Green and sustainable cellulose-based shape memory composites with excellent conductivity for temperature warning

Nanocellulose-based nanogenerators for sensor applications: A review

With the rapid development of the Internet of Things, nanogenerator as a green energy collection technology has attracted great attention in various fields. Specifically, the natural renewable nanocellulose as a raw material can significantly improve the environmental friendliness of the nanocellulose-based nanogenerators, which also makes the nanocellulose based nanogenerators expected to further develop in areas such as wearable devices and sensor networks. This paper mainly reports the application of nanocellulose in nanogenerator, focusing on the sensor. The types, sources and preparation methods of nanocellulose are briefly introduced. At the same time, the special structure of nanocellulose highlights the advantages of nanocellulose in nanogenerators. Then, the application of nanocellulose-based nanogenerators in sensors is introduced. Finally, the future development prospects and shortcomings of this nanogenerator are discussed.

Magnetite embedded κ-carrageenan-based double network nanocomposite hydrogel with two-way shape memory properties for flexible electronics and magnetic actuators

Shape memory hydrogels attract increasing attention as flexible strain sensors due to their shape recovery property that can improve the lifetime of the sensor. Herein, we have designed a magnetic shape memory hydrogel based on Fe₃O₄ nanoparticles, carrageenan, and poly (acrylamide-co-acrylic acid) with self-adhesive and conductive properties. The resulting double network hydrogel showed promising actuator and strain sensor applications. Electrical conductivity was observed in this hydrogel without using additional ions. The presence of magnetite nanoparticles increased the tensile strength and temporary shape fixity ratio to around 6.5 MPa and 94.3 %, respectively. The excellent cantilever and catheter-like behavior of the hydrogels were illustrated through magnetic routing by an external magnet. Also, these hydrogels demonstrated suitable performance in the 500 cycles strain sensing tests before and after their initial shape recovery.

Polyurethane Shape Memory Polymer/pH-Responsive Hydrogel Hybrid for Bi-Function Synergistic Actuations

Stimuli-responsive actuating hydrogels response to the external stimulus with complex deformation behaviors based on the programmable anisotropic structure design are one of the most important smart soft materials, which have great potential applications in artificial muscles, smart values, and mini-robots. However, the anisotropic structure of one actuating hydrogel can only be programmed one time, which can only provide single actuating performance, and subsequently, has severely limited their further applications. Herein, we have explored a novel SMP/hydrogel hybrid actuator through combining polyurethane shape memory polymer (PU SMP) layer and pH-responsive polyacrylic-acid (PAA) hydrogel layer by a napkin with UV-adhesive. Owing to both the super-hydrophilicity and super-lipophilicity of the cellulose-fiber based napkin, the SMP and the hydrogel can be bonded firmly by the UV-adhesive in the napkin. More importantly, this bilayer hybrid 2D sheet can be programmed by designing a different temporary shape in heat water which can be fixed easily in cool water to achieve various fixed shapes. This hybrid with a fixed temporary shape can achieve complex actuating performance based on the bi-functional synergy of temperature-triggered SMP and pH-responsive hydrogel. The relatively high modulus PU SMP achieved high to 87.19% and 88.92% shape-fixing ratio, respectively, correspond to bending and folding shapes. The hybrid actuator can actuate with the 25.71 °/min actuating speed. Most importantly, one SMP/hydrogel bi-layer hybrid sheet was repeatedly programmed at least nine times in our research to fix various temporary 1D, 2D and 3D shapes, including bending, folding and spiraling shapes. As a result, only one SMP/hydrogel hybrid can provide various complex stimuli-responsive actuations, including the reversable bending-straightening, spiraling-unspiraling. A few of the intelligent devices have been designed to simulate the movement of the natural organisms, such as bio-mimetic "paw", "pangolin" and "octopus". This work has developed a new SMP/hydrogel hybrid with excellent multi-repeatable (≥9 times) programmability for high-level complex actuations, including the 1D to 2D bending and the 2D to 3D spiraling actuations, which also provides a new strategy to design other new soft intelligent materials and systems.

