Stretchable, Compressible and Conductive Metal-Coated PDMS Sponges

Development of flexible, highly conductive electrodes or interconnects with excellent mechanical stability has been a challenging issue in device fabrication for wearable electronics, flexible displays, etc. Researchers at College of Chemistry and Environmental Engineering, Shenzhen University, P. R. China have developed 3D stretchable, compressible and electrically conductive conductors by surface modification of poly(dimethylsiloxane) (PDMS) sponges with poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride] (PMETAC) polymers followed by electroless deposition of metals.

The PDMS sponges were fabricated using sugar templating method. The sugar cubes were immersed in a mixture of Sylgard 184 and curing agent, degassed in a vacuum desiccator for 2 h followed by baking at 65 °C for 3 h, removal of sugar template by immersion in water at 60 °C for 24 h and drying at 100 °C for 2 h. The PDMS sponges were activated by air plasma treatment for 5 min, functionalized with vinyltrimethoxysilane (VTMS) via silanization followed by in situ free radical polymerization with METAC monomer and potassium persulfate as initiator, leading to the formation of PMETAC-modified PDMS sponges with 3D-interconnected porous structures. Electroless deposition of metals enables the formation of metal-coated PDMS sponges. The schematic of the fabrication process and images of electroless Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponges are shown in Fig. 1.

Fig. 1 (a) Schematic illustration of the fabrication of metal-coated PDMS sponges; (b, c) Optical images of the PDMS sponge and Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponge

The PDMS sponges consist of highly interconnected 3-D porous structures and the sponge-like structure is retained even after coating them with Cu, Ag/Cu and Au/Cu by electroless deposition. The coated metal particles exhibit a close-packed arrangement on the surface of PDMS sponges. The elastomeric property of PDMS sponges enable them to be stretched and compressed while metal coating makes them conductive, thus making them suitable for the fabrication of electrically conductive, stretchable and compressible electrodes. The metal-coated PDMS sponges exhibit remarkable mechanical stability and electrical conductivity, which is evidenced by the overlap of I–V characteristics of Ag/Cu-PDMS sponges while stretching or compressing them from 0% to 50% as well as by the continuous glowing of LED lamps at different extents of stretching, bending, and twisting (Fig. 2). In addition, the metal-coated PDMS sponges remain conducting even after cutting them to two pieces, which indicates that the metal coating is uniform not only on the outer side but also on the inner side of PDMS sponges.

Fig. 2 Flexible LED circuits made of Ag/Cu-PDMS sponge interconnects: (a, b) I–V characteristics of the LED circuit at different (a) tensile; and (b) compressive strains; (c-e) Optical images of the LED circuits with two LEDs at different extent of (c) stretching; (d) bending; and (e) twisting.

The ability of metal-coated PDMS sponges to offer no change in resistance at 40% tensile strain and only ≈20% increments at 50% strain after 5000 cycles of stretching suggests that they can be used for stretchable, compressible, and bendable interconnects or soft electronic contacts.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: S.Q. Liang et al., 3D Stretchable, Compressible, and Highly Conductive Metal-Coated Polydimethylsiloxane Sponges, Adv. Mater. Technol. 2016, 1600117, DOI: 10.1002/admt.201600117

 

Ni(OH)2 Nanosheet Ink for Wearable Energy Storage Devices

Flexible electronics have received considerable attention, particularly in energy storage devices. Among the various techniques available for their fabrication, solution based methods such as ink-jet printing, screen printing, and roll-to-roll printing assume significance in terms of their low-cost, high processing speed, and scalability. Nevertheless, formulation of inks containing suitable active materials with better dispersion and stability remains a challenge. Researchers at Nanjing Tech University, China, Nanyang Technological University, Singapore, Lanzhou University, China, Nanjing University of Posts and Telecommunications, China and Northwestern Polytechnical University, China have developed a facile method to prepare a highly concentrated ink comprised of 2D ultrathin Ni(OH)₂ nanosheets, which can be easily coated on commercial printing paper as well as carbon fiber yarns (CFYs). Using CFY@Ni(OH)₂ as a weavable electrode, they have fabricated wearable energy storage devices.

