The shape – morphing performance of magnetoactive soft materials

Magnetoactive soft materials (MSMs) are soft polymeric composites filled with magnetic particles that are an emerging class of smart and multifunctional materials with immense potentials to be used in various applications including but not limited to artificial muscles, soft robotics, controlled drug delivery, minimally invasive surgery, and metamaterials. Advantages of MSMs include remote contactless actuation with multiple actuation modes, high actuation strain and strain rate, self-sensing, and fast response etc. Having broad functional behaviours offered by the magnetic fillers embedded within non-magnetic matrices, MSMs are undoubtedly one of the most promising materials in applications where shape-morphing, dynamic locomotion, and reconfigurable structures are highly required. This review article provides a comprehensive picture of the MSMs focusing on the materials, manufacturing processes, programming and actuation techniques, behaviours, experimental characterisations, and device-related achievements with the current state-of-the-art and discusses future perspectives. Overall, this article not only provides a comprehensive overview of MSMs’ research and development but also functions as a systematic guideline towards the development of multifunctional, shape-morphing, and sophisticated magnetoactive devices

Chiral design of tough spring-shaped hydrogels for smart umbrellas

Developing hydrogel artificial muscles to mimic the motion of natural muscles has long attracted scientists from the perspective of materials science for potential applications in soft robotics. However, rational design of hydrogel artificial muscles with large stroke, rapid actuation speed, and high work capacity remains a major challenge. Herein, we reported two kinds of chiral spring-shaped hydrogels that were prepared via consecutive shaping process (e.g., stretching, twisting, folding, coiling, and fixing). By switching the chirality of coil, homochiral muscle and heterochiral muscle were obtained, respectively. Homochiral muscle could rapidly expand to 560% with an average speed of 6.7 % s−1 in response to NIR irradiation, whose maximum work capacity reached 45 J kg−1. On contrary, heterochiral muscle contracted 69% within 1 min under NIR irradiation with a maximum work capacity of 33 J kg−1. Interestingly, the parasol containing homochiral muscles opened autonomously during dehydration process, while the umbrellas containing heterochiral muscle could opened rapidly when water was applied. This work provided an innovative strategy for developing tough hydrogel muscles with opposite chiralities

An additively manufactured silicone polymer: Thermo-viscoelastic experimental study and computational modelling

The additive manufacturing (AM) is a new paradigm across various disciplines of engineering sciences. Despite significant advances in the areas of hard material printings, the options for 3D printed soft materials are still limited. Most of the existing 3D printed polymers are in the areas of acrylics and polyurethanes or their composites. Recently emerged Digital Light Synthesis (DLS) technology hugely accelerates the additive manufacturing of soft polymers. A DLS-inspired 3D printer uses a continuous building technique instead of a layer-by-layer approach, where the curing process is activated by an ultra-violet (UV) light. In this contribution, a DLS-based digitally printed silicone (SIL30) is experimentally characterized. To understand polymer's temperature-dependent mechanical responses, an extensive thermo-viscoelastic experimental characterisation at various strain rates under tensile deformation and temperature fields from -20° C to 60° C is performed. The study reveals significant effects of time-and temperature-dependency on the mechanical responses of the 3D printed silicone. Motivated by the thermo-mechanical results of the polymer, a thermodynamically consistent constitutive model at large strain is devised. Afterwards, the model is calibrated to the data that results in the identification of relevant parameters. The model predicts the experimental results with a good accuracy. 3D printed soft polymers are major candidates in designing complex and intricate architectured metamaterials for biomedical applications. Hence, a comprehensive thermo-mechanical experimental study and subsequent constitutive modelling will facilitate in designing and simulating more complex cellular metamaterials using 3D printed silicones

Additive manufacturing of sustainable biomaterials for biomedical applications

Biopolymers are promising environmentally benign materials applicable in multifarious applications. They are especially favorable in implantable biomedical devices thanks to their excellent unique properties, including bioactivity, renewability, bioresorbability, biocompatibility, biodegradability, and hydrophilicity. Additive manufacturing (AM) is a flexible and intricate manufacturing technology, which is widely used to fabricate biopolymer-based customized products and structures for advanced healthcare systems. Three-dimensional (3D) printing of these sustainable materials is applied in functional clinical settings including wound dressing, drug delivery systems, medical implants, and tissue engineering. The present review highlights recent advancements in different types of biopolymers, such as proteins and polysaccharides, which are employed to develop different biomedical products by using extrusion, vat polymerization, laser, and inkjet 3D printing techniques in addition to normal bioprinting and four-dimensional (4D) bioprinting techniques. This review also incorporates the influence of nanoparticles on the biological and mechanical performances of 3D-printed tissue scaffolds. This work also addresses current challenges as well as future developments of environmentally friendly polymeric materials manufactured through the AM techniques. Ideally, there is a need for more focused research on the adequate blending of these biodegradable biopolymers for achieving useful results in targeted biomedical areas. We envision that biopolymer-based 3D-printed composites have the potential to revolutionize the biomedical sector in the near future

