Soft Composites Research Articles

Using structural tensors for inelastic material modeling in the finite strain regime – A novel approach to anisotropic damage

The idea of the paper is to apply the well established structural tensors approach to complex inelastic material modeling in the finite strain regime. Using this concept, a very general and new anisotropic damage model is derived which is the major novel contribution of the paper. In this framework, a second order damage tensor plays the role of a structural tensor. The damage model is coupled to finite elasto-plasticity such that its first purpose is to model damage-induced anisotropy in ductile materials, in particular metals. Further structural tensors could be introduced to represent initial anisotropy of materials such as soft tissues, composites or thermoplastics. The main results of the derivation are the suitable choice of the Helmholtz free energy function, the development of yield and damage potentials as well as the formulation of physically reasonable evolution equations for the plastic deformation, the damage tensor as well as plastic and damage hardening. It is important to mention that all tensorial internal variables are symmetric and referred to the undeformed configuration. To the knowledge of the authors, the interpretation of a structural tensor of the undeformed configuration as damage tensor is new. Two-point tensors such as the plastic part of the deformation gradient show up in the theoretical derivation but are never computed. The plastic spin remains undetermined which has the advantage that a statement about its evolution is not needed. Further, the evolution equation for the plastic deformation includes only six non-linear scalar equations instead of nine. The numerical implementation of the model is straightforward. The number of material parameters remains moderate. A strategy to identify them by means of standard experiments is discussed. Nevertheless, the validation of the model by means of own experiments or experimental results from the literature is left for further work.

Switchable acoustic metamaterial design through instability-driven microstructure transformations in soft composites

Soft materials can sustain large deformations, and, thus, can be used as a platform for tunable and switchable systems capable of wave manipulations. The fascinating elastic instability phenomenon in soft materials can be used to design new (meta-) materials with switchable microstructures, properties, and functions. Examples include the emergence of tunable band gaps at low-frequency ranges, the appearance of negative group velocity, and extreme wave slowing-down. Here, we investigate the elastic instability phenomenon in soft heterogeneous materials, and the applications of the phenomenon to design of soft acoustic metamaterials. The deformable composites typically combine soft matrix and stiffer phases (such as fibers or inclusions). We will start by considering the effect of experimentally observed wrinkling on elastic waves band-gaps in soft laminates. Next, we will discuss the emergence of the negative group velocity in 3-D fiber composite brought to the “marginally stable” state. Then, we will turn to the so-called auxetic or negative Poisson's ratio materials comprising of soft- matrix-void-stiff-inclusion systems, and illustrate the mechanisms leading to the emergence of low-frequency band gaps. Finally, a new type of instabilities giving rise to anti-symmetric domain formations will be illustrated on the examples of 3-D printed soft composites.

Freeform fabrication of fiber-reinforced hydrogels using direct-write far-field electrospinning

Fiber-reinforced hydrogels are a class of soft composites that are seeing increased use in tissue engineering, regenerative medicine, and bio-printing. These hydrogels exhibit mechanical and biological properties that are similar to those found in living tissues. While advancements are being made on the biological feasibility of fiber-reinforced hydrogels, there are limits to the geometries realized using electrospinning techniques. To this end, the objective of this paper is to develop a freeform fabrication process that addresses two key limitations of existing fiber-reinforced hydrogel manufacturing processes, viz., their inability to manufacture (i) tall structures in the tens of millimeters height range; and (ii) structures with external overhangs. The freeform fabrication process involves the use of a direct-write far-field electrospinning print-head that enables the printing of specific electrospun fiber layers based on the cross-sectional geometry of the part. These fiber layers are then stacked to create a three-dimensional fiber pre-form. Finally, the fiber pre-form is submerged in specific hydrogel and cross-linking solutions to realize the fiber-reinforced hydrogel part of interest. In order to create an external overhang, a sacrificial support material is incorporated using hydrophobic fiber pre-forms to resist the infiltration of the hydrogel solution during the submersion step. The process has been successfully demonstrated both for thin parts with contours, and vasculature-mimicking tubular structures. Mechanical testing of the ∼1.6 wt. % fiber-reinforced hydrogels shows an order of magnitude improvement in the tensile properties when compared to published results for pure hydrogel.

