Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Hydrogel-Templated Transfer-Printing of Conductive Nanonetworks for Wearable Sensors on Topographic Flexible Substrates Tae-Hyung Kang,† Hochan Chang,† Dongwon Choi,†,‡ Soonwoo Kim,†,∥ Jihee Moon,†,§ Jung Ah Lim,† Ki-Young Lee,† and Hyunjung Yi*,† †
Post-Silicon Semiconductor Institute, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea School of Electrical Engineering and §Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea ∥ Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea ‡
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S Supporting Information *
ABSTRACT: Transfer-printing enables the assembly of functional nanomaterials on unconventional substrates with a desired layout in a controllable manner. However, transferprinting to substrates with complex surfaces remains a challenge. Herein, we show that hydrogels serve as effective template material platforms for the assembly and transferprinting of conductive nanonetwork patterns for flexible sensors on various topographic surfaces in a very simple yet versatile manner. The non-adherence, nanoporous structure, and molding capability of the hydrophilic hydrogel enable the assembly of conductive nanonetwork patterns on the hydrogel surface and transfer of the nanonetworks onto various flexible and topographic substrates. Flexible strain sensors and pressure sensors that monitor finger motions and arterial pulses are successfully demonstrated using the hydrogel-templated approach. The rich chemistry of polymeric networks, facile molding capability, and biocompatibility of hydrogels could be further combined with additive technology for hydrogels and electronic materials for emerging four-dimensional functional materials and soft bioelectronics. KEYWORDS: Transfer-printing, conductive nanonetworks, hydrogels, flexible devices, inkjet printing, wearable sensors platform for biological and chemical materials in the fields of tissue engineering, drug delivery, soft robotics, and bio- and chemical sensors.14−22 Moreover, in the field of molecular biology, hydrogels, particularly agarose-based hydrogels (agarose hydrogel) and polyacrylamide-based hydrogels (PAAm hydrogel), have been widely utilized in electrophoresis for separating small biomolecules such as oligonucleotides from each other and from larger ones such as proteins and large nucleic acid polymers.23,24 Considering that small watersoluble molecules can easily diffuse out through the hydrophilic pores of hydrogels, whereas larger molecules cannot penetrate the hydrogel matrix, we can extend such analogy toward separating electronic nanomaterials from small molecules such as surfactant and dispersants used for stabilizing nanomaterials. Moreover, when such hydrophobic electronic materials are localized on the hydrophilic hydrogel surface in a designed pattern, the localized pattern of nanomaterials could be transferred on hydrophobic substrates. However, although hydrogels have been interfaced with
he increasingly diverse substrates for flexible electronics, soft electronics, and wearable sensors often require the separation of fabrication substrates from the application substrates to overcome process limits imposed by unconventional substrates.1−4 The transfer-printing of inorganic nanomaterials of excellent electronic properties enables us to fabricate functional nanomaterials and nanostructures on unconventional substrates with desired layout in a controllable manner.5,6 Various inorganic materials and carbon-based materials as well as integrated devices and circuits have been successfully transfer-printed for flexible and soft electronics.6−9 For the successful transfer-printing of nanomaterials, the control of the interaction of nanomaterials with donor and receiver substrates is of paramount importance and several transfer methods have been developed using thermal, kinetic, and geometrical effects.10−13 However, despite these successful efforts, it still remains a challenge to produce functional nanomaterials and devices on topographic surfaces in a simple and versatile manner. Hydrogels are three-dimensional, hydrophilic polymer networks that absorb large amount of water. The hydrophilic porous nature, molding capability, and rich chemistry of polymeric networks of hydrogels provide an attractive matrix
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© XXXX American Chemical Society
Received: February 21, 2019 Revised: April 19, 2019
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DOI: 10.1021/acs.nanolett.9b00764 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Procedures for the hydrogel-templated transfer-printing of conductive nanonetworks. (a) SWNT solutions were inkjet-printed on the hydrogel surfaces, and surfactants were allowed to diffuse away through the hydrogel pores to result in hydrophobic SWNT nanonetworks on the surface of the hydrophilic hydrogel. SWNT nanonetworks were then transferred to hydrophobic substrates either by contacting preformed substrates to or by curing elastomeric solutions on the surface of the hydrogel containing the nanonetworks. (b) Procedures for the transferprinting of nanonetworks on microstructured substrates. The molding capabilities of both the hydrogel template and the curable elastomeric solutions were exploited. (c) A transfer method can be chosen depending on the surface morphologies of the final substrates for the nanonetworkbased devices. The upper photograph shows the serpentine patterns of nanonetworks contact-transferred on a poly glove. The lower photograph shows the line patterns of nanonetworks of different thicknesses molding-transferred using PDMS.
