Characterization of mechanical stress in the occurrence of cortical opacification in age-related cataracts using three-dimensional finite element model of the human lens and RNA-seq.

Cataract is the leading cause of blindness across the world. Age-related cataract (ARC) is the most common type of cataract, but its pathogenesis is not fully understood. Using three-dimensional finite element modeling combining experimental biotechnology, our study demonstrates that external forces during accommodation cause mechanical stress predominantly in lens cortex, basically matching the localization of opacities in cortical ARCs. We identified the cellular senescence and upregulation of PIEZO1 mRNA in HLECs under mechanical stretch. This mechano-induced senescence in HLECs might be mediated by PIEZO1-related pathways, portraying a potential biomechanical cause of cortical ARCs. Our study updates the fundamental insight towards cataractogenesis, paving the way for further exploration of ARCs pathogenesis and nonsurgical treatment.

Designing Ultratough Single-Network Hydrogels with Centimeter-Scale Fractocohesive Lengths via Inelastic Crack Blunting.

Fractocohesive length, defined as the ratio of fracture toughness to work of fracture, measures the sensitivity of materials to fracture in the presence of flaws. The larger the fractocohesive length, the more flaw-tolerant and crack-resistant the hydrogel. For synthetic soft materials, the fractocohesive length is short, often on the scale of 1 mm. Here, highly flaw-insensitive (HFI) single-network hydrogels containing an entangled inhomogeneous polymer network of widely distributed chain lengths are designed. The HFI hydrogels demonstrate a centimeter-scale fractocohesive length of 2.21 cm, which is the highest ever recorded for synthetic hydrogels, and an unprecedented fracture toughness of ≈13 300 J m-2. The uncommon flaw insensitivity results from the inelastic crack blunting inherent to the highly inhomogeneous network. When the HFI hydrogel is stretched, a large number of short chains break while coiled long chains can disentangle, unwind, and straighten, producing large inelastic deformation that substantially blunts the crack tip in a plastic manner, thereby deconcentrating crack-tip stresses and blocking crack extension. The flaw-insensitive design strategy is applicable to various hydrogels such as polyacrylamide and poly(N,N-dimethylacrylamide) hydrogels and enables the development of HFI soft composites.

A novel intraocular pressure predicting method based on hyperelastic mechanical model of cornea.

Measuring intraocular pressure (IOP) is crucial and remains challenging in diagnosing glaucoma, as it is associated with cornea deformation during inflation. In this study, a three-dimensional analytical model based on hyperelastic constitutive relationship to predict correlation between cornea vertex displacement and the IOP is proposed. The analytical model is validated by rigorous experiments. Rabbit corneas were selected for this study and their mechanical properties were obtained using uniaxial tensile tests. To mimic the environment in which the cornea exists, an artificial anterior chamber equipped with water-injection pipelines was constructed to study the relationship between the corneal vertex displacement with IOP value in practical situation. The experimental results of rabbits corneas prove that the IOP can be deduced based on the measured corneal vertex displacement by the analytical model. Furthermore, subtle difference occurs when comparing the calculated human IOPs with those measured by medical equipment, demonstrating that the proposed method is suitable for monitoring the IOP of human. This novel IOP predicting method provides new inspiration for the design of eyepieces, as well as the preoperative preparation for laser surgery and evaluation of corneal damage.

A Bayesian method with nonlinear noise model to calibrate constitutive parameters of soft tissue.

The measured mechanical responses of soft tissue exhibit large variability and errors, especially for the softest brain tissue, while calibrating its constitutive parameters in a deterministic way remains a common practice. Here we implement a Bayesian method considering the nonlinear noise model to calibrate constitutive parameters of brain tissue. A probability model is first developed based on the measured experimental data, likelihood function, and prior function, from which the posterior distributions of model parameters are formulated. The likelihood function considers the nonlinear behaviors of the constitutive response and noise distribution of the experimentally measured data. Meanwhile, the prior predictive distribution is computed to check the probability model preliminarily. Secondly, the Markov Chain Monte Carlo (MCMC) method is used to compute the posterior distributions of model parameters, enabling assessment of parameter uncertainty, correlation, and model calibration error. Finally, the posterior predictive distributions of the overall response, constitutive response, and noise response are computed to validate the probabilistic model, all of which are consistent with the corresponding data. Furthermore, the effect of the prior distribution, experimental data, and noise model on posterior distribution is studied. Our study provides a general approach to calibrating constitutive parameters of soft tissue despite errors and large variability in experimental data.