Tunable Young's Moduli of Soft Composites Fabricated from Magnetorheological Materials Containing Microsized Iron Particles.

This study experimentally investigates the field-dependent Young’s moduli of soft composites, which are fabricated from two different magnetic-responsive materials; magnetorheological elastomer (MRE) and magnetorheological fluid (MRF). Four factors are selected as the main factors affecting Young’s modulus of soft composites: the amount of MRF, the channel pattern, shore hardness and carbonyl iron particle (CIP) concentration of the MRE layer. Five specimens are manufactured to meet the investigation of four factors. Prior to testing, the scanning electron microscopy (SEM) image is taken to check the uniform dispersion of the carbonyl iron particle (CIP) concentration of the MRE layer, and a magnetic circuit is constructed to generate the effective magnetic field to the specimen fixed at the universal tensile test machine. The force–displacement curve is directly measured from the machine and converted to the stress–strain relationship. Thereafter, the Young’s modulus is determined from this curve by performing linear regression analysis with respect to the considered factors. The tunability of the Young’s moduli of the specimens is calculated based on the experimental results in terms of two performance indicators: the relative percentage difference of Young’s modulus according to the magnetic field, and the normalized index independent of the zero-field modulus. In the case of the relative percentage difference, the specimens without MRF are the smallest, and the ones with the highest CIP concentration are the largest. As a result of comparing the normalized index of each factor, the change in shore hardness and channel pattern have little effect on the tunability of Young’s moduli, and the amount of MRF injected and CIP concentration of MRE have a large effect. The results of this study are expected to provide basic guidelines for fabricating soft composites whose field-dependent Young’s moduli can be tuned by several factors with different effects.

Numerical Investigation of Auxetic Textured Soft Strain Gauge for Monitoring Animal Skin.

Recent advances in hyperelastic materials and self-sensing sensor designs have enabled the creation of dense compliant sensor networks for the cost-effective monitoring of structures. The authors have proposed a sensing skin based on soft polymer composites by developing soft elastomeric capacitor (SEC) technology that transduces geometric variations into a measurable change in capacitance. A limitation of the technology is in its low gauge factor and lack of sensing directionality. In this paper, we propose a corrugated SEC through surface texture, which provides improvements in its performance by significantly decreasing its transverse Poisson’s ratio, and thus improving its sensing directionality and gauge factor. We investigate patterns inspired by auxetic structures for enhanced unidirectional strain monitoring. Numerical models are constructed and validated to evaluate the performance of textured SECs, and to study their performance at monitoring strain on animal skin. Results show that the auxetic patterns can yield a significant increase in the overall gauge factor and decrease the stress experienced by the animal skin, with the re-entrant hexagonal honeycomb pattern outperforming all of the other patterns.

Surface Engineering of Liquid Metal Nanodroplets by Attachable Diblock Copolymers.

Liquid metal (LM) micro/nano droplets have promising applications in various fields such as flexible electronics, catalysis, and soft composites as well as biomedicines. However, the preparation of highly stable LM nanodroplets suspension based on eutectic gallium/indium (EGaIn) alloys is still challenging. Herein, we report a general and robust strategy to fabricate EGaIn nanodroplets stabilized by polymer brushes (polymer brushes/EGaIn nanodroplets) via in situ attachment of well-defined diblock copolymers with short poly(acrylic acid) (PAA) anchoring segments. Under ultrasonication, the anchoring PAA block is in situ attached onto the gallium oxide "skin" layer of EGaIn nanodroplets to form polymer brushes. The attachable diblock copolymer surfactants allow for highly efficient formation of EGaIn nanodroplets in high yield and with narrow size distribution. The polymer brushes/EGaIn nanodroplets contain very low fractions of attached polymer (<1 wt %) and exhibit high colloidal stability (>30 days) and good redispersibility. Precise control of polymer architecture by atom-transfer radical polymerization was employed to prepare various block copolymers for suspensions in a variety of solvents.

Fiber-Reinforced Viscoelastomers Show Extraordinary Crack Resistance That Exceeds Metals.