3D/4D printing of cellulose nanocrystals-based biomaterials: Additives for sustainable applications

Cellulose nanocrystals (CNCs) have gained significant attraction from both industrial and academic sectors, thanks to their biodegradability, non-toxicity, and renewability with remarkable mechanical characteristics. Desirable mechanical characteristics of CNCs include high stiffness, high strength, excellent flexibility, and large surface-to-volume ratio. Additionally, the mechanical properties of CNCs can be tailored through chemical modifications for high-end applications including tissue engineering, actuating, and biomedical. Modern manufacturing methods including 3D/4D printing are highly advantageous for developing sophisticated and intricate geometries. This review highlights the major developments of additive manufactured CNCs, which promote sustainable solutions across a wide range of applications. Additionally, this contribution also presents current challenges and future research directions of CNC-based composites developed through 3D/4D printing techniques for myriad engineering sectors including tissue engineering, wound healing, wearable electronics, robotics, and anti-counterfeiting applications. Overall, this review will greatly help research scientists from chemistry, materials, biomedicine, and other disciplines to comprehend the underlying principles, mechanical properties, and applications of additively manufactured CNC-based structures

Hard magnetics in ultra-soft magnetorheological elastomers enhance fracture toughness and delay crack propagation

Pre-existing flaws in highly stretchable elastomers trigger fracture under large deformations. For multifunctional materials, fracture mechanics may be influenced by additional physical phenomena. This work studies the implications of hard magnetics on the fracture behaviour of ultra-soft magnetorheological elastomers (MREs). We experimentally demonstrate that MREs with remanent magnetisation have up to a 50 % higher fracture toughness than non pre-magnetised samples. Moreover, we report crack closure due to the magnetic field as a mechanism that delays the opening of cracks in pre-magnetised MREs. To overcome experimental limitations and provide further understanding, a phase-field model for the fracture of MREs is conceptualised. The numerical model incorporates magneto-mechanical coupling to demonstrate that the stress concentration at the crack tip is smaller when the MRE is pre-magnetised. Overall, this work unveils intriguing applications for functional actuators, with better fracture behaviour and potential better performance under cyclic loading

Mathematical formulations for elastic magneto-electrically coupled soft materials at finite strains: Time-independent processes

Recently, among other smart and multifunctional materials, magneto-electric soft materials are expected to open a new horizon with myriad of potential applications such as wireless energy harvesting, spintronics and nonvolatile memories, magneto-electric random access memory, to mention a few. Magneto-electric coupling can be defined as the ability of a material to electrically polarize upon the application of a magnetic field and conversely, to magnetize under the application of an electric field. In contrast to traditional multi-ferroic hard materials, magneto-electric soft materials are of largely deformable where electric and magnetic fields and mechanical deformations are intricately coupled at finite strians. In this contribution, we will emphasis to formulate generalised mathematical frameworks of finitely deformed magneto-electric soft materials. After elaborating fundamental and governing equations, some homogeneous and non-homogeneous classical boundary value problems are studied under magneto-electrically coupled loads

A review on magneto-mechanical characterizations of magnetorheological elastomers

Magnetorheological elastomers (MREs) are a class of recently emerged smart materials whose moduli are largely influenced when exposed to an external magnetic field. The MREs are particulate composites, where micro-sized magnetic particles are dispersed inside a non-magnetic polymeric matrix. These elastomers are known for changing their mechanical and rheological properties in the presence of a magnetic field. This change in properties is widely known as the magnetorheological (MR) effect. The MR effect depends on a number of factors such as type of matrix materials, type, concentration and distribution of magnetic particles, use of additives, working modes, and magnetic field strength. The investigation of MREs’ mechanical properties in both off-field and on-field (i.e. the absence and presence of a magnetic field) is crucial to deploy them in real engineering applications. The common magneto-mechanical characterization experiments of MREs include static and dynamic compression, tensile, and shear tests in both off-field and on-field. This review article aims to provide a comprehensive overview of the magneto-mechanical characterizations of MREs along with brief coverage of the MRE materials and their fabrication methods

Experimental study and phenomenological modelling of flaw sensitivity of two polymers used as dielectric elastomers

The extreme stretching of dielectric elastomers in sensors, actuators and energy harvesting devices is a common phenomenon where the materials are prone to fracture under the influence of flaws and notches. In this work, we have investigated the length of flaw sensitivities of two widely used dielectric materials, acrylic (VHB) and silicone (Ecoflex) elastomers under a pure shear loading and established that the length of flaw sensitivity of acrylic is almost double than that of silicone. Therefore, the acrylic elastomer is safer to operate for small notches as compared to the silicone material. However, within the flaw-sensitive length, failure stretch, fracture toughness and failure stress are more for Ecoflex than those for VHB. It is found that the failure stretch and the fracture toughness decrease drastically after the length of flaw sensitivities for both materials. Also, the failure stress keeps on decreasing with an increase in notch length for both materials. Afterwards, a simple phenomenological relation is proposed for fitting experimental results under a pure shear loading with only two parameters. The mathematical relation is valid for both the materials and covers the notch sensitivity with a good agreement