Quantum Dot Hybridization of Piezoelectric Polymer Films for Non-Transfer Integration of Flexible Biomechanical Energy Harvesters.

This work presents a low-temperature engineering strategy, from quantum dot (QD) synthesis to fabrication of a hybrid from a homogeneous dispersion to thermal annealing with elaborate use of a small organic molecule dopamine, for achieving a kind of ZnO QD-hybridized piezoelectric polymer film directly integrated into a flexible electrode and a plastic substrate. This strategy is the key for non-transfer assembly of flexible piezoelectric nanogenerators (FPENGs) with both mechanical robustness and high electrical performance via direct lamination. The rational addition of dopamine plays multiple roles of (1) significantly decreasing the size of ZnO particles to a QD level (3.77 nm), (2) formation of a stable and homogeneous dispersion of a ZnO QDs/piezoelectric polyvinylidene fluoride-co-hexafluoropropylene copolymer for uniform hybridization of a piezoelectric film, and (3) increment of the piezoelectric phase via induced crystallization at a low annealing temperature. This dopamine-assisted low-temperature annealing strategy for a hybrid piezoelectric film with a high d33 value (∼31.56 pC/N, 30.56% larger than that of a pure piezoelectric polymer film) required no additional high-voltage polarization treatment and effectively avoided the delamination, distortion, or melt phenomenon between the piezoelectric layer, flexible electrode, and plastic protective layer caused by the high temperature and thermal stress. The obtained FPENGs showed significantly enhanced output performance and mechanical robustness under repeated impact and large amounts of strain conditions. Their specific output voltage and charge density were stably maintained at 7.16 V and 2.40 nC/cm2, which were 30.7 and 50.0% higher than those of FPENGs based on a pure piezoelectric polymer film, respectively. They were further used as biomechanical energy harvesters for generating electricity to charge capacitor energy storage devices for power electronics and self-powered sensors for visual motion-detecting systems, indicating their promising applications in both wearable technology and smart homes.

Three-Dimensional Conformal Porous Microstructural Engineering of Textile Substrates with Customized Functions of Brick Materials and Inherent Advantages of Textiles.

The conventional use of textiles as substrates for the incorporation of brick materials (i.e., polymers and nanomaterials) is ubiquitously developed with primary purposes for introducing desired technical/functional performance rather than maintaining the aesthetic/decorative characteristics and inherent advantages (i.e., flexibility and permeability) of textiles. Such kinds of modified textiles with typical solid coating layers, however, are becoming more and more unsuitable for some emerging applications, such as smart wearable devices. Herein, we presented a brand-new kind of modified textiles with brick materials formed contouring to the nonplanar fiber surfaces of a fabric substrate as a three-dimensional (3D) conformal layer of porous microstructures by a unique breath figure self-assembling strategy of employing water microdroplet arrays as soft dynamic templates that can be controlled, formed, and removed spontaneously. In this paper, the main influential factors such as solution concentration, relative humidity, temperature, brick materials, and fabric substrates were studied systematically to control and adjust the formation of 3D conformal porous microstructures (3CPMs). The obtained 3D conformal porous microstructured textiles (3CPMTs) hierarchically combining the inherent texture features of the porous network of textiles and honeycomb porous microstructures templated from water microdroplet arrays not only possess new functions of introduced brick materials (such as triboelectric performance and wettability) and maintain the excellent inherent advantages (such as flexibility, air permeability, water vapor permeability, and unique texture features) of fabrics but also enhance the tensile strength and thermal insulation performance of substrates. Taking advantage of the introduced functions, they can be either used for conventional applications (i.e., oil/water separation) with enhanced performance or explored for new applications (i.e., self-powered sensors with textile breathability and comfort) with truly wearable potential. We believe this efficient, robust, and versatile strategy opens up numerous possibilities for designing and developing a broad range of advanced multifunctional textiles upon end uses.

An Adhesive Surface Enables High-Performance Mechanical Energy Harvesting with Unique Frequency-Insensitive and Pressure-Enhanced Output Characteristics.

Viscoelastic polymer adhesives (VPAs) are common materials broadly used in adhesive tapes for bonding objects tightly in daily life. This work presents a conceptually new strategy of using contact electrification (rather than strong adhesion) of VPAs to directly convert mechanical energy to electric energy, generally showing 202-419% of the electric energy generated by conventional mechanical energy harvesters under the same triggering conditions. More notably, the VPA-based generators (VPAGs) possess unique frequency-insensitive and pressure-enhanced output characteristics. The output power of a VPAG not only does not show regular degradation of performance with the decrease of triggering frequency, but also can be further enhanced by simple introduction of a second VPA layer with a smaller area to increase the applied pressure without the requirement of rising applied force. The average output power density of a VPAG with a second layer of 0.5 cm × 0.5 cm can reach 216.7 µW cm-2 , which is ≈150% larger than that of a VPAG without a second VPA layer. This research is of significance to harvesting the random, irregular, and low-frequency (bio-)mechanical energy that widely exists but is wasted in the environment for both stable electric energy generation and electronic device operation.