Soft fiber-reinforced polymers (FRPs), consisting of rubbery matrices and rigid fabrics, are widely utilized in industry because they possess high specific strength in tension while allowing flexural deformation under bending or twisting. Nevertheless, existing soft FRPs are relatively weak against crack propagation due to interfacial delamination, which substantially increases their risk of failure during use. In this work, a class of soft FRPs that possess high specific strength while simultaneously showing extraordinary crack resistance are developed. The strategy is to synthesize tough viscoelastic matrices from acrylate monomers in the presence of woven fabrics, which generates soft composites with a strong interface and interlocking structure. Such composites exhibit fracture energy, Γ, of up to 2500 kJ m-2 , exceeding the toughest existing materials. Experimental elucidation shows that the fracture energy obeys a simple relation, Γ = W · lT , where W is the volume-weighted average of work of extension at fracture of the two components and lT is the force transfer length that scales with the square root of fiber/matrix modulus ratio. Superior Γ is achieved through a combination of extraordinarily large lT (10-100 mm), resulting from the extremely high fiber/matrix modulus ratios (104 -105 ), and the maximized energy dissipation density, W. The elucidated quantitative relationship provides guidance toward the design of extremely tough soft composites.

Ultra-High Actuation Stress Polymer Actuators as Light-Driven Artificial Muscles.

Remotely addressable actuators are of great interest in fields like microrobotics and smart textiles because of their simplicity, integrity, flexibility, and lightweight. However, most of the existing actuator systems are composed of complex assemblies and/or offer a low response rate. Here, the actuation performance of a light-driven, highly oriented film based on ultra-high molecular weight polyethylene (UHMW-PE), containing a photo-responsive additive, 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol (BZT), is reported. The material exhibits a fast (<1 s) and reversible photo-induced thermal response upon exposure to UV light, which results in an exceptionally high actuation stress (∼70 MPa) at a low strain (<0.1%). The proposed actuation mechanism originates from light absorption by BZT and energy transfer into heat, in combination with the intrinsic high stiffness (∼80 GPa) and a negative thermal expansion (NTE) of the oriented polymer films. This unique set of properties of this actuator, in particular the very high specific actuation stress, compared to existing organic and inorganic actuators, and the remote optical actuation, promises impact in fields related to soft robotics, composites, medical devices, optics, prosthetics, and smart textiles.

Soft Actuators Based on Liquid-Vapor Phase Change Composites.

Liquid-vapor phase change materials (PCMs), capable of significant volume change, are emerging as attractive actuating components in forming advanced soft composites for robotic applications. However, the novel and functional design of these PCM composites is significantly limited due to the lacking of the fundamental understanding of the mechanical properties, which further inhibits the broad applications of PCM based materials in the engineering structures requiring large deformation and high loading capacity. In this study we fabricate PCM-elastomer composites exhibiting large deformation and high output stress. Thermomechanical properties of these composites are experimentally and theoretically investigated, demonstrating enhanced deformation and loading capacity due to the induced vapor pressure. By controlling the distribution and content of the PCM inclusions, structures with tunable deformability under a relatively small strain in comparison with traditional soft materials are fabricated. Accompanying with the asymmetrical friction and deformation, complex locomotion and adaptable grabbing function are achieved with excellent performance.

Soft composite based hyperelastic model for anisotropic tissue characterization

Soft tissues are complex anisotropic composite systems comprising of multiple differently oriented layers of fiber embedded within a soft matrix. To date, soft tissues have been mainly characterized using simplified linear elastic material models, isotropic viscoelastic and hyperelastic models, and transversely isotropic models. In such models, the effect of fiber volume fraction (FVF), fiber orientation, and fiber-matrix interactions are missing, inhibiting accurate characterization of anisotropic tissue properties. The current work addresses this literature gap with the development of a novel soft composite based material framework to model tissue anisotropy. In this model, the fiber and matrix are considered as separate hyperelastic materials, and fiber-matrix interaction is modeled using multiplicative decomposition of the deformation gradient. The effect of the individual contribution of the fibers and matrix are introduced into the numerical framework for a single soft composite layer, and fiber orientation effects are incorporated into the strain energy functions. Also, strain energy formulations are developed for multiple soft composite layers with varying fiber orientations and contributions, describing the biomechanical behavior of an entire anisotropic tissue block. Stress-strain relationships were derived from the strain energy equations for a uniaxial mechanical test condition. To validate the model parameters, experimental models of soft composites tested under uniaxial tension were characterized using the novel anisotropic hyperelastic model (R2 = 0.983). To date, such a robust anisotropic hyperelastic composite framework has not been developed, which would be indispensable for experimental characterization of tissues and for improving the fidelity of computational biological models in future.