electronic materials to produce electrically and ionically conductive composites or ionic devices or to serve as smart adhesive for fabricating nanomembrane-based devices,25−27 so far, the possibility of hydrogels to assemble patterns of nanonetworks and transfer-print on topographic surfaces has not been explored. Herein, we show that hydrogel materials serve as versatile template material platforms for the transfer-printing of conductive nanonetworks on various types of substrates and surface morphologies via hydrophobic and hydrophilic interactions and molding capability of hydrogels. Patterns of one-dimensional electronic nanomaterials such as surfactantdispersed single-walled carbon nanotubes (SWNTs) were inkjet-printed on the surface of the hydrogels, and both physically cross-linked hydrogels and chemically cross-linked hydrogels were able to retain printed SWNTs and simultaneously remove surfactants to form nanonetworks of SWNTs on the surface of the hydrogel. Moreover, the non-adherence of the surface of the hydrogel, the nanoporous structure and the molding capability of the hydrogel enabled the facile transfer of the pattern of nanonetworks onto various receiving substrates including topographic substrates. We were able to transfer-print the nanonetworks for flexible devices by either forming contacts between appropriate substrates and the hydrogel surface containing the patterned nanonetworks or curing elastomeric solutions directly on the surface of the hydrogel. Wearable strain sensors that differentiated various hand motions and flexible pressure sensors that monitored arterial pulses were demonstrated using the hydrogel-templatebased approach. We envision that the facile molding capability, biocompatibility, and high diversity in chemical and micro-
structural properties of hydrogels and low-cost and lowtemperature process involving hydrogels could provide a breakthrough toward realizing, in a very simple yet versatile way, complex electronic devices on unconventional substrates not only for digital healthcare and intelligent human-machine interfaces but also for neuroscience and engineering, tissue engineering, regenerative medicine, and personalized medicine.28−39 Figure 1a schematically illustrates the procedure for the hydrogel-templated transfer-printing of the nanonetworks. Here, the agarose as a physically cross-linked hydrogel and PAAm as a chemically cross-linked hydrogel were chosen as templating hydrogel materials based on their controllable pore sizes, appropriate mechanical properties for facile handling, and excellent anti-biofouling.14,23 A hydrogel template material was first prepared, and then a solution of one-dimensional nanomaterials with lengths greater than the pore size of the hydrogel was inkjet-printed on the surface of the hydrogel solution (Figure 1a). Note in this regard that the hydrophilic porous nature of the hydrogel allowed for the simultaneous assembly of nanonetworks and the removal of surfactants or dispersants used for stabilizing nanomaterials in solution. This spontaneous washing capability of the hydrogel obviated a chemical or thermal removal post-process that has been considered to be a significant obstacle to the device applications of aqueous solution-dispersed nanomaterials. Therefore, as model one-dimensional electronic nanomaterials, SWNTs were employed not only because of their excellent electronic properties and high mechanical and chemical stability but also because the removal of dispersant used for stabilizing SWNTs is inevitable for high-performance devices B
DOI: 10.1021/acs.nanolett.9b00764 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. Physically cross-linked agarose hydrogels and chemically cross-linked PAAm hydrogels as template materials. (a) Schematic illustrations of structural aspects of agarose hydrogel (physically cross-linked) and PAAm hydrogel (chemically cross-linked). (b) Scanning electron micrographs of 0.8% (upper panels) and 2.5% (lower panels) agarose hydrogels. (c) Scanning electron micrographs of 5% (upper panels) and 10% (lower panels) PAAm hydrogels. (d) Representative stress−strain curves of agarose and PAAm hydrogels of various compositions. (e) Compressive moduli and compressive strengths of agarose and PAAm hydrogels of various compositions. Mean value ± SD were obtained from three different samples.