The Ni(OH)₂ nanosheets were prepared by co-precipitation method. Since the precipitated Ni(OH)₂ nanosheets tend to aggregate, they were exfoliated by ultrasonication. The Ni(OH)₂ nanosheets dispersed in water, ethanol, and DMF exhibit excellent stability. The Ni(OH)₂ ink (20 mg/mL in water) (Fig. 1(a)) was directly coated on printing paper using a brush. The desired thickness of the Ni(OH)₂ coating can be tuned by a careful choice of the number of cycles. The Ni(OH)₂ coated paper is highly flexible and it can be easily wound around a glass rod (Fig. 1(b)). Selective removal of Ni(OH)₂ from the coated paper (Fig. 1(c)) using 3M HCl enables patterning of suitable designs (Fig. 1(d)).

Fig. 1 (a) Ni(OH)₂ ink (20 mg/mL in water); (b, c) flexible and plain printing papers coated with Ni(OH)₂ nanosheets; and (d) selective removal of Ni(OH)₂ from the coated printing paper using 3M HCl for patterning of suitable designs

The Ni(OH)₂ nanosheet ink was also coated on carbon fiber yarn (CFY). Ethanol was used as the solvent to improve wettability of CFY with the ink. Nafion (Nf) was used as an ionic binder to promote the interaction between Ni(OH)₂ nanosheets and CFY. The CFY@Nf-Ni(OH)₂ electrode was prepared by impregnation-dyeing method (Fig. 2(a)) in which the CFY was repetitively impregnated in Nf-Ni(OH)₂ ink (20 mg/mL in ethanol) for 1 min for each cycle and dried at 80 °C. The CFY@Nf-Ni(OH)₂ maintains excellent flexibility (Fig. 2(b)). Morphological features reveal that the Ni(OH)₂ is uniformly coated on CFY (300 nm thick for 6 impregnations) (Figs. 2(c) and 2(d)).

Fig. 2 (a) Schematic illustration of the preparation of CFY@Nf-Ni(OH)₂ by impregnation-dyeing method; (b) Photograph of highly bended CFY@Nf-Ni(OH)₂; (c) SEM image of a single yarn CF@Nf-Ni(OH)₂; (d) Magnified image of (c) 300 nm thick Ni(OH)₂ coated layer obtained after 6 impregnations.

Galvanostatic charge–discharge curves of CFY@Nf-Ni(OH)₂ indicate that a maximum  specific volumetric capacitance (CV) is obtained after 6 times of impregnation whereas gravimetric capacitance (Cg) is decreased from 1010.0 to 640.0 F/g with an increase in impregnation times from 2 to 10. Repetitive impregnations though enable an increase in the mass loading of Ni(OH)₂ nanosheets, it leads to a reduction in the conductivity of CFY@Nf-Ni(OH)₂. The CFY@Nf-Ni(OH)₂ electrode exhibits good cycling stability, which is evidenced by its 89% capacitance retention after 5000 charge–discharge cycles.

A hybrid supercapacitor was fabricated using CFY@Nf-Ni(OH)₂ as positive electrode, CFY@CPs (carbon particles) as negative electrode and poly(vinyl alcohol-KOH gel as the solid-state electrolyte, enclosed in a thermal plastic tube to seal the device (Fig. 3). The hybrid supercapacitor exhibits a wide potential window of 1.5 V and delivers a high energy density of 11.3 mWh/cm³ at a power density of 0.3 W/cm³.

Fig. 3 Schematic of the fabrication of yarn-based hybrid supercapacitor

The CFY-based hybrid supercapacitor can be woven in the form of a glove (Fig. 4), which at different bending angles is capable of retaining 96% capacitance after 5000 bending–unbending cycles. This behaviour seems to be promising for their potential use as energy storage devices in e-textiles.

Fig. 4 (a) Photographs of yarn-based hybrid supercapacitor woven in the form of a glove at different bending states; (b) Capacitance retention of the hybrid supercapacitor at different bending angles.

The Ni(OH)₂ nanosheet inks seem to be promising for large-scale production of high-performance energy storage devices for flexible electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to Peipei Shi et al., Highly Concentrated, Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices, Adv. Mater. 2017, 1703455, DOI: 10.1002/adma.201703455

Designing All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties

Flexible electronic devices are gaining momentum in various applications including touch screen panels, solar cells, wearable devices, etc. Susceptibility to the environmental conditions limits their performance. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China have suggested a novel strategy for the design of all-weather flexible superhydrophobic, electrically conductive paper with flame-resistant property.