Electrospun cellulose acetate nanofibrous composites for multi-responsive shape memory actuators and self-powered pressure sensors

Soft actuators and sensors have attracted extensive scientific interest attributed to their great potential applications in various fields, but the integration of actuating and sensing functions in one material is still a big challenge. Here, we developed an electrospun cellulose acetate (CA)/carbon nanotube nanofiborous composite with both functional applications as multi-responsive shape memory actuators and triboelectric nanogenerator (TENG) based sensors. Attributed to excellent thermo- and light-induced shape memory performance, the CA nanofiborous composites showed high heavy-lift capability as light driven actuators, able to lift burdens 1050 times heavier than their own weight. The CA nanofiborous membranes based TENG exhibited high output performance with open-circuit voltage, short-circuit density, and instantaneous power density about 103.2 V, 7.93 mA m^-2 and 0.74 W m^-2, respectively. The fabricated TENG based pressure sensor exhibited a high sensitivity of 3.03 V kPa^-1 below 6.8 kPa and 0.11 V kPa^-1 in the pressure range from 6.8 to 65 kPa, which can be effectively used to monitor human motion state and measure wind velocity. It is expected that the electrospun composites with actuating and sensing functions will show prosperous applications prospects in soft robotics.

Self-healing, self-adhesive, and stretchable conductive hydrogel for multifunctional sensor prepared by catechol modified nanocellulose stabilized poly(α-thioctic acid)

Self-healing, self-adhesive, and stretchable bio-based conductive hydrogels exhibit properties similar to those of biological tissues, making them an urgent requirement for emerging wearable devices. The primary challenge lies in devising straightforward strategies to accomplish all the aforementioned performances and achieve equilibrium among them. This study used the natural compound thioctic acid (TA) and modified cellulose to prepare conductive hydrogels with stretchability, healing, and self-adhesion through a simple one-step strategy. Metastable poly(TA) was obtained through ring-opening polymerization of lithiated TA, followed by the introduction of dopamine-grafted cellulose nanofibers (DCNF) to stabilize poly(TA) and prepare PTALi/DCNF hydrogels with the aforementioned properties. The hydrogels demonstrated remarkable conductivity, attributed to the existence of Li + ions, with a maximum conductivity of 17.36 mS/cm. The self-healing capacity of the hydrogels was achieved owing to the presence of disulfide bond in TA. The introduction of DCNF can effectively stabilize poly(TA), endow the hydrogel with self-adhesion ability, improve the mechanical properties, and further enhance the formability of hydrogels. Generally, bio-based PTALi/DCNF hydrogels with stretchability, self-healing, self-adhesion, and conductivity are obtained through a simple strategy and used as a sensor with a wide response range and high sensitivity. Hydrogels have significant potential for application in wearable electronic devices, electronic skins, and soft robots.

Preparation and research of PCL/cellulose composites: Cellulose derived from agricultural wastes

For the rational use of agricultural wastes, bagasse, orange peel and wheat bran were used to fabricate bio-based polymer materials. Cellulose was extracted from the three different agricultural wastes, and poly(ε-caprolactone) (PCL) was used as the matrix material. PCL was mixed with nanocrystalline cellulose (CNC), extracted bagasse cellulose (GC), orange peel cellulose (JC) and wheat bran cellulose (MC) by solution casting. Morphology and structure of the extracted cellulose were studied by Scanning Electron Microscope, Fourier Infrared spectrometer, thermogravimetry and X-ray diffractometer. The influence of GC, JC, MC on the crystallization process and mechanical properties of PCL was investigated by DSC and tensile test. Experimental results show that the addition of CNC, GC, JC, MC increases the crystallization temperature of PCL, accelerates the crystallization process of PCL, and improves the tensile property of PCL.

