Materials, processing, and structural strategies for encapsulation in stretchable and flexible optoelectronics

In our research, development of encapsulating material of light emitting diodes (LEDs) is the mainly purpose. In the first part (Chapter 2), optical hybrid materials with high refractive index were synthesis by in-situ production of titanium dioxide (TiO2) directly in a commercial-grade silicone resin via a sol-gel reaction. The optical transparency of the prepared TiO2/silicone hybrid film was investigated by UV-visible spectroscopy. These hybrid films with various TiO2 contents exhibited different refractive indices; the refractive index could reach 1.66 for the hybrid with 30 wt% of the TiO2. In addition, refractive index, thermal stability of the cured hybrid materials was also investigated. In the second part (Chapter 3), novel ZrO2/silicone hybrid materials (ZrO2/AB) useful for the encapsulation of light-emitting diodes (LEDs) are synthesized by an in situ sol-gel reaction of zirconium propoxide directly in a commercial-grade silicone resin by an in situ sol-gel reaction. By the chelation of the acetic acid, the ZrO2/AB hybrids exhibit high transparency owing to the well-dispersed ZrO2 particles in the silicone material, and their refractive index value also increased with increasing weight percentage of the ZrO2 in the hybrid. These high-refractive-index ZrO2/AB hybrids were then used as encapsulating materials to improve the luminous flux of the LEDs. From the results of the luminous flux measurement, the LED encapsulated with the pure silicone material has luminous flux of 3.97 lm. After encapsulated with the ZrO2/AB hybrid, the luminous flux of the LED was enhanced to a value of 4.35, which revealing that the increase in the refractive index of the encapsulating material by the incorporation of the ZrO2 could effectively improved the luminous flux of the LED, and these novel ZrO2/AB hybrids could also be considered as a suitable candidate as the encapsulating material for the LED. In the third part (Chapter 4), ZrO2 particles are synthesized directly in a commercial-grade silicone resin (AB) with the addition of a vinyl monomer (glycidyl methacrylate, GMA) by an in situ sol-gel reaction to obtain the silicone hybrid material (ZrO2/ABG). In addition to use the acetic acid as chelating agent, the addition of the GMA is able to retard the gelation rate of the metal alkoxide compound as well as the growing rate of ZrO2 during the sol-gel reaction, and also enhance the interfacial strength of the inorganic fillers with the silicone matrix by reacting with the silicone resin through a hydrosilation reaction via its vinyl group. The ZrO2/ABG hybrids thus exhibit excellent transparency and high refractive index. These ZrO2/ABG hybrids were then used as encapsulating materials to help the LEDs reach a higher luminous flux. Furthermore, according to the optical theory and the structure of the high-power LEDs, a simple simulation model was developed to estimate the luminous flux of the LEDs encapsulated with the ZrO2/ABG hybrids and the influence of the refractive index of the passivation layer on the luminous flux of the LEDs. Finally, with the help of this simulation model, an optimum combination of the encapsulating material and the passivation layer with appropriate refractive index values was obtained for achieving the highest luminous flux of LED. In the fourth part (Chapter 5), the purpose is to improve thermal mechanical property of the encapsulating materials in a very large temperature difference condition., thus decreasing the influence of the encapsulation on the reliability performance of the LED. The PBA(polybutylacrylate) oliogomer was firstly synthesized by a telomerization using 2-mercaptoethanol as a chain transfer agent. By the absorption to the inorganic particles and the carbonyl groups, the ZrO2 particles were produced on the oligomer chains of the PBA via an in situ sol-gel reaction to obtain the ZrO2-PBA composite particles, and subsequently mixed with a commercial-grade silicone resin to obtain the ZrO2-PBA/silicone hybrid materials (ZrO2-PBA/AB). The well-dispersed ZrO2-PBA domains lead to a high transparency and high refractive index of the ZrO2-PBA/AB hybrids, resulting in a higher luminous flux than the pristine silicone material. In addition to discuss the transmittance, refractive index, thermal resistance, and the water uptake of the ZrO2-PBA/AB hybrids, thermal stress evaluation of the LEDs encapsulated with the ZrO2-PBA/AB hybrids was also carried out in a thermal shock experiment with a temperature change from -35 °C to 125 °C within 15 mins. The results show that these ZrO2-PBA/AB hybrids have a lower thermal stress and exhibit a better mechnical resistance against the thermal shock, and the reliability performance of the LEDs are also greatly improved as encapsulated with the ZrO2-PBA/AB hybrids.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