Design of High-Performance Wearable Energy and Sensor Electronics from Fiber Materials.

A fiber material is composed of a group of flexible fibers that are assembled in a certain dimensionality. With its good flexibility, high porosity, and large surface area, it demonstrates a great potential in the development of flexible and wearable electronics. In this work, a kind of nickel/active material-coated flexible fiber (NMF) electrodes, such as Ni/MnO2/reduced graphene oxide (rGO) NMF electrodes, Ni/carbon nanotube (CNT) NMF electrodes, and Ni/G NMF electrodes, is developed by a new general method. In contrast with previous approaches, it is for the first time that porous and rich hydrophilic structures of fiber materials have been used as the substrate to fully absorb active materials from their suspension or slurry and then to deposit a Ni layer on active material-coated fiber materials. The proposed processes of active material dip-coating and then Ni electroless plating not only greatly enhance the electrical conductivity and functional performance of fiber materials but also can be applied to an extensive diversity of fiber materials, such as fabrics, yarns, papers, and so on, with outstanding flexibility, lightweight, high stability, and conductivity for making kinds of energy and sensor devices. As demonstration, a two-dimensional (2D) Ni/MnO2/rGO NMF electrode is obtained for supercapacitors, showing excellent electrochemical performance for energy storage. Then, Ni/CNT NMF electrodes with different dimensionalities, including one-dimensional fiber-shaped, 2D plane, and three-dimensional spatial, are fabricated as various tensile and compressive strain sensors for observation of human's movements and health. Finally, a 2D Ni/graphene NMF electrode is developed for assembling triboelectric nanogenerators for mechanical energy harvesting. Benefiting from wearable property of the textile substrates, the obtained NMF electrodes are expected to be designed into kinds of wearable devices for the future practical applications. The NMF electrode designed in this work provides a simple, stable, and effective approach for designing and fabricating wearable energy and sensor electronics from fiber materials.

Design of Novel Wearable, Stretchable, and Waterproof Cable-Type Supercapacitors Based on High-Performance Nickel Cobalt Sulfide-Coated Etching-Annealed Yarn Electrodes.

Rapid advances in functional electronics bring tremendous demands on innovation toward effective designs of device structures. Yarn supercapacitors (SCs) show advantages of flexibility, knittability, and small size, and can be integrated into various electronic devices with low cost and high efficiency for energy storage. In this work, functionalized stainless steel yarns are developed to support active materials of positive and negative electrodes, which not only enhance capacitance of both electrodes but can also be designed into stretchable configurations. The as-made asymmetric yarn SCs show a high energy density of 0.0487 mWh cm-2 (10.19 mWh cm-3 ) at a power density of 0.553 mW cm-2 (129.1 mW cm-3 ) and a specific capacitance of 127.2 mF cm-2 under an operating voltage window of 1.7 V. The fabricated SC is then made into a stretchable configuration by a prestraining-then-releasing approach using polydimethylsiloxane (PDMS) tube, and its electrochemical performance can be well maintained when stretching up to a high strain of 100%. Moreover, the stretchable cable-type SCs are stably workable under water-immersed condition. The method opens up new ways for fabricating flexible, stretchable, and waterproof devices.

Three-Dimensionally Conformal Porous Polymeric Microstructures of Fabrics for Electrothermal Textiles with Enhanced Thermal Management.

Three-dimensionally conformal porous microstructured fabrics (3CPMFs) are a new kind of modified fabrics with three-dimensionally conformal porous microstructures of introduced materials recently developed for wearable technology. They can effectively introduce customized functional performance based on the choice of brick materials, while at the same time maintain the excellent inherent properties of textiles. In this paper, based on the introduction of polystyrene with low thermal conductivity at only 8 × 10-4 g cm-2, we developed a kind of polyester fabric-based 3CPMF with enhanced thermal insulation, while maintaining its unique fabric texture, flexibility, moisture permeability, and light weight. It was demonstrated to be a good textile material for the fabrication of wearable electrothermal textile (ET) devices with enhanced thermal management. Compared to pristine fabric-based ET devices, this kind of 3CPMF-based ET devices can obtain higher temperatures under the same input power to provide thermal comfort for human beings, while saving more electric power to achieve the same thermal equilibrium temperature. We believe that, based on the choice of different functional materials and textiles, a wide range of 3CPMFs with customized functionalities and properties can be designed and developed for the realization of a brand-new class of truly wearable devices with desired functional performance and daily garment-like safety and comfort.

