Table of contents

Integrating different functions in one material system is a fundamental challenge, especially if those functions seem to exclude each other. Understanding function-structure relationships and developing a competence in the system approach for multifunctionality enables many modern applications, which can improve quality of life and address important global challenges. The interdisciplinary field recently extends to computational engineering approaches for virtual material design and to advanced fabrication schemes taking advantage of digitalisation. In this way development cycles can be shortened and products based on multifunctional materials can become more and more adaptive and individualised. It is against this backdrop that Multifunctional Materials has been launched, in consultation with the scientific community, to become a selective journal focused on conceptual novelty that will uniquely bring together all aspects of this rapidly developing field.

Fabrics have conventionally been passive materials with static properties, leveraging the mechanical, optical, and thermal properties of networks of fibers. Recently, however, the emergence of functional fibers with dynamic properties has begun to disrupt this conventional definition. Engineers have begun to explore new materials and manufacturing processes for actuating, sensing, and variable-stiffness fibers, and the integration of these functional fibers into dynamic, robotic fabrics. This review discusses recent developments in functional fibers and speculates on their utility in future robotic fabrics.

Terfenol-D has one of the largest magnetoelastic coefficients at room temperature, making it a good candidate for future magnetoelectric memory devices. However, few studies exist on the spin configuration in nanoscale Terfenol-D single domain structures. There are two exchange stiffness parameters of Terfenoal-D reported in the literature. In this paper, we use micromagnetic simulations to evaluate the influence of these two exchange parameters on the spin states of ellipsoidal shapes. Analytical results indicate that these two parameters produce significantly different spin states. Therefore, experiments to more accurately measure the exchange stiffness constant of Terfenol-D are needed.

For conducting polymer actuators to be practically useful, they need to be able to generate large forces and displacements and respond quickly. The simplest way to generate larger forces is to produce thicker actuators, but this approach has a negative impact on the response time. The effects of polypyrrole film thickness and voltage scan rate on the electrochemical actuation strain rate are investigated in this study. The rate of oxidative charging is shown to follow a standard Fickian diffusion model suggesting that the migration of ions into the polymer from the electrolyte is the dominant rate-determining mechanism. The migration rate is slow with full oxidation requiring several minutes for film thicknesses of just 10 μm. The free strains generated were found to be directly proportional to the oxidative charge passed. The isotonic actuation strains were additionally reduced by increasing applied stress and this effect was attributed to the increase in Young's modulus that occurs during polypyrrole oxidation. A simple model is presented that predicts the change in modulus during oxidation and gives reasonable estimates of the isotonic actuation for PPy actuators of different thickness and when subjected to different stresses.

Phosphotungstic acid (PTA) and the phosphotungstinates have been shown to be beneficial additives for polypyrrole (PPy) electropolymerization. The goal of this work was to study the PTA concentration effect on the electrodeposition of PPy doped with dodecylbenzenesulfonate (DBS), in view of electronic, linear actuation and sensory properties. Cyclic voltammetry, square wave potential steps and square wave amperometry conducted with isometric and isotonic electro-chemo-mechanical deformation measurements were performed in aqueous lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte. The films deposited from 0.01 M PTA showed the highest strain—as much as 20%. The highest conductivities, however, were obtained with films deposited from 0.005 M PTA—in range of 44 S cm⁻¹, accompanied by a high specific capacitance of 223 F g⁻¹. The highest PTA concentration (0.1 M) resulted in qualitatively different film properties and behavior.

In this work, we investigate polycrystalline Ni and FeGa magnetostrictive microstructures on pre-poled (011)-cut single crystal [Pb(Mg_1/3Nb_2/3)O₃]_1−x-[PbTiO₃]_x (PMN-PT, x ≈ 0.31) with linear strain profile versus applied electric field. Magnetostrictive microstructure arrays with various geometries are patterned on PMN-PT. Functionalized magnetic beads are trapped by localized stray fields originating from the microstructures. With an applied electric field, the magnetic domains are actuated, inducing the motion of the coupled particles with sub-micrometer precision. This work shows promise of using energy-efficient electric-field-controlled magnetostrictive micro- and nanostructures for manipulating magnetic beads via a linear strain response. The work also demonstrates the viability of cells suspended in solution on these structures when subject to applied electric fields, proving the cytocompatibility of the platform for live cell sorting applications.