Tough and fatigue-resistant polymer networks by crack tip softening.

Soft materials fail by crack propagation under external loads. While fracture toughness of a soft material can be enhanced by orders of magnitude, its fatigue threshold remains insusceptible. In this work, we demonstrate a crack tip softening (CTS) concept to simultaneously improve the toughness and threshold of a single polymeric network. Polyacrylamide hydrogels have been selected as a model material. The polymer network is cured by two kinds of crosslinkers: a normal crosslinker and a light-degradable crosslinker. We characterize the pristine sample and light-treated sample by shear modulus, fracture toughness, fatigue threshold, and fractocohesive length. Notably, we apply light at the crack tip of a sample so that the light-sensitive crosslinkers degrade, resulting in a CTS sample with a softer and elastic crack tip. The pristine sample has a fracture toughness of 748.3 ± 15.19 J/m2 and a fatigue threshold of 9.3 J/m2. By comparison, the CTS sample has a fracture toughness of 2,774.6 ± 127.14 J/m2 and a fatigue threshold of 33.8 J/m2. Both fracture toughness and fatigue threshold have been enhanced by about four times. We attribute this simultaneous enhancement to stress de-concentration and elastic shielding at the crack tip. Different from the "fiber/matrix composite" concept and the "crystallization at the crack tip" concept, the CTS concept in the present work provides another option to simultaneously enhance the toughness and threshold, which improves the reliability of soft devices during applications.

Wireless Rehabilitation Training Sensor Arrays Made with Hot Screen-Imprinted Conductive Hydrogels with a Low Percolation Threshold.

Herein, we propose a highly sensitive wireless rehabilitation training ball with a piezoresistive sensor array for patients with Parkinson's disease (PD). The piezoresistive material is a low percolation threshold conductive hydrogel which is formed with polypyrrole (PPy) nanofibers (NFs) as a conductive filler derived from a polydopamine (PDA) template. The proton acid doping effect and molecular template of PDA are essential for endowing PPy NFs with a high aspect ratio, leading to a low percolation threshold (∼0.78 vol %) and a low Young's 004Dodulus of 37.69 kPa and hence easy deformation. The piezoresistive sensor exhibited a static and dynamic stability of 10,000 s and 15,000 cycle times, respectively. This stability could be attributed to the increased hydrophilicity of conductive fillers, enhancing the interfacial strength between the conductive filler and the matrix. The interaction between the PDA-PPy NFs and the hydrogel matrix endows the hydrogel with toughness and ensures the stability of the device. Additionally, the microdome structure of the conductive hydrogel, produced by hot screen-imprinting, dramatically improves the sensitivity of the piezoresistive sensor (∼856.14 kPa-1). The microdome conductive hydrogel can distinguish a subtle pressure of 15.40 Pa compared to the control hydrogel without a microstructure. The highly sensitive piezoresistive sensor has the potential to monitor the hand-grip force, which is not well controlled by patients with PD. The rehabilitation training ball assembled with a sensor array on the surface and a wireless chip for communication inside is built and used to monitor the pressure in real time through the WeChat applet. Thus, this work has significantly broadened the application of hydrogel-based flexible piezoresistive sensors for human activity monitoring, which provides a promising strategy to realize next-generation electronics.

A versatile hydrogel network-repairing strategy achieved by the covalent-like hydrogen bond interaction.

Hydrogen bond engineering is widely exploited to impart stretchability, toughness, and self-healing capability to hydrogels. However, the enhancement effect of conventional hydrogen bonds is severely limited by their weak interaction strength. In nature, some organisms tolerate extreme conditions due to the strong hydrogen bond interactions induced by trehalose. Here, we report a trehalose network-repairing strategy achieved by the covalent-like hydrogen bonding interactions to improve the hydrogels' mechanical properties while simultaneously enabling them to tolerate extreme environmental conditions and retain synthetic simplicity, which proves to be useful for various kinds of hydrogels. The mechanical properties of trehalose-modified hydrogels including strength, stretchability, and fracture toughness are substantially enhanced under a wide range of temperatures. After dehydration, the modified hydrogels maintain their hyperelasticity and functions, while the unmodified hydrogels collapse. This strategy provides a versatile methodology for synthesizing extremotolerant, highly stretchable, and tough hydrogels, which expand their potential applications to various conditions.