Three-Dimensionally Conformal Porous Microstructured Fabrics via Breath Figures: A Nature-Inspired Approach for Novel Surface Modification of Textiles.

Breath figures (BFs) are a kind of water droplet arrays that can be formed by condensing aqueous vapor onto a cold surface, such as dewy phenomenon on a spider web. This study developed a BF-inspired approach for direct introduction of desired materials onto the textile surfaces with three-dimensionally conformal porous microstructures by the evaporation of solution-coated fabric under high humidity environment, which brings a brand-new kind of modified textiles, three-dimensionally conformal porous microstructured fabrics (CPMFs). Such kind of CPMFs can possess customized multifunctional properties of introduced materials, and meanwhile maintain the inherent properties and unique texture features of fabrics. This nature-inspired BF approach is robust and versatile for customized preparation of CPMFs based on different fabrics with different common polymers. Moreover, it is also feasible for one-step functionalization of CPMFs by the incorporation of nanoparticles (such as titanium dioxide nanoparticles, TiO2 NPs) into the porous microstructures during the BF process. Comparing to the sample modified without porous microstructures, the resultant TiO2 NPs-incorporated CPMFs show an obviously enhanced performance on photocatalytic degradation of pollutants under the same ultraviolet irradiation conditions.

Breath Figure Micromolding Approach for Regulating the Microstructures of Polymeric Films for Triboelectric Nanogenerators.

A triboelectric nanogenerator (TENG) is an innovative kind of energy harvester recently developed on the basis of organic materials for converting mechanical energy into electricity through the combined use of the triboelectric effect and electrostatic induction. Polymeric materials and their microstructures play key roles in the generation, accumulation, and retainment of triboelectric charges, which decisively determines the final electric performance of TENGs. Herein we report a simple and efficient breath figure (BF) micromolding approach to rapidly regulate the surface microstructures of polymeric films for the assembly of TENGs. Honeycomb porous films with adjustable pore size and dimensional architectures were first prepared by the BF technique through simply adjusting the concentration of the polymer solution. They were then used as negative molds for straightforward synthesis of polydimethylsiloxane (PDMS) films with different microlens arrays (MLAs) and lens sizes, which were further assembled for TENGs to investigate the influence of film microstructures. All MLA-based TENGs were found to have an obviously enhanced electric performance in comparison with a flat-PDMS-film-based TENG. Specifically, up to 3 times improvement in the electric performance can be achieved by the MLA-based TENG with optimal surface microstructures over flat-PDMS-film-based TENG under the same triggering conditions. A MLA-based TENG was further successfully used to harvest the waste mechanical energy generated by different human body motions, including finger tapping, hand clapping, and walking with a frequency ranging from 0.5 to 5.5 Hz.

Binary breath figures for straightforward and controllable self-assembly of microspherical caps.

The intense interest surrounding asymmetrical microparticles originates from their unique anisotropic properties and promising applications. In this work, direct self-assembly of polymeric microspherical caps without the assistance of any additives has been achieved by using low-surface-tension methanol (MeOH) and high-surface-tension water as binary breath figures (BFs). With the evaporation of polystyrene (PS) solution containing low-boiling-point solvent in the binary vapors, the formed MeOH BFs could quickly diffuse into solution, while water BFs tended to remain at the solution surface. This led to the formation of a gradient nonsolvent layer at the vapor/solution interface, which induced the formation of nuclei and guided further asymmetrical growth of polymer particles. After the spontaneous removal of MeOH, water and residual solvent by evaporation, polymeric microspherical caps were left on the substrate. Through controlling the proportion of water introduced by adjusting the ratios of MeOH and water, polymeric microspherical caps with a range of controllable shapes (divided at different positions of a sphere) were successfully obtained. The formation mechanism was explained based on the difference of vapor pressure, surface tension and miscibility between the employed solvents and nonsolvents. A solvent possessing a high vapor pressure, low surface tension and good miscibility with MeOH contributed to the formation of microspherical caps. This flexible, green and straightforward technique is a nondestructive strategy, and avoids complicated work on design, preparation and removal of hard templates and additives.

Formation of nanoscale networks: selectively swelling amphiphilic block copolymers with CO2-expanded liquids.

