Monolithic Three-Dimensional Functionally Graded Hydrogels for Bioinspired Soft Robots Fabrication

Selaginella lepidophylla-Inspired Multi-Stimulus Cooperative Control MXene-Based Flexible Actuator

Predictable bending deformation, high cycle stability, and multimode complex motion have always been the goals pursued in the field of flexible robots. In this study, inspired by the delicate structure and humidity response characteristics of Selaginella lepidophylla, a new multilevel assisted assembly strategy was developed to construct MXene-CoFe2O4 (MXCFO) flexible actuators with different concentration gradients, to achieve predictable bending deformation and multi-stimulus cooperative control of the actuators, revealing the intrinsic link between the gradient change and the bending deformation ability of the actuator. The thickness of the actuator shows uniformity compared with the common layer-by-layer assembly strategy. And, the bionic gradient structured actuator shows high cycle stability, and it maintains excellent interlayer bonding after bending 100 times. The flexible robots designed based on the predictable bending deformation and the multi-stimulus cooperative response characteristics of the actuator initially realize conceptual models of humidity monitoring, climbing, grasping, cargo transportation, and drug delivery. The designed bionic gradient structure and unbound multi-stimulus cooperative control strategy may show great potential in the design and development of robots in the future.

Magnetostriction Enhancement in Midrange Modulus Magnetorheological Elastomers for Sensor Applications

Magnetorheological elastomer (MRE), which is capable of exhibiting magnetostriction in the presence of a magnetic field, has a great potential to be used for the development of sensor devices. Unfortunately, to date, many works focused on studying low modulus of MRE (less than 100 kPa) which can hamper their potential application in sensors due to short lifespan and low durability. Thus, in this work, MRE with storage modulus above 300 kPa is to be developed to enhance magnetostriction magnitude and reaction force (normal force). To achieve this goal, MREs are prepared with various compositions of carbonyl iron particles (CIPs), in particular, MRE with 60, 70 and 80 wt.% of CIP. It is shown that both the magnetostriction percentage and normal force increment are achieved as the concentration of CIPs increases. The highest magnetostriction magnitude of 0.075% is obtained with 80 wt.% of CIP, and this increment is higher than that of moderate stiffness MRE developed in the previous works. Therefore, the midrange range modulus MRE developed in this work can copiously produce the required magnetostriction value and potentially be implemented for the design of forefront sensor technology.

A Hydrogel-Based Self-Sensing Underwater Actuator

Soft robots made of hydrogels are suited for underwater exploration due to their biocompatibility and compliancy. Yet, reaching high dexterity and actuation force for hydrogel-based actuators is challenging. Meanwhile, real-time proprioception is critical for feedback control. Moreover, sensor integration to mimic living organisms remains problematic. To address these challenges, we introduce a hydrogel actuator driven by hydraulic force with a fast response (time constant 0.83 s). The highly stretchable and conductive hydrogel (1400% strain) is molded into the PneuNet shape, and two of them are further assembled symmetrically to actuate bi-directionally. Then, we demonstrate its bionic application for underwater swimming, showing 2 cm/s (0.19 BL/s) speed. Inspired by biological neuromuscular systems' sensory motion, which unifies the sensing and actuation in a single unit, we explore the hydrogel actuator's self-sensing capacity utilizing strain-induced resistance change. The results show that the soft actuator's proprioception can monitor the undulation in real-time with a sensitivity of 0.2%/degree. Furthermore, we take a finite-element method and first-order differential equations to model the actuator's bending in response to pressure. We show that such a model can precisely predict the robot's bending response over a range of pressures. With the self-sensing actuator and the proposed model, we expect the new approach can lead to future soft robots for underwater exploration with feedback control, and the underlying mechanism of the undulation control might offer significant insights for biomimetic research.

