong, U. Quick, Large-Area Assembly of a Single-Crystal Monolayer of Spherical Particles by Unidirectional Rubbing. Adv. Mater. 2014, 26, 4633−4638. (42) Park, C. J.; Koh, K.; Jeong, U. Structural Color Painting by Rubbing Particle Powder. Sci. Rep. 2015, 5, 8340. (43) Koh, K.; Hwang, H.; Park, C.; Lee, J. Y.; Jeon, T. Y.; Kim, S.-H.; Kim, J. K.; Jeong, U. Large-Area Accurate Position Registry of Microparticles on Flexible, Stretchable Substrates Using Elastomer Templates. ACS Appl. Mater. Interfaces 2016, 8, 28149−28158. (44) Lee, W. J.; Han, S. W.; You, I.; Choi, S.-E.; Jeong, U.; Kim, J. W. Conductive Magnetic-Patchy Colloidal Microparticles for a High Performance Pressure Sensor. Chem. Commun. 2016, 52, 12334− 12337. (45) Hwang, H.; Choi, S.-E.; Han, S. W.; You, I.; Jeong, E. S.; Kim, S.; Yang, H.; Lee, S.; Choo, J.; Kim, J. W.; Jeong, U. Cut-and-Paste Transferrable Pressure Sensing Cartridge Films. Chem. Mater. 2018, 30, 6410−6419. (46) You, I.; Choi, S. E.; Hwang, H.; Han, S. W.; Kim, J. W.; Jeong, U. E-Skin Tactile Sensor Matrix Pixelated by Position-Registered Conductive Microparticles Creating Pressure-Sensitive Selectors. Adv. Funct. Mater. 2018, 28, 1801858. (47) Kumar, R.; Shin, J.; Yin, L.; You, J. M.; Meng, Y. S.; Wang, J. All-Printed, Stretchable Zn-Ag2O Rechargeable Battery via Hyperelastic Binder for Self-Powering Wearable Electronics. Adv. Energy Mater. 2017, 7, 1602096. (48) Shin, M.; Song, W. J.; Son, H. B.; Yoo, S.; Kim, S.; Song, G.; Choi, N. S.; Park, S. Highly Stretchable Separator Membrane for Deformable Energy-Storage Devices. Adv. Energy Mater. 2018, 8, 1801025. (49) Oh, J. Y.; Kim, S.; Baik, H. K.; Jeong, U. Conducting polymer dough for deformable electronics. Adv. Mater. 2016, 28, 4455−4461. (50) Kim, O.; Shin, T. J.; Park, M. J. Fast low-voltage electroactive actuators using nanostructured polymer electrolytes. Nat. Commun. 2013, 4, 2208. (51) Pinchuk, L.; Wilson, G. J.; Barry, J. J.; Schoephoerster, R. T.; Parel, J.-M.; Kennedy, J. P. Medical applications of poly (styreneblock-isobutylene-block-styrene)(“SIBS”). Biomaterials 2008, 29, 448−460.
(25) Xiong, J.; Li, S.; Ye, Y.; Wang, J.; Qian, K.; Cui, P.; Gao, D.; Lin, M. F.; Chen, T.; Lee, P. S. A Deformable and Highly Robust Ethyl Cellulose Transparent Conductor with a Scalable Silver Nanowires Bundle Micromesh. Adv. Mater. 2018, 30, 1802803. (26) Park, M.; Im, J.; Shin, M.; Min, Y.; Park, J.; Cho, H.; Park, S.; Shim, M.-B.; Jeon, S.; Chung, D.-Y.; Bae, J.; Park, J.; Jeong, U.; Kim, K. Highly stretchable electric circuits from a composite material of silver nanoparticles and elastomeric fibres. Nat. Nanotechnol. 2012, 7, 803−809. (27) Vural, M.; Behrens, A. M.; Ayyub, O. B.; Ayoub, J. J.; Kofinas, P. Sprayable elastic conductors based on block copolymer silver nanoparticle composites. ACS Nano 2015, 9, 336−344. (28) Lee, S.; Shin, S.; Lee, S.; Seo, J.; Lee, J.; Son, S.; Cho, H. J.; Algadi, H.; Al-Sayari, S.; Kim, D. E.; Lee, T. Ag nanowire reinforced highly stretchable conductive fibers for wearable electronics. Adv. Funct. Mater. 