Hydrogen-bonding topological remodeling modulated ultra-fine bacterial cellulose nanofibril-reinforced hydrogels for sustainable bioelectronics

Flexible plasmon metasensor devices describe the use of multiple Ag/Al2O3/mica layers for tunable plasmonic resonances and are a promising research direction. Here, we report on a flexible Ag/Al2O3/mica multilayer platform and its excellent performance on flexible biosensors and photon-emitting devices. In our approach, muscovite (mica) was adopted as a single-crystal substrate due to its optical transparency and mechanical flexibility. The Ag/Al2O3/mica multilayer film is characterized by X-ray diffraction and transmission electron microscopy. Optical, plasmonic, and biosensing studies of Ag/Al2O3/mica multilayers are performed for detailed understanding. A combination of optical absorption, numerical simulations, and optical reflectance measurements has confirmed the biosensor performance. Two kinds of flexible plasmonic device applications are reported here, including (1) plasmonic biosensors with high refractive index sensitivities and (2) significantly enhanced spontaneous photoluminescence (PL) of monolayer tungsten disulfide (WS2) spectra. We found that the PL emission under 0.4 mm-1 curvature bending state increased to 16% compared to the unbent state and redshift of 60 meV/% strain in the emission of WS2 monolayer. Furthermore, the Ag/Al2O3/mica multilayer film displays robust stability and strong endurance up to a bending curvature of 0.4 mm-1. This study shows great potential to be used for biosensors and flexible optoelectronics.

The piezoelectric characteristics of transducer crystals may be expressed in terms of electromechanical coupling coefficient, mechanical compliance coefficient, and dielectric constant, all referring to the mode to be applied, and the density. The modes considered are face shear (utilized in the torque “Bimorph”), lateral expansion, thickness expansion, and thickness shear. Properties not readily expressed in circuit terms but of the highest practical importance are dielectric and mechanical strength and permissible temperature rise. Rochelle salt offers a coupling coefficient of 0.5 or more, combined with fairly high mechanical compliance and high dielectric constant for face shear and lateral expansion. Quartz combines a rather low coupling of about 0.1 with low electric compliance; these factors are disadvantageous for most transducer applications but may be outweighed by excellent stability. The newer crystals, ammonium dihydrogen phosphate (ADP) and lithium sulfate monohydrate (LH), combine fair stability against atmospheric conditions with intermediate dielectric constant and couplings about 0.3, the former for face shear and lateral expansion, the latter for thickness and volume expansion. Neutral potassium tartrate (DKT) in face shear or lateral expansion is indicated for transducer uses requiring fairly high coupling, up to 0.25, with a low temperature coefficient of resonant frequency. Polarized barium titanate ceramic takes its place besides the single crystals. It combines low mechanical compliance and very high dielectric constant with coupling of 0.20 for the lateral expansion and 0.45 for the thickness expansion. It may be shaped into curved transducer elements. A comparative table for these and some other substances will be given.

Organic piezoelectric materials have garnered increasing interest for biomechanical applications; however, their practical appeal has been limited by low piezoelectricity and low mechanical compliance. Recently, liquid–liquid interface polar engineering has emerged as a promising strategy to overcome these limitations. By utilizing polarity asymmetry at the liquid–liquid interface, this strategy enables precise control over molecular assembly and phase separation, thereby inducing high piezoelectric polarization of organic materials while maintaining their intrinsic mechanical compliance. This breakthrough paves the way for developing organic piezoelectric materials that exhibit both high piezoelectricity and high mechanical compliance, advancing their potential in biomechanical sensing, actuating, and energy harvesting.

In the advancing landscape of technology and novel material development, additive manufacturing (AM) is steadily making strides within the biomedical sector. Moving away from traditional, one-size-fits-all implant solutions, the advent of AM technology allows for patient-specific scaffolds that could improve integration and enhance wound healing. These scaffolds, meticulously designed with a myriad of geometries, mechanical properties, and biological responses, are made possible through the vast selection of materials and fabrication methods at our disposal. Recognizing the importance of precision in the treatment of bone defects, which display variability from macroscopic to microscopic scales in each case, a tailored treatment strategy is required. A patient-specific AM bone scaffold perfectly addresses this necessity. This review elucidates the pivotal role that customized AM bone scaffolds play in bone defect treatment, while offering comprehensive guidelines for their customization. This includes aspects such as bone defect imaging, material selection, topography design, and fabrication methodology. Additionally, we propose a cooperative model involving the patient, clinician, and engineer, thereby underscoring the interdisciplinary approach necessary for the effective design and clinical application of these customized AM bone scaffolds. This collaboration promises to usher in a new era of bioactive medical materials, responsive to individualized needs and capable of pushing boundaries in personalized medicine beyond those set by traditional medical materials.