Hydroxyapatite nanowires (HNs), Ketjen black (KB), and polydimethylsiloxane (PDMS) were used to fabricate the all-weather flexible electrically conductive paper. The HNs were prepared by mixing an aqueous solution (500 ml) containing 2.2 g of CaCl₂, 10 g of NaOH and 2.8 g of NaH₂PO₄·2H₂O with an ethanolic solution (140 g) containing 100 g of oleic acid under stirring followed by hydrothermal treatment at 180 °C for 24 h in a Teflon-lined stainless steel autoclave. The HNs were dispersed in ethanol to form a colloidal suspension. KB was dispersed in ethanol under ultrasonication to obtain a KB colloidal suspension. The colloidal suspension of KB was mixed with the colloidal suspension of HNs under stirring for 10 min. Vacuum-assisted filtration was adopted to fabricate the KB + HNs paper and it was peeled off after drying at 90 °C for 5 min. The KB + HNs paper was immersed in dilute PDMS solution (ratio of PDMS:curing agent:ethylacetate is 10:1:100) at room temperature for 30 min and subsequently cured at 100 °C for 1 h (Fig. 1(a)). The as-prepared KB+HNs+PDMS paper (KHP paper) exhibits a rough morphology (Fig. 1(b)), higher water contact angle (>150°) and low sliding angle (<10°) (Fig. 1(c)) high flexibility (twisted and bent without breaking for 500 cycles) (Fig. 1(d)) and electrically conductive (Fig. 1(e)).

Fig. 1 (a) Schematic illustration of various stages involved in the preparation; (b) morphology; (c) water contact angle; (d) flexibility; and (e) electric conductivity of flexible electrically conductive KB + HNs + PDMS paper.

The high water contact angle (>150°) and a low sliding angle (<10°) enables water droplets to bounce off from the surface and automatically rolled away even at a small tilting angle, thus keeping the KHP paper to keep dry (Fig. 2(a)). The water repellent ability of KHP paper is retained under highly corrosive conditions (pH: 2-13), when heating up to 300 °C for 12 h and when exposed to humid conditions (50 -90% RH) for 24 h. The KHP paper also exhibits self-cleaning ability, which is evidenced by the easy removal of soil by water droplets (Figs. 2(b-e)). Real-time electrical performance of the KHP paper upon wetting, monitored by measuring the resultant current upon applying a potential of 3 V indicates that the water droplets remains stable on the surface without wetting (Figs. 2(f) and 2(g)) and the electrical current was steady from 0 to 10 s (Fig. 2 h and 2(i)). Real-time electrical performance underwater (Fig. 2(j)) also indicates the ability of the KHP paper to exhibit a good stability, as evidenced by the brightness of the LED lamp from 0 to 120 s (Figs. 2(k) and 2(l)).

Fig. 2 (a) Bouncing-off of water droplets; (b-e) self-cleaning ability; (f-l) Real-time monitoring of electrical conductivity: (f-i) with a few water droplets; (j-l) after total immersion in water

The electrothermal effect of the KHP paper is ascertained by applying a direct voltage to the paper covered by copper foils at the edges and measuring change in surface temperature using an infrared thermal imaging camera (Fig. 3(a)). The surface temperature is increased quickly within 10 s and then leveled off (Fig. 3(b)); the higher the applied voltage, the higher the surface temperature of KHP paper. The ability of the KHP paper to retain the rapid thermoresponsive behavior for five cycles indicated its recyclability (Fig. 3(c)). Due to its electrothermal effect, the KHP paper in capable of quickly evaporating tiny water droplet within 128 s (Fig. 3(d)) and deicing of ice within 23 s (Fig. 3(e)).

Fig. 3 (a) Schematic illustration of the surface temperature measurement of the KHP paper; (b) Change in surface temperature with time; (c) stability upon repeated heating/cooling cycles; (d) evaporation of tiny water droplet (3 µL); and (e) deicing of ice.

The KHP paper also exhibits flame retarding characteristics and its electrical conductivity is increased from 11.92 mA to 13.39 mA after exposure to flame for 60 s and stabilized to 13.34 A after 7 min (Fig. 4(a)). Real-time monitoring of the electrical current and the brightness of an LED lamp up to 7 min confirm the ability of the KHP paper to retain electrical conductivity even  under extreme condition of combustion (Fig. 4(b)).