Biomimetic ultra-strong, ultra-tough, degradable cellulose-based composites for multi-stimuli responsive shape memory

Fabrication of ultra-strong, ultra-tough, sustainable, and degradable bio-based composites is urgently needed but remains challenging. Here, a biomimetic sustainable, degradable, and multi-stimuli responsive cellulose/PCL/Fe₃O₄ composite with ultra-strong mechanical strength and ultra-high toughness was developed. To prepare the proposed composites, the soft poly(ε-caprolactone) (PCL) side chain was grafted onto the rigid cellulose backbone, then the cellulose graft copolymer (EC-g-PCL) reacted with rigid hexamethylenediamine modified Fe₃O₄ nanoparticle (Fe₃O₄-NH₂) to construct the crosslinking network using MDI-50 as a crosslinker. Given by the construction of crosslinking network and the "hard" and "soft" interactive structure, the composites showed ultra-strong mechanical strength (25.7 MPa) and ultra-high toughness (107.0 MJ/m³), and the composite specimen could lift a weight of approximately 21,200 times its mass. The composites also exhibited rapid degradation ability with high degradation efficiency. In addition, the composites showed excellent thermal responsive shape memory property with a shape recovery ratio above 96 %. Most importantly, the Fe₃O₄ nanoparticles endowed the composites with photothermal conversion property, the composites exhibited superior NIR light-triggered shape memory capability. The EC-g-PCL/Fe₃O₄ composites with ultra-strong mechanical strength and ultra-high toughness have promising applications in heavy-lift, object transportation, and self-tightening knots.

High lignin, light-driven shape memory polymers with excellent mechanical performance

With the gradual global standardization of carbon emission policies, the development of renewable resources to replace traditional fossil resources is assuming increasing importance. Lignin is the most abundant natural source of aromatic compounds and has the potential to replace petroleum-based aromatic hydrocarbons. In this work, the rigid benzene ring structure and excellent photothermal properties of lignin were exploited to produce light-driven lignin-based shape memory polymers (ELEPs) that contain high proportions of lignin and have good mechanical properties. Enzymatically hydrolyzed lignin (EL), epoxy soybean oil (ESO) and polyethylene glycol (PEG 400) were copolymerized and cured to form ELEPs, which have a disordered three-dimensional network. An increase in the proportion of EL from 40 to 60 wt% enhanced the mechanical properties, as reflected by an increase in tensile strength from 11.3 to 30.8 MPa and in the glass transition temperature (T_g) from 93 to 115.7 °C. Under simulated solar irradiation (2000 W m^-2), ELEP50, which contains 50 wt% lignin and has a T_g of 105 °C, reached a surface temperature as high as 105 °C and achieved shape memory within 20 s. The shape fixation ratio (R_f) and shape recovery ratio (R_r) were stably >98 % and >97 %, respectively, over eight cycles in a bending-recovery experiment. The unique light-driven shape memory properties of ELEPs provide a method for high value utilization of EL, and the design strategy offers new ideas for producing novel intelligent materials.

Recent advances in dynamic covalent bond-based shape memory polymers

Dynamic covalent bond-based shape memory polymers (DCB-SMPs) are one of most important SMPs which have a wide potential application prospect. Different from common strong covalent bonds, DCBs own relatively weak bonding energy, similarly to the supramolecular interactions of noncovalent bonds, and can dynamically combine and dissociate these bonds. DCB-SMP solids, which can be designed to respond for different stimuli, can provide excellent self-healing, good reprocessability, and high mechanical performance, because DCBs can obtain dynamic cross-linking without sacrificing ultrahigh fixing rates. Furthermore, besides DCB-SMP solids, DCB-SMP hydrogels with responsiveness to various stimuli also have been developed recently, which have special biocompatible soft/wet states. Particularly, DCB-SMPs can be combined with emerging 3D-printing techniques to design various original shapes and subsequently complex shape recovery. This review has summarized recent research studies about SMPs based on various DCBs including DCB-SMP solids, DCB-SMP hydrogels, and the introduction of new 3D-printing techniques using them. Last but not least, the advantages/disadvantages of different DCB-SMPs have been analyzed via polymeric structures and the future development trends in this field have been predicted.