3D Printing of Conductive Hydrogel-Elastomer Hybrids for Stretchable Electronics.

Electronically conductive hydrogels integrated with dielectric elastomers show great promise in a wide range of applications, such as biomedical devices, soft robotics, and stretchable electronics. However, one big conundrum that impedes the functionality and performance of hydrogel-elastomer-based devices lies in the strict demands of device integration and the requirements for devices with satisfactory mechanical and electrical properties. Herein, the digital light processing three-dimensional (3D) printing method is used to fabricate 3D functional devices that bridge submillimeter-scale device resolution to centimeter-scale object size and simultaneously realize complex hybrid structures with strong adhesion interfaces and desired functionalities. The interconnected poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) network endows the PAAm hydrogel with high conductivity and superior electrical stability and poly(2-hydroxyethyl acrylate) functions as an insulating medium. The strong interfacial bonding between the hydrogel and elastomer is achieved by incomplete photopolymerization that ensures the stability of the hybrid structure. Lastly, applications of stretchable electronics illustrated as 3D-printed electroluminescent devices and 3D-printed capacitive sensors are conceptually demonstrated. This strategy will open up avenues to fabricate conductive hydrogel-elastomer hybrids in next-generation multifunctional stretchable electronics.

Functional hydrogel coatings.

Hydrogels-natural or synthetic polymer networks that swell in water-can be made mechanically, chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.

Study of non-uniform axial magnetic field induced deformation of a soft cylindrical magneto-active actuator.

Magneto-active polymers (MAPs) can undergo rapid and noticeable deformation through external wireless magnetic stimulation, offering a possibility to develop potential applications such as in actuators, flexible micro-grippers, soft robots, etc. In this paper, a theoretical model is presented to depict the relationship between nonlinear deformation and the external mechanical load applied on cylindrical samples in the presence of magnetic fields generated by an electromagnet. The geometrical and electromagnetic properties of the electromagnet are explicitly modeled in COMSOL Multiphysics based on the measured data. Furthermore, a finite element model is constructed to obtain detailed information (such as strain field), which cannot be obtained in experiments. The theoretical and simulation results fit quite well with the experimental results, showing the accuracy of the model construction. The proposed designing approach and model provide guidelines for researchers to enrich the applications of MAPs.

Mechanically Robust and UV-Curable Shape-Memory Polymers for Digital Light Processing Based 4D Printing.

4D printing is an emerging fabrication technology that enables 3D printed structures to change configuration over "time" in response to an environmental stimulus. Compared with other soft active materials used for 4D printing, shape-memory polymers (SMPs) have higher stiffness, and are compatible with various 3D printing technologies. Among them, ultraviolet (UV)-curable SMPs are compatible with Digital Light Processing (DLP)-based 3D printing to fabricate SMP-based structures with complex geometry and high-resolution. However, UV-curable SMPs have limitations in terms of mechanical performance, which significantly constrains their application ranges. Here, a mechanically robust and UV-curable SMP system is reported, which is highly deformable, fatigue resistant, and compatible with DLP-based 3D printing, to fabricate high-resolution (up to 2 µm), highly complex 3D structures that exhibit large shape change (up to 1240%) upon heating. More importantly, the developed SMP system exhibits excellent fatigue resistance and can be repeatedly loaded more than 10 000 times. The development of the mechanically robust and UV-curable SMPs significantly improves the mechanical performance of the SMP-based 4D printing structures, which allows them to be applied to engineering applications such as aerospace, smart furniture, and soft robots.

In Situ and Intraoperative Detection of the Ureter Injury Using a Highly Sensitive Piezoresistive Sensor with a Tunable Porous Structure.

Iatrogenic ureteral injury, as a commonly encountered problem in gynecologic, colorectal, and pelvic surgeries, is known to be difficult to detect in situ and in real-time. Consequently, this injury may be left untreated, thereby leading to serious complications such as infections, renal failure, or even death. Here, high-performance tubular porous pressure sensors were proposed to identify the ureter in situ intraoperatively. The electrical conductivity, mechanical compressibility, and sensor sensitivity can be tuned by changing the pore structure of porous conductive composites. A low percolation threshold of 0.33 vol % was achieved due to the segregated conductive network by pores. Pores also lead to a low effective Young's modulus and high compressibility of the composites and thus result in a high sensitivity of 448.2 kPa-1 of sensors, which is consistent with the results of COMSOL simulation. Self-mounted on the tip of forceps, the sensors can monitor tube pressures with different frequencies and amplitudes, as demonstrated using an artificial pump system. The sensors can also differentiate ureter pulses from aorta pulses of a Bama minipig in situ and in real-time. This work provides a facile, cost-effective, and nondestructive method to identify the ureter intraoperatively, which cannot be effectively achieved by traditional methods.