based on SWNTs.40 Once the solution of nanomaterials was inkjet-printed onto the surface of the hydrogel, the resulting one-dimensional nanomaterials remained on the surface of the hydrogel while small surfactants or dispersants diffused out into the hydrogel through pores, forming a nanonetwork pattern on the surface of the hydrogel. As a result of the hydrophilic and antifouling natures and the nanoporous structure of the hydrogel, the nanonetwork of hydrophobic electronic nanomaterials was simply “sitting” on the surface of the hydrogel, and minimally interacting with it. This localization of the hydrophobic nanonetwork and the weak interaction with hydrophilic hydrogel allowed for the assembled nanonetworks to be easily transferred to various hydrophobic substrates for device fabrications either by bringing a preformed substrate into contact (“contact transfer”) with or pouring a curable elastomeric solution (“molding transfer”) onto the surface of the hydrogel containing the printed nanonetworks. When nanonetworks needed to be defined on a smooth or relatively flat surface, both contacttransfer and molding-transfer methods worked, as illustrated in Figure 1a. For realizing nanonetworks on a very rough or microstructured surface where direct-contact transfer was challenging due to the poor contact between the assembling and the topographic receiving substrates,41 the moldingtransfer method was found to be particularly useful. The processes used to form nanonetworks on microstructures are illustrated in Figure 1b. First, hydrogels used to serve as the template material were molded on a surface showing a desired morphology. Once the hydrogel solidified, the hydrogel film
was detached from the mold to serve as a template with a microstructured surface morphology. Nanonetworks were printed onto the microstructured hydrogel surface, and then another moldable solution such as an elastomeric polydimethylsiloxane (PDMS) solution was poured, cured, and detached from the hydrogel template to produce patterned nanonetworks on microstructured elastomeric substrates. Therefore, we were able to realize nanonetworks on various types of substrates and surface morphologies using the hydrogel template-based transfer-printing approach, as illustrated in Figure 1c. This approach differed from various previously reported “transfer-based” approaches for fabricating nanonetwork-based flexible electrodes and devices in that the molding capability of the hydrogel template material enabled the material to provide nanonetworks to devices of various surface morphologies in addition to flat surfaces.40,42 Moreover, the hydrophilic porous structures of the hydrogels simultaneously removed surfactants and dispersants during the assembly. This unique and powerful capability of hydrogels to promote the templated assembly of nanonetworks suggests that hydrogels should be applicable to a wide variety of device substrates, surface morphologies, and transfer methods. The microstructures of agarose and PAAm hydrogels of varying compositions were first investigated because the microstructures and related pore size were important parameters of hydrogels for the templated assembly of nanonetworks. Agarose is known to form a physical gel by lateral association of the polysaccharide chains into rigid helical bundles, whereas PAAm hydrogel is known to form a chemical C
DOI: 10.1021/acs.nanolett.9b00764 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 3. Contact transfer and molding transfer of conductive nanonetwork patterns. (a, b) Photographs of SWNT nanonetwork patterns of various shapes and dimensions on (a) 2.5% agarose and (b) 10% PAAm hydrogels. (c) Photograph of transferred SWNT nanonetworks on PET and PDMS films. Inset: photograph of a transferred SWNT nanonetworks on a PDMS slab. (d) Photographs showing procedures for forming SWNT nanonetworks on a rough surface. The number indicates the number of printing layers of rectangular pattern (1 mm × 5 mm). (e, f) Scanning electron micrographs of the (e) sandpaper used as a mold for the hydrogel template and (f) cured PDMS containing the transferred SWNT nanonetwork. Comparison of the scanning electron micrographs confirms the molding transfer capable of producing SWNT nanonetworks with desired microstructures. (g) Sheet resistance values of nanonetworks printed in various numbers of layers and transferred using different types of hydrogel and transfer methods. Mean value ± SD were obtained from five different samples.
gel by cross-linking acrylamide chains (Figure 2a).14,23 Scanning electron micrographs of cross-sections of freezedried agarose and PAAm hydrogels of various compositions are presented in Figures 2b,c and S1. The fabricated agarose hydrogels (Figure 2b) showed fibrous microstructures. The 0.8% agarose hydrogel showed very fine fibers, whereas for 2.5% agarose, the fibers formed relatively dense networks. For both compositions, the fibrous bundles formed porous structures of largely varying pore diameters from tens of nanometers to several micrometers, but 0.8% agarose showed relatively larger features (Figure S1). In contrast, the PAAm
hydrogels (Figure 2c) showed cellular microstructures having more uniformly sized and larger structures than those of the agarose hydrogels. The cell size of the 5% PAAm hydrogel was estimated to be ∼3.3 μm ± 1.09 μm. For the 10% PAAm hydrogel, it showed larger cells of 9.9 ± 3.19 μm containing much smaller pores of 345 ± 209 nm inside the larger cells. The mechanical properties of various compositions of the hydrogels were also investigated because the hydrogel needed to be mechanically robust throughout the printing, transferring, and molding processes. Various hydrogels were subjected to a compression test. The resulting representative stress−strain D
DOI: 10.1021/acs.nanolett.9b00764 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 4. Hydrogel-templated tranfer-printing for versatile fabrication of flexible sensors on various surface morphologies. (a) Schematic illustration of the fabrication of highly stretchable strain sensors via the contact transfer of serpentine-shaped nanonetworks. (b) Photographs of the stretchable strain sensor on a PDMS slab. (c) Changes in the resistance of the nanonetwork-based strain sensor for various strain levels. (d) Stability of the strain sensor. (e) Responses of each strain sensor to various hand motions. Bending of each finger yielded an increase (decrease) in the resistance (current level) of its corresponding strain sensor. Each hand motion thus produced a unique pattern of resistance levels. Inset: photographs of strain sensors on the index finger and middle finger locations of a poly glove (lower left) and various hand motions (upper middle). (f) Response of the pressure sensors to various loading levels. Inset: photograph of the fabricated flexible pressure sensor and a schematic showing the structure and the corresponding transfer method. The pressure sensor consisted of two layers: a top PDMS layer with a microstructured SWNT nanonetwork and a bottom PET layer with a flat SWNT nanonetwork. One layer of nanonetwork was inkjet-printed for both the top and the bottom layers. (g) Response of the pressure sensor up to 50 kPa. The sensitivity in the low-pressure range (