Electronic skins (e-skins) seek to go beyond the natural human perception, e.g., by creating magnetoperception to sense and interact with omnipresent magnetic fields. However, realizing magnetoreceptive e-skin with spatially continuous sensing over large areas is challenging due to increase in power consumption with increasing sensing resolution. Here, by incorporating the giant magnetoresistance effect and electrical resistance tomography, we achieve continuous sensing of magnetic fields across an area of 120 × 120 mm2 with a sensing resolution of better than 1 mm. Our approach enables magnetoreceptors with three orders of magnitude less energy consumption compared to state-of-the-art transistor-based magnetosensitive matrices. A simplified circuit configuration results in optical transparency, mechanical compliance, and vapor/liquid permeability, consequently permitting its imperceptible integration onto skins. Ultimately, these achievements pave the way for exceptional applications, including magnetoreceptive e-skin capable of undisturbed recognition of fine-grained gesture and a magnetoreceptive contact lens permitting touchless interaction.

Homogeneous organic–inorganic hybrid materials with excellent thermal and optical performance were successfully prepared via the mixing of (3‐methacryloxypropyl)trimethoxysilane‐capped acrylic resin with titania synthesized by sol–gel approaches. The effects of the titania content, amount of water, and pH in the sol–gel and the ratio of the solvents (butyl acetate and ethanol) on the structures and properties of the hybrid materials were studied. More titania, more water, and base catalysis in the sol–gel process and a higher ratio of butyl acetate to ethanol for the hybrids led to a larger size and a more compact structure of the titania phase. Increasing the titania content dramatically improved the glass‐transition temperature, UV‐shielding properties, and refractive index for the hybrid materials, and it did not reduce their optical transparency. In general, better thermal stability and optical properties were achieved for the hybrid materials prepared with less water, lower pH values (e.g., acidic or neutral conditions), and a lower ratio of butyl acetate to ethanol. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 637–649, 2005

Efficient heat dissipation is vital for emerging high‐power electronic systems, where interfacial thermal resistance remains one of the primary limitations to reliable operation. Here, we report a mechanically perforated, vertically aligned graphene paper/epoxy composite produced through an integrated perforation–stacking–cutting process. The perforation strategy not only suppresses graphene restacking but also creates interlayer phonon bridges, thereby establishing continuous through‐plane heat conduction pathways while simultaneously strengthening matrix–filler interfacial coupling. By optimizing the filler volume fraction, the composite delivers an ultrahigh through‐plane thermal conductivity of 456 W m −1 K −1 combined with a low compressive modulus of 4.8 MPa, ensuring excellent conformability and reduced interfacial resistance. The PGPE 28 sample exhibits a total thermal resistance of 0.23 K cm 2 W −1 and outstanding reliability, maintaining structural and thermal stability after 10 000 thermal‐shock cycles and 8 h of continuous operation at 60 W. When deployed at the device level, PGPE 28 markedly lowers the steady‐state temperature of LED and CPU modules compared with commercial thermal interface materials (HD90000 and Shin‐Etsu 7921) and retains robust performance under fan‐failure conditions. This work provides a scalable route for manufacturing graphene‐based TIMs that simultaneously achieve high thermal conductivity and mechanical compliance, offering a promising solution for thermal management.

The dramatic growth of smart wearable electronics has generated a demand for conductive hydrogels due to their tunability, stimulus responsiveness, and multimodal sensing capabilities. However, the substantial trade-off between mechanical and electrical properties hinders their multifunctionality. Here, we report a double-network hydrogel composite that features a conductive "highway" constructed using magnetic-field-aligned nickel nanowires and liquid metal. The liquid metal fills the gaps between the aligned nickel nanowires. Such interconnected structures can form efficient conductive paths at low filler content, resulting in high conductivity (1.11 × 104 S/m) and mechanical compliance (Young's modulus, 89 kPa; toughness, 721 kJ/m3). When used as a wearable sensor, the hydrogel displays a high sensitivity and fast response for wireless motion detection and human-machine interaction. Furthermore, by exploiting its outstanding conductivity and electrical heating capacity, the hydrogel integrates electromagnetic shielding and thermal management functionalities. Owing to these all-around properties, our design offers a broader platform for expanding hydrogel applications.