Composite Hydrogels in Three-Dimensional in vitro Models.

3-dimensional (3D) in vitro models were developed in order to mimic the complexity of real organ/tissue in a dish. They offer new possibilities to model biological processes in more physiologically relevant ways which can be applied to a myriad of applications including drug development, toxicity screening and regenerative medicine. Hydrogels are the most relevant tissue-like matrices to support the development of 3D in vitro models since they are in many ways akin to the native extracellular matrix (ECM). For the purpose of further improving matrix relevance or to impart specific functionalities, composite hydrogels have attracted increasing attention. These could incorporate drugs to control cell fates, additional ECM elements to improve mechanical properties, biomolecules to improve biological activities or any combinations of the above. In this Review, recent developments in using composite hydrogels laden with cells as biomimetic tissue- or organ-like constructs, and as matrices for multi-cell type organoid cultures are highlighted. The latest composite hydrogel systems that contain nanomaterials, biological factors, and combinations of biopolymers (e.g., proteins and polysaccharide), such as Interpenetrating Networks (IPNs) and Soft Network Composites (SNCs) are also presented. While promising, challenges remain. These will be discussed in light of future perspectives toward encompassing diverse composite hydrogel platforms for an improved organ environment in vitro.

Effects of polyether titanate coupling agent on the flame retardancy and mechanical properties of soft poly(vinyl chloride)/basic magnesium carbonate composites

AbstractIn this paper, a new polyether titanate coupling agent (Ti4000) was used to modify basic magnesium carbonate (BMC), and Ti4000 modified BMC (Ti4000‐BMC) was characterized by X‐ray powder diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and X‐ray photoelectron spectroscopy. Then, PVC composites were prepared by adding BMC or Ti4000‐BMC into soft PVC matrix via melt blending, and their flame retardancy was examined by limiting oxygen index (LOI), UL‐94 test, and cone calorimeter test. The results show that BMC can effectively improve flame resistance and suppress smoke production of pure soft PVC. As a result, the LOI is increased from 23.9% for pure soft PVC to 29.2% for Ti4000‐BMC/PVC composite with 50 phr Ti4000‐BMC (50%Ti4000‐BMC/PVC), and the composite passes the V0 rating in UL‐94 test, the peak heat release rate, total heat release, peak smoke production rate, and total smoke production are decreased from 343.10 kW/m2, 94.50 MJ/m2, 0.17 m2/s, and 27.20 m2/kg for pure soft PVC to 234.20 kW/m2, 70.20 MJ/m2, 0.02 m2/s, and 2.30 m2/kg for 50%Ti4000‐BMC/PVC composite, respectively. The analysis of char residues suggests that Ti4000‐BMC/PVC composites form the most compact char layer structure, indicating that the soft PVC composites show better char forming ability. In addition, the yield strength is increased from 4.4 MPa for pure soft PVC to 12.9 MPa for 50%Ti4000‐BMC/PVC composite, with an increase of 193.2%, and it shows good reinforcement effect on the soft PVC matrix.

Efficient Syntheses of 2D Materials from Soft Layered Composites Guided by Yield Prediction Model: Potential of Experiment‐Oriented Materials Informatics

AbstractExfoliation is a common route to synthesize 2D materials such as monolayered and few‐layered nanosheets. However, it is not easy to control the breaking‐down process. This work shows a new exfoliation strategy for generation of nanosheets from soft layered composites in high yield. Soft layered composites of inorganic hosts and organic guests are exfoliated into nanosheets in organic dispersion media. A simple yield prediction model is constructed on small‐scale experimental data by sparse modeling, a data‐scientific approach, combined with chemical perspective. The prediction model indicates the guest–medium combinations enabling high‐ and low‐yield syntheses of the nanosheets. The high‐ and low‐yield syntheses are actually achieved on different types of layered transition metal oxides in a minimum number of experiments. The number of experiments is reduced by 94