Active and Deformable Organic Electronic Devices based on Conductive Shape Memory Polyimide

Moisture-Triggered 71000-Fold Stiffness Change Materials Via Crystallization and Hydrogen Bonding

Stiffness change materials have been widely used in soft robots, intelligent adhesives and biomedical minimally invasive devices which can respond to specific stimuli. Compared with stimuli such as light, electricity, magnetism, temperature, and pressure, moisture is a non-toxic, readily available, and inexhaustible triggered source in the atmosphere. However, it is challenging to achieve a stiffness variable material with a large stiffness change using moisture. Traditional sensitive materials usually swell to expand the volume or dissolve to a liquid-like state when absorbing moisture, which is unfavorable for real applications. Herein, we demonstrate a polyvinylamine (PVAm)/polyethylenimine (PEI) composite film with variable stiffness change by moisture absorption and release. PVAm is a semicrystalline polymer with high stiffness and PEI forms intermolecular hydrogen bonding with primary amine groups on PVAm. The synergistic effect of hydrogen bonding and crystallization was maximized when 15 wt % PEI was added to the composite system, resulting in a large stiffness change of up to 7.1 × 104 (0.022 MPa versus 1560.85 MPa) of PVAm/PEI composite film under 25 °C, 95% RH. The crystallinity structure and hydrogen bonding can be broken and reformed by adjusting humidity. As promising variable stiffness polymers, the developed PVAm/PEI composite film is demonstrated as a reconfigurable multitool for shape memory and locking on demand.

Transient Superhydrophilic Surface Modification of Polyimide by Metal Ion Beam Irradiation

Polyimide is commonly used as a substrate for flexible electronic devices because of its excellent thermal, physical, and electrical properties. To enhance the adhesion between substrates and electrodes, it is necessary to improve the hydrophilic properties of the polyimide. Various surface treatments, such as plasma treatment, laser ablation, and ultraviolet treatments, have been applied for this purpose. In this study, we demonstrated that Cu and Ti ion beam irradiation can temporarily create a superhydrophilic surface on polyimide after irradiation. When Cu or Ti ions bombarded the polyimide, the contact angle changed systematically with the beam current density and over time. We present atomic force microscopy (AFM) data for polyimide irradiated with Cu and Ti ions at different beam current densities and discuss the possible mechanisms behind the changes in the contact angle.

A review of chitosan-based shape memory materials: Stimuli-responsiveness, multifunctionalities and applications

Shape memory polymers (SMPs), as a type of smart materials, possess the unique shape memory and deformation recovery abilities. Hence, SMPs have been attracted extensive attentions and widely used in fields of electric devices, aerospace structures and biomedical engineering. Chitosan (CS), as a renewable natural biomass material, exhibits the excellent biocompatibility, biodegradability and antibacterial activities. Using biomass CS as SMPs matrix materials could greatly enhance the environmental friendliness and adaptability, promoting the applications in fields of biomedical engineering and smart devices. This paper provides a detailed overview of current research progress about CS-based SMPs, including diverse stimuli responsiveness, multifunctionalities and various applications. Though, the research on CS-based SMPs is still in the early stage, which exhibits extensive prospect and potential, and could be of significance in advancing smart biomedical technologies.

Sputtered anti-reflection layer on transparent polyimide - substrate improves adhesion strength to - copper layer: effects of layer thickness and sputtering power

In order to shield the electronic circuits on a transparent polyimide (PI) substrate, an anti-reflection (AR) layer was deposited on a PI film via DC reactive magnetron sputtering. The effects of sputtering power and thickness of AR layer on the optical property and adhesion strength of the PI were investigated. The composition of the AR layer influences the bonding between layers. Sufficient thickness of the AR layer is essential to strengthen the adhesion between the PI and copper (Cu) layers. The sputtered AR layer on the PI also improves the barrier property for water vapor. The AR layer-sputtered PI substrates remain transparent and exhibit high peel strength to the Cu layer, suggesting their potential applications as reliable transparent substrates for modern electronic devices.