Organic electrochemical transistors (OECTs) require electrolytes that unite high ionic conductivity and mechanical compliance with intrinsic biocompatibility and sustainability, a combination difficult to achieve by conventional aqueous liquids, ionic liquids, or synthetic hydrogels. Here, we demonstrate that a natural aloe vera gel can serve as a single-component quasi-solid electrolyte for high-performance OECTs, leveraging its native minerals (Ca2+, K+, Na+, Mg2+ ions) and high-water content. Rheological tests confirm a soft, viscoelastic gel network, and electrical measurements reveal an ionic conductivity of ∼6.1 mS/cm with interfacial capacitance of ∼58.5 μF/cm2. In OECT devices, the aloe electrolyte yields high-performance transistors: peak transconductance of ∼11 mS and ON-current of ∼6.5 mA, comparable to conventional phosphate buffered saline (PBS) gating. Transient analysis confirmed rapid switching kinetics, with high stability demonstrated by >96% drain current retention after 100 cycles. These results establish natural aloe vera gel as a unique, non-diffusion-limited, inherently biocompatible and biodegradable gating medium, overturning expectations of slow solid-state performance and charting a path toward truly sustainable ("green") bioelectronic systems.

Poly(amic acid) (PAA) was prepared by the reaction of 4,4-(hexafluoroisopropylidene)diphthalic anhydride with m-xylylenediamine in N,N-dimethylacetamide. Hybrid films were obtained from solutions of the precursor polymer blended with varying amounts of pristine saponite (SPT) clay (0–25 wt%). The cast PAA film was heat-treated at different temperatures to create polyimide (PI) hybrid films, which showed excellent optical transparencies and were almost colorless. The wide-angle X-ray diffraction and transmittance electron microscopy results for the PI hybrid films showed a substantial increase in the agglomeration of the clay particles as the clay loading was increased from 10–25 wt%. This finding suggests that in hybrid materials with low clay contents, the clay particles are better dispersed in the matrix polymer and do not agglomerate significantly. The addition of some amount of clay was sufficient to improve the thermal and oxygen barrier properties of the PI, with a maximum improvement observed at 20 wt% SPT. Open image in new window

Core–shell types of polymer-based nanoparticles (NPs) are attractive from the standpoint of integrating the key features of core and shell into hybrid or composite materials, such as controllable particle size, surface functionality and entrapment of particular particles. The focus of core–shell design over the years has evolved toward more innovative, simple and cost-effective methods. This chapter provides an overview of recent progress in the fabrication methodologies and functional modulations of polymer-based core–shell NPs in colloid and materials science. Discussion and comparisons are presented, along with a summary and outlook of these methods for core–shell hybrid architecture.

To achieve high accuracy and effectiveness in sensing and modulating neural activity, efficient charge-transfer biointerfaces and a high spatiotemporal resolution are required. Ultrathin bioelectrode arrays exhibiting mechanical compliance with biological tissues offer such biointerfaces. However, their thinness often leads to a lack of mechano-electrical stability or sufficiently high electrochemical capacitance, thus deteriorating their overall performance. Here, we report ultrathin (∼115 nm) bioelectrode arrays that simultaneously enable ultraconformability, mechano-electrical stability and high electrochemical performance. These arrays show high opto-electrical conductivity (2060 S cm-1@88% transparency), mechanical stretchability (110% strain), and excellent electrochemical properties (24.5 mC cm-2 charge storage capacity and 3.5 times lower interfacial impedance than commercial electrodes). The improved mechano-electrical and electrochemical performance is attributed to the synergistic interactions within the poly(3,4-ethylenedioxythiophene) sulfonate (PEDOT:PSS)/graphene oxide (GO) interpenetrating network (PGIN), where π-π and hydrogen bonding interactions improve conductive pathways between PEDOT chains and enhance the charge-transfer mobility. This ultrathin bioelectrode is compatible with photolithography processing and provides spatiotemporally precise signal mapping capabilities for sensing and modulating neuromuscular activity. By capturing weak multichannel facial electromyography signals and applying machine learning algorithms, we achieve high accuracy in silent speech recognition. Moreover, the high transparency of the bioelectrode allows simultaneous recording of electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS) signals, facilitating dual-mode brain activity analysis with both high temporal and high spatial resolution.