Structural Flexibility in Triboelectric Nanogenerators: A Review on the Adaptive Design for Self-Powered Systems

There is an increasing need for structural flexibility in self-powered wearable electronics and other Internet of Things (IoT), where adaptable triboelectric nanogenerators (TENGs) play a key role in realizing the true potential of IoT by endowing the latter with self-sustainability. Thus, in this review, the topic was restricted to the adaptive design of TENGs with structural flexibility that aims to promote the sustainable operation of various smart electronics. This review begins with an emphatical discussion of the concept of flexible electronics and TENGs, and continues with the introduction of TENG-based self-powered intelligent systems while placing the emphasis on self-powered flexible intelligent devices. Self-powered healthcare sensors, e-skins, and other intelligent wearable electronics with enhanced intelligence and efficiency in practical applications due to the integration with TENGs are illustrated, along with an emphasis on the design strategy of structural flexibility of TENGs and the associated integration schemes. This review aims to cover recent achievements in the field of self-powered systems, and provides information on how flexibility or adaptability in TENGs can be adopted, their types, and why they are required in promoting advanced IoT applications with sustainability and intelligence algorithms.

Biodegradable floating hydrogel baits as larvicide delivery systems against mosquitoes

Biological methods for mosquito larvae control are completely biodegradable and have null or limited effects on nontarget organisms. However, commercially available products have a low residual activity, with the consequent need for multiple applications that inevitably increase costs and the risk of resistance phenomena insurgence. Smart delivery systems made of hydrogels proved their efficacy in increasing the action duration of biolarvicides up to several months, but the lack of an efficient baiting mechanism to strongly attract the target pest remains a problem in practical applications. In this work, we investigated two novel hydrogel-based formulations of completely natural composition for baiting and killing larvae of Aedes albopictus mosquitos. The proposed materials consist of charged crosslinked polysaccharides (chitosan and cellulose) and are specifically manufactured to float in water, simulating organic matter usually present at breeding sites. Within the hydrogels' matrix, yeast colonies of Saccharomyces cerevisiae were embedded as phagostimulants alongside a biolarvicide (Bacillus thuringiensis var. israelensis (Bti)). Despite the similar chemical nature and structure, chitosan-based hydrogels exhibited a markedly superior baiting potential compared to those made of cellulose and also succeeded in efficiently killing mosquito larvae just after a few hours from administration. We are confident that the proposed smart delivery hydrogel made of chitosan can be an enabling tool to attract mosquito larvae towards biopesticides of different nature without delocalizing active ingredients away from the breeding site and to simultaneously increase their residual activity, thus holding the potential of minimizing environmental pollution related to pest control and vector-borne disease prevention.

Structure-Processing-Property Relationships of 3D Printed Porous Polymeric Materials

Imparting porosity to 3D printed polymeric materials is an attractive option for producing lightweight, flexible, customizable objects such as sensors and garments. Although methods currently exist to introduce pores into 3D printed objects, little work has explored the structure-processing-property relationships of these materials. In this study, photopolymer/sacrificial paraffin filler composite inks were produced and printed by a direct ink writing (DIW) technique that leveraged paraffin particles as sacrificial viscosity modifiers in a matrix of commercial elastomer photocurable resin. After printing, paraffin was dissolved by immersion of the cured part in an organic solvent at elevated temperature, leaving behind a porous matrix. Rheometry experiments demonstrated that composites with between 40 and 70 wt % paraffin particles were able to be successfully 3D printed; thus, the porosity of printed objects can be varied from 43 to 73 vol %. Scanning electron microscopy images demonstrated that closed-cell porous structures formed at low porosity values, whereas open-cell structures formed at and above approximately 53 vol % porosity. Tensile tests revealed a decrease in elastic modulus as the porosity of the material was increased. These tests were simulated using finite element analysis (FEA), and it was found that the Neo-Hookean model was appropriate to represent the 3D printed porous material at lower and higher void fractions within a 75% strain, and the Ogden model also gave good predictions of porous material performance. The transition between closed- and open-cell behaviors occurred at 52.4 vol % porosity in the cubic representative volume elements used for FEA, which agreed with experimental findings that this transition occurred at approximately 53 vol % porosity. This work demonstrates that the tandem use of rheometry, FEA, and DIW enables the design of complex, tailorable 3D printed porous structures with desired mechanical performance.