The KHP paper exhibits superhydrophobicity, better flexibility, enhanced mechanical properties, good electrical conductivity, high thermal stability, suitable electrothermal effect and good flame retardancy. Due to its ability to perform well under extreme conditions (underwater as well as in flame), the KHP electrically conductive paper seems to be promising for applications in flexible electronic devices.

T.S.N. Sankara Narayanan

Fig. 4 Real-time monitoring of electrical conductivity of the KHP paper in flame up to 7 min : (a) change in current; and (b) brightness of LED lamps.

For more information, the reader may kindly refer: Fei-Fei Chen et al., Hydroxyapatite Nanowire-Based All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties, ACS Appl. Mater. Interfaces, DOI: 10.1021/acsami.7b09484

 

 

Smart Dental Braces – Pushing the boundary of personalized health care electronics to the next level

Batteries contribute to the overall weight and size of implantable devices such as cardiac peacemakers and neuro-stimulators. The rigid encapsulation and requirement for proper insulation from corrosive materials limit the widespread utility of batteries for implantable devices. Besides weight and design aspects, the batteries have to be biocompatible and offer a high performance. Hence, development of light weight, physically flexible, biocompatible, high performance batteries are highly warranted to meet the demands of advanced personalized health care applications.

Researchers at King Abdullah University of Science and Technology (KAUST), Saudi Arabia, lead by Prof. Muhammad M. Hussain, have demonstrated a transfer-less method to develop a flexible, light-weight, biocompatible, high performance lithium ion batteries (LIBs) for implantable devices. They have also proposed a strategy for integrating the LIBs with flexible electronics and embedding them in a three-dimensional (3D) printed dental brace for orthodontics application (npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7)

Polydimethylsiloxane (PDMS) was used as the carrier substrate. A bulk LIB (thickness: 130 μm) was flipped on to the PDMS substrate. The Si (at the base of the LIB) was etched using xenon difluoride (XeF₂) at a rate of 67 nm/s for 36 min (50 cycles). The surface roughness was monitored using atomic force microscopy (AFM) to optimize the conditions of XeF₂ etching. The thinned LIB was removed from the PDMS (Fig. 1(a)). Complete removal of the Si substrate from the thinned LIB results in a free standing and physically flexible active stack (thickness: 30 μm). It consists of SiO₂ (insulation layer), Al (cathode current collector), lithium cobalt oxide (cathode), lithium phosphorous oxynitride (electrolyte), Ti (anode current collector) and protective layers at the top surface (Fig. 1(b)). In spite of a large reduction in thickness (from 130 μm to 30 μm), the flexible LIB experienced only a lower strain (five times less) when compared to the bulk, for a 10 mm bending. Cell cultures grown on LIB for 3 to 5 days exhibited a healthy proliferation of the cells, which confirmed the biocompatibility of LIB. The flexible LIB possess a light weight (236 μg for each microcell of 2.25 × 1.7 mm) and exhibits a very high energy density (200 mWh/cm³) with a capacity retention of up to 70% after 120 cycles.

Fig. 1 (a) Schematic of the fabrication of flexible LIBs; and (b) cross sectional morphology of the flexible thinned battery and its components

Polyethylene terephthalate (PET) was used as the base for the flexible electronic device. It was metallized with Al (thickness: 0.023 mm). Interconnections were patterned using 1.06 μm ytterbium-doped fiber laser. Flip-chip technology and stencil printing allowed placement of the components. A conductive silver epoxy was used to bond the LIBs and LEDs. The LIB in the flexible electronic device exhibits minimal strain since most of the stress is experienced by the PET film. Transparent orthodontic brace was prepared by 3D printing using a clear resin. Finally, the flexible electronic device is integrated with the 3D printed orthodontic brace. The schematic of the integration of the flexible thinned LIBs with the flexible electronics and 3D printed dental braces is shown in Fig. 2(a). A pictorial representation of how the device fits in conformably onto the human dental arch is shown in Fig. 2(b).

Fig. 2 (a) Schematic of the integration of the flexible thinned LIBs with flexible electronics and 3D printed dental braces; and (b) Pictorial representation of how the device fits in conformable manner onto the human dental arch.

T.S.N. Sankara Narayanan

 

 

For more information, the reader may kindly refer: Arwa T. Kutbee et al., npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7