2015, 25, 3114−3121. (29) Song, J. H.; Kim, Y. T.; Cho, S.; Song, W. J.; Moon, S.; Park, C. G.; Park, S.; Myoung, J. M.; Jeong, U. Surface-Embedded Stretchable Electrodes by Direct Printing and their Uses to Fabricate Ultrathin Vibration Sensors and Circuits for 3D Structures. Adv. Mater. 2017, 29, 1702625. (30) Wang, Y.; Zhu, C.; Pfattner, R.; Yan, H.; Jin, L.; Chen, S.; Molina-Lopez, F.; Lissel, F.; Liu, J.; Rabiah, N. I.; Chen, Z.; Chung, J. W.; Linder, C.; Toney, M. F.; Murmann, B.; Bao, Z. A highly stretchable, transparent, and conductive polymer. Sci. Adv. 2017, 3, e1602076. (31) Bhattacharya, S.; Datta, A.; Berg, J. M.; Gangopadhyay, S. Studies on surface wettability of poly (dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength. J. Microelectromech. Syst. 2005, 14, 590−597. (32) Stoyanov, H.; Kollosche, M.; Risse, S.; Waché, R.; Kofod, G. Soft conductive elastomer materials for stretchable electronics and voltage controlled artificial muscles. Adv. Mater. 2013, 25, 578−583. (33) Moon, S.; Park, H. K.; Song, J. H.; Cho, S.; Kim, J. C.; Kim, J.; Hwang, H.; Kim, H. S.; Jeong, U. Metal Deposition on a SelfGenerated Microfibril Network to Fabricate Stretchable Tactile Sensors Providing Analog Position Information. Adv. Mater. 2018, 30, 1801408. (34) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Stretchable active-matrix organic light-emitting diode display using printable elastic conductors. Nat. Mater. 2009, 8, 494−499. (35) Liang, J.; Li, L.; Chen, D.; Hajagos, T.; Ren, Z.; Chou, S.-Y.; Hu, W.; Pei, Q. Intrinsically stretchable and transparent thin-film transistors based on printable silver nanowires, carbon nanotubes and an elastomeric dielectric. Nat. Commun. 2015, 6, 7647. (36) Shin, M.; Song, J. H.; Lim, G. H.; Lim, B.; Park, J. J.; Jeong, U. Highly stretchable polymer transistors consisting entirely of stretchable device components. Adv. Mater. 2014, 26, 3706−3711. (37) Shin, M.; Oh, J. Y.; Byun, K. E.; Lee, Y. J.; Kim, B.; Baik, H. K.; Park, J. J.; Jeong, U. Polythiophene nanofibril bundles surfaceembedded in elastomer: A route to a highly stretchable active channel layer. Adv. Mater. 2015, 27, 1255−1261. (38) Oh, J. Y.; Shin, M.; Lee, T. I.; Jang, W. S.; Min, Y.; Myoung, J.M.; Baik, H. K.; Jeong, U. Self-seeded growth of poly (3hexylthiophene)(P3HT) nanofibrils by a cycle of cooling and heating in solutions. Macromolecules 2012, 45, 7504−7513. (39) Xu, J.; Wang, S.; Wang, G.-J. N.; Zhu, C.; Luo, S.; Jin, L.; Gu, X.; Chen, S.; Feig, V. R.; To, J. W. F.; Rondeau-Gagne, S.; Park, J.; Schroeder, B. C.; Lu, C.; Oh, J. Y.; Wang, Y.; Kim, Y.-H.; Yan, H.; Sinclair, R.; Zhou, D.; Xue, G.; Murmann, B.; Linder, C.; Cai, W.; Tok, J. B.-H.; Chung, J. W.; Bao, Z. Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 2017, 355, 59−64. (40) Wang, S.; Xu, J.; Wang, W.; Wang, G.-J. N.; Rastak, R.; MolinaLopez, F.; Chung, J. W.; Niu, S.; Feig, V. R.; Lopez, J.; Lei, T.; Kwon, S.-K.; Kim, Y.; Foudeh, A. M.; Ehrlich, A.; Gasperini, A.; Yun, Y.; Murmann, B.; Tok, J. B.-H.; Bao, Z. Skin electronics from scalable 72
DOI: 10.1021/acs.accounts.8b00488 Acc. Chem. Res. 2019, 52, 63−72