Polymeric films with nanoscale networks were prepared by selectively swelling an amphiphilic diblock copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), with the CO(2)-expanded liquid (CXL), CO(2)-methanol. The phase behavior of the CO(2)-methanol system was investigated by both theoretical calculation and experiments, revealing that methanol can be expanded by CO(2), forming homogeneous CXL under the experimental conditions. When treated with the CO(2)-methanol system, the spin cast compact PS-b-P4VP film was transformed into a network with interconnected pores, in a pressure range of 12-20 MPa and a temperature range of 45-60 °C. The formation mechanism of the network, involving plasticization of PS and selective swelling of P4VP, was proposed. Because the diblock copolymer diffusion process is controlled by the activated hopping of individual block copolymer chains with the thermodynamic barrier for moving PVP segments from one to another, the formation of the network structures is achieved in a short time scale and shows "thermodynamically restricted" character. Furthermore, the resulting polymer networks were employed as templates, for the preparation of polypyrrole networks, by an electrochemical polymerization process. The prepared porous polypyrrole film was used to fabricate a chemoresistor-type gas sensor which showed high sensitivity towards ammonia.

Constructing honeycomb micropatterns on nonplanar substrates with high glass transition temperature polymers.

In Qiao's previous report, only star polymers with T(g) (glass transition temperature) below 48°C were found forming homogeneous honeycomb coatings on the nonplanar substrates. The polymers with high T(g) are believed not able to duplicate nonplanar substrate due to their brittleness. This article presents a comprehensive study on the construction of macroporous polymeric films on various nonplanar substrates with static breath figure (BF) technique, using linear polymers with high T(g). Two kinds of linear polymers with high T(g), polystyrene-b-poly(acrylic acid) and polystyrene without polar end groups, are employed to prepare 3-dimensional macroporous films on different nonplanar substrates. Scanning electronic microscopy views on the side wall in addition to views in-plane prove that polymer films with BF array perfectly replicated the surface features of these substrates. The formation processes of macropores on these substrates are analyzed in detail, and it demonstrates that neither molecular topography nor T(g) of polymers is the critical factor contouring nonplanar substrate. A new hypothesis involving polymer plasticization and conformation during the solvent evaporation is formulated.

Fabrication of multi-level carbon nanotube arrays with adjustable patterns.

Multi-level carbon nanotube (CNT) arrays with adjustable patterns were prepared by a combination of the breath figure (BF) process and chemical vapor deposition. Polystyrene-b-poly(acrylic acid)/ferrocene was dissolved in carbon disulfide and cast onto a Si substrate covered with a transmission electron microscope grid in saturated relative humidity. A two-level microporous hybrid film with a block copolymer skeleton formed on the substrate after evaporation of the organic solvent and water. One level of ordered surface features originates from the contour of the hard templates; while the other level originates from the condensation of water droplets (BF arrays). Ultraviolet irradiation effectively cross-linked the polymer matrix and endowed the hybrid film with improved thermal stability. In the subsequent pyrolysis, the incorporated ferrocene in the hybrid film was oxidized and turned the polymer skeleton into the ferrous inorganic micropatterns. Either the cross-linked hybrid film or the ferrous inorganic micropatterns could act as a template to grow the multi-level CNT patterns, e.g. isolated and honeycomb-structured CNT bundle arrays perpendicular to the substrate.

Fabrication of robust micro-patterned polymeric films via static breath-figure process and vulcanization.

Here, we present the preparation of thermally stable and solvent resistant micro-patterned polymeric films via static breath-figure process and sequent vulcanization, with a commercially available triblock polymer, polystyrene-b-polyisoprene-b-polystyrene (SIS). The vulcanized honeycomb structured SIS films became self-supported and resistant to a wide range of organic solvents and thermally stable up to 350°C for 2h, an increase of more than 300K as compared to the uncross-linked films. This superior robustness could be attributed to the high degree of polyisoprene cross-linking. The versatility of the methodology was demonstrated by applying to another commercially available triblock polymer, polystyrene-b-polybutadiene-b-polystyrene (SBS). Particularly, hydroxy groups were introduced into SBS by hydroboration. The functionalized two-dimensional micro-patterns feasible for site-directed grafting were created by the hydroxyl-containing polymers. In addition, the fixed microporous structures could be replicated to fabricate textured positive PDMS stamps. This simple technique offers new prospects in the field of micro-patterns, soft lithography and templates.

Breath figure lithography: A facile and versatile method for micropatterning.

We describe a facile method to micropattern solid substrates: breath figure lithography (BFL). A honeycomb structured gold mask was prepared by sputter-coating a micro-porous polymer film with BF arrays, and then inductively coupled plasma reactive ion etching (ICP-RIE) transferred the patterns onto silicon wafer. The large etching rate selectivity between golden mask and substrate plays an important role in the effective transfer of the patterns. The versatility of the method was demonstrated by forming micropatterns on various solid substrates with adjustable sizes. Furthermore, the micropatterns on solid substrate could be replicated by PDMS stamp.