Disordered hyperuniform heterogeneous materials are new, exotic amorphous states of matter that behave like crystals in the manner in which they suppress volume-fraction fluctuations at large length scales, and yet are statistically isotropic with no Bragg peaks. It has recently been shown that disordered hyperuniform dielectric two-dimensional (2D) cellular network solids possess complete photonic band gaps comparable in size to photonic crystals, while at the same time maintaining statistical isotropy, enabling waveguide geometries not possible with photonic crystals. Motivated by these developments, we explore other functionalities of various 2D ordered and disordered hyperuniform cellular networks, including their effective thermal or electrical conductivities and elastic moduli. We establish the multifunctionality of a class of such low-density networks by demonstrating that they maximize or virtually maximize the effective conductivities and elastic moduli. This is accomplished using the machinery of homogenization theory, including optimal bounds and cross-property bounds, and statistical mechanics. We rigorously prove that anisotropic networks consisting of sets of intersecting parallel channels in the low-density limit, ordered or disordered, possess optimal effective conductivity tensors. For a variety of different disordered networks, we show that when short-range and long-range order increases, there is an increase in both the effective conductivity and elastic moduli of the network. Moreover, we demonstrate that the effective conductivity and elastic moduli of various disordered networks derived from disordered 'stealthy' hyperuniform point patterns possess virtually optimal values. We note that the optimal networks for conductivity are also optimal for the fluid permeability associated with slow viscous flow through the channels as well as the mean survival time associated with diffusion-controlled reactions in the channels. In summary, we have identified ordered and disordered hyperuniform low-weight cellular networks that are multifunctional with respect to transport (e.g., heat dissipation and fluid transport), mechanical and electromagnetic properties, which can be readily fabricated using 3D printing and lithographic technologies.

Structures and devices with reversible shape change (RSC) are highly desirable in many applications such as mechanical actuators, soft robotics, and artificial muscles. In this paper, we propose to use 3D grayscale printing method to create reversible self-folding structures. The grayscale pattern was used to control the light intensity distribution of a UV projector in a digital light processing 3D printer such that the same photo irradiation time leads to different curing degrees and thus different crosslinking densities at different locations in the polymer during 3D printing. After leaching the uncured oligomers inside the loosely crosslinked network, bending deformation could be induced due to the volume shrinkage. The bending deformation was reversed if the bent structure absorbed acetone and swelled. Using this method, we designed and created RSC structures such as reversible pattern transformation and self-expanding/shrinking structures, auxetic metamaterial, structures mimicking the blossom of a flower. The grayscale 4D printing method provides us a simple and efficient way to create active structures and has great potential in the application of smart structures, composite materials, soft robotics and endovascular stent.

Carbon fibres (CFs), originally made for use in structural composites, have also been demonstrated as high capacity Li-ion battery negative electrodes. Consequently, CFs can be used as structural electrodes; simultaneously carrying mechanical load and storing electrical energy in multifunctional structural batteries. To date, all CF microstructural designs have been generated to realise a targeted mechanical property, e.g. high strength or stiffness, based on a profound understanding of the relationship between the graphitic microstructure and the mechanical performance. Here we further advance this understanding by linking CF microstructure to the lithium insertion mechanism and the resulting electrochemical capacity. Different PAN-based CFs ranging from intermediate- to high-modulus types with distinct differences in microstructure are characterised in detail by SEM and HR-TEM and electrochemical methods. Furthermore, the mechanism of Li-ion intercalation during charge/discharge is studied by in situ confocal Raman spectroscopy on individual CFs. Raman G band analysis reveals a Li-ion intercalation mechanism in the high-modulus fibre reminiscent of that in crystalline graphite. Also, the combination of a relatively low capacity of the high-modulus CFs (ca. 150 mAh g⁻¹) is shown to be due to that the formation of a staged structure is frustrated by an obstructive turbostratic disorder. In contrast, intermediate-modulus CFs, which have significantly higher capacities (ca. 300 mAh g⁻¹), have Raman spectra indicating a Li-ion insertion mechanism closer to that of partly disordered carbons. Based on these findings, CFs with improved multifunctional performance can be realised by tailoring the graphitic order and crystallite sizes.

We present electrostrictive materials with excellent properties for vibrational energy harvesting applications. The developed materials consist of a porous carbon black composite, which is processed using water-in-oil emulsions. In combination with an insulating layer, the investigated structures exhibit a high effective relative dielectric permittivity (up to 182 at 100 Hz) with very low effective conductivity (down to 2.53 10⁻⁸ S m⁻¹). They can generate electrical energy in response to mechanical vibrations with a power density of 0.38 W m⁻³ under an applied bias electric field of 32 V. They display figures or merit for energy harvesting applications well above reference polymer materials in the field, including fluorinated co- and ter-polymers synthetized by heavy chemical processes. The production process of the present materials is based on non hazardous and low-cost chemicals. The soft dielectric materials are highly flexible (Young's modulus of ∼1 MPa) making them also suited for highly sensitive capacitive sensors.

Multifunctional structures such as mechanical load-bearing batteries and supercapacitors require electrolytes that possess both mechanical robustness and high ionic conductivity. In this study, we use additive manufacturing to build three-dimensional interpenetrating structures as model systems for structural electrolytes. Maxwell truss structures with varying solid volume fractions were fabricated by printing thermoplastic molds using fused filament fabrication, injecting and curing epoxy resins, and then etching away the mold. These unit cells were then subject to uniaxial compression to characterize mechanical stiffness, and intercalated with liquid electrolyte with a form-fitting test cell to measure system ionic conductivity. Finite element simulations of the truss structures provide good agreement with the experimental data, and are then used to calculate shear properties that would be difficult to measure experimentally. The results show that the present truss systems provide superior multifunctional properties compared to prior structural polymer electrolyte systems, and suggest that segregated truss structures are a promising approach for creating multifunctional systems.