3D Printing Method for Tough Multifunctional Particle-Based Double-Network Hydrogels.

3D printing of hydrogels finds widespread applications in biomedicine and engineering. Artificial cartilages and heart valves, tissue regeneration and soft robots, require high mechanical performance of complex structures. Although many tough hydrogels have been developed, complicated synthesis processes hinder their fabrication in 3D printing. Here, a strategy is proposed to formulate hydrogel inks, which can be printed into various strong and tough particle-based double-network (P-DN) hydrogels of arbitrary shapes without any rheological modifiers. These hydrogel inks consist of microgels and a hydrogel precursor. The microgels are individual highly cross-linked networks. They are prepared by swelling dried microparticles in the hydrogel precursor that consists of monomers, initiators, and cross-linkers. Microgels regulate the rheological properties of the hydrogel ink and enable the direct printing. After printing and curing, the precursor forms a sparsely cross-linked network that integrates the microgels, leading to a P-DN hydrogel. The proposed hydrogel inks allow 3D printing of multifunctional hydrogel structures with high mechanical performance and strong adhesion to diverse materials. This strategy will open new avenues to fabricate multifunctional devices in tissue engineering and soft robotics.

Self-powered soft robot in the Mariana Trench.

The deep sea remains the largest unknown territory on Earth because it is so difficult to explore1-4. Owing to the extremely high pressure in the deep sea, rigid vessels5-7 and pressure-compensation systems8-10 are typically required to protect mechatronic systems. However, deep-sea creatures that lack bulky or heavy pressure-tolerant systems can thrive at extreme depths11-17. Here, inspired by the structure of a deep-sea snailfish15, we develop an untethered soft robot for deep-sea exploration, with onboard power, control and actuation protected from pressure by integrating electronics in a silicone matrix. This self-powered robot eliminates the requirement for any rigid vessel. To reduce shear stress at the interfaces between electronic components, we decentralize the electronics by increasing the distance between components or separating them from the printed circuit board. Careful design of the dielectric elastomer material used for the robot's flapping fins allowed the robot to be actuated successfully in a field test in the Mariana Trench down to a depth of 10,900 metres and to swim freely in the South China Sea at a depth of 3,224 metres. We validate the pressure resilience of the electronic components and soft actuators through systematic experiments and theoretical analyses. Our work highlights the potential of designing soft, lightweight devices for use in extreme conditions.

3D printing of highly stretchable hydrogel with diverse UV curable polymers.

Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer-based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high-water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.

A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer.

Soft ionic conductors, such as hydrogels and ionogels, have enabled stretchable and transparent ionotronics, but they suffer from key limitations inherent to the liquid components, which may leak and evaporate. Here, novel liquid-free ionic conductive elastomers (ICE) that are copolymer networks hosting lithium cations and associated anions via lithium bonds and hydrogen bonds are demonstrated, such that they are intrinsically immune from leakage and evaporation. The ICEs show extraordinary mechanical versatility including excellent stretchability, high strength and toughness, self-healing, quick self-recovery, and 3D-printability. More intriguingly, the ICEs can defeat the conflict of strength versus toughness-a compromise well recognized in mechanics and material science-and simultaneously overcome the conflict between ionic conductivity and mechanical properties, which is common for ionogels. Several liquid-free ionotronics based on the ICE are further developed, including resistive force sensors, multifunctional ionic skins, and triboelectric nanogenerators (TENGs), which are not subject to limitations of previous gel-based devices, such as leakage, evaporation, and weak hydrogel-elastomer interfaces. Also, the 3D printability of the ICEs is demonstrated by printing a series of structures with fine features. The findings offer promise for a variety of ionotronics requiring environmental stability and durability.

A Wearable Capacitive Sensor Based on Ring/Disk-Shaped Electrode and Porous Dielectric for Noncontact Healthcare Monitoring.