Research Progress and Application of Polyimide-Based Nanocomposites

Polyimide (PI) is one of the most dominant engineering plastics with excellent thermal, mechanical, chemical stability and dielectric performance. Further improving the versatility of PIs is of great significance, broadening their application prospects. Thus, integrating functional nanofillers can finely tune the individual characteristic to a certain extent as required by the function. Integrating the two complementary benefits, PI-based composites strongly expand applications, such as aerospace, microelectronic devices, separation membranes, catalysis, and sensors. Here, from the perspective of system science, the recent studies of PI-based composites for molecular design, manufacturing process, combination methods, and the relevant applications are reviewed, more relevantly on the mechanism underlying the phenomena. Additionally, a systematic summary of the current challenges and further directions for PI nanocomposites is presented. Hence, the review will pave the way for future studies.

Electrothermally Actuated Semitransparent Shape Memory Polymer Composite with Application as a Wearable Touch Sensor

A semitransparent shape memory polymer (SMP):silver nanowire (AgNW) composite is demonstrated to be capable of low-temperature actuation, thus making it attractive for wearable electronics applications that require intimate contact with the human body. We demonstrate that the SMP:AgNW composite has tunable electrical and optical transparency through variation of the AgNW loading and that the AgNW loading did not significantly change the mechanical behavior of the SMP. The SMP composite is also capable of electrical actuation through Joule heating, where applying a 4 V bias across the AgNWs resulted in full shape recovery. The SMP was found to have high strain sensitivity at both small (<1%) and large (over 10%) applied strain. The SMP could sense strains as low as 0.6% with a gauge factor of 8.2. The SMP composite was then utilized as a touch sensor, able to sense and differentiate tapping and pressing. Finally, the composite was applied as a wearable ring that was thermally actuated to conformably fit onto a finger as a touch sensor. The ring sensor was able to sense finger tapping, pressing, and bending with high signal-to-noise ratios. These results demonstrate that SMP:AgNW composites are a promising design approach for application in wearable electronics.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Surface-microstructured cellulose films toward sensitive pressure sensors and efficient triboelectric nanogenerators

To achieve environmental sustainability, cellulose-based functional materials have been extensively used in advanced electronic devices, such as pressure sensor and triboelectric nanogenerator (TENG). Here, we fabricate the surface-microstructured cellulose films (M-CFs) by facile regeneration and hot pressing combined with screen mesh templating. Through simple carbonization, the M-CFs are further converted into the surface-microstructured carbonized cellulose films (M-CCFs) with a good conductivity but maintain the original array concave-pits on surface. These constructed microstructures, which are tunable via controlling the screen mesh's aperture, endow the assembled electronics with adjustable and improved working performance. The pressure sensors with M-CCFs as active materials exhibit an enhanced sensitivity in a wide working range and promising potentials for applications in motions detection and healthcare. The TENGs with M-CFs as tribo-positive friction layers demonstrate higher electrical output and an efficient energy harvesting. Our work provides novel insights into the design and construction of cellulose-based functional films for eco-friendly advanced applications.

Fractal-Based Stretchable Circuits via Electric-Field-Driven Microscale 3D Printing for Localized Heating of Shape Memory Polymers in 4D Printing

Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-field-driven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

Templateless, Plating-Free Fabrication of Flexible Transparent Electrodes with Embedded Silver Mesh by Electric-Field-Driven Microscale 3D Printing and Hybrid Hot Embossing

Flexible transparent electrodes (FTEs) with an embedded metal mesh are considered a promising alternative to traditional indium tin oxide (ITO) due to their excellent photoelectric performance, surface roughness, and mechanical and environmental stability. However, great challenges remain for achieving simple, cost-effective, and environmentally friendly manufacturing of high-performance FTEs with embedded metal mesh. Herein, a maskless, templateless, and plating-free fabrication technique is proposed for FTEs with embedded silver mesh by combining an electric-field-driven (EFD) microscale 3D printing technique and a newly developed hybrid hot-embossing process. The final fabricated FTE exhibits superior optoelectronic properties with a transmittance of 85.79%, a sheet resistance of 0.75 Ω sq^-1 , a smooth surface of silver mesh (R_a  ≈ 18.8 nm) without any polishing treatment, and remarkable mechanical stability and environmental adaptability with a negligible increase in sheet resistance under diverse cyclic tests and harsh working conditions (1000 bending cycles, 80 adhesion tests, 120 scratch tests, 100 min ultrasonic test, and 72 h chemical attack). The practical viability of this FTE is successfully demonstrated with a flexible transparent heater applied to deicing. The technique proposed offers a promising fabrication strategy with a cost-effective and environmentally friendly process for high-performance FTE.