Wearable sensors are gradually enabling decentralized healthcare systems. However, these sensors need to be closely attached to skin, which is unsuitable for long-term dynamic health monitoring of the patients, such as infants or persons with burn injuries. Here, a wearable capacitive sensor based on the capacitively coupled effect for healthcare monitoring in noncontact mode is reported. It consists of a ring-shaped top electrode, a disk-shaped bottom electrode, and a porous dielectric layer with low permittivity. This unique design enhanced the capacitively coupled effect of the sensor, which enables a high noncontact detectivity of capacitance change. When an object approaches the sensor, its capacitance change (ΔC/C i = -38.7%) is 3-5 times higher than that of previously reported sensors. Meanwhile, the sensor is insensitive to the stretching strain and pressure (ΔC/C i < 5%) due to the unique ring-shaped electrode and the incompressible closed cells of the porous dielectric material, respectively. Finally, various human physiological signals (pulse and respiratory) are recorded in noncontact mode, where a person wears loose and soft clothes implanted with the sensor. Thus, it is promising to build smart healthcare clothes based on it to develop wearable decentralized healthcare systems.

Dual pH-Responsive Hydrogel Actuator for Lipophilic Drug Delivery.

As one of the most promising drug delivery carriers, hydrogels have received considerable attention in recent years. Many previous efforts have focused on diffusion-controlled release, which allows hydrogels to load and release drugs in vitro and/or in vivo. However, it hardly applies to lipophilic drug delivery due to their poor compatibility with hydrogels. Herein, we propose a novel method for lipophilic drug release based on a dual pH-responsive hydrogel actuator. Specifically, the drug is encapsulated and can be released by a dual pH-controlled capsule switch. Inspired by the deformation mechanism of Drosera leaves, we fabricate the capsule switch with a double-layer structure that is made of two kinds of pH-responsive hydrogels. Two layers are covalently bonded together through silane coupling agents. They can bend collaboratively in a basic or acidic environment to achieve the "turn on" motion of the capsule switch. By incorporating an array of parallel elastomer stripes on one side of the hydrogel bilayer, various motions (e.g., bending, twisting, and rolling) of the hydrogel bilayer actuator were achieved. We conducted an in vitro lipophilic drug release test. The feasibility of this new drug release method is verified. We believe this dual pH-responsive actuator-controlled drug release method may shed light on the possibilities of various drug delivery systems.

Toward Highly Thermal Stable Perovskite Solar Cells by Rational Design of Interfacial Layer.

Heat is crucial to the long-term stability of perovskite solar cells (PVSCs). Herein, thermal stability of PVSCs based on metal oxide (MO) and polymer (P) was investigated. Firstly, chemical decomposition behavior of perovskite films was characterized and analyzed, revealing that chemically active MO would accelerate the decomposition of methylamine lead iodide (MAPbI3). Secondly, thermal-induced stress, resulting from the mismatched thermal expansion coefficients of different layers of PVSCs, and its effect on the mechanical stability of perovskite films were studied. Combining experiment and simulation, we conclude that "soft" (low modulus) and thick (>20 nm) interfacial layers offer better relaxation of thermal-induced stress. As a result, PVSCs employing thick polymer interfacial layer offer a remarkably improved thermal stability. This work offers not only the degradation insight of perovskite films on different substrates but also the path toward highly thermal stable PVSCs by rational design of interfacial layers.

Magnetic-Assisted Transparent and Flexible Percolative Composite for Highly Sensitive Piezoresistive Sensor via Hot Embossing Technology.

A highly transparent and flexible percolative composite with magnetic reduced graphene oxide@nickel nanowire (mGN) fillers in EcoFlex matrix is proposed as a sensing layer to fabricate high-performance flexible piezoresistive sensors. Large excluded volume and alignment of mGN fillers contribute to low percolation threshold (0.27 vol %) of mGN-EcoFlex composites, leading to high electrical conductivity of 0.003 S m-1, optical transmittance of 71.8%, and low Young's modulus of 122.8 kPa. Large-scale microdome templates for sensors are prepared by hot embossing technology cost-effectively and COMSOL Multiphysics is utilized to optimize the sensor performances. Piezoresistive sensors fabricated experimentally show superior average sensitivity of 1302.1 kPa-1 with a low device-to-device variation of 3.74%, which provides a new way to achieve transparent, highly sensitive, and large-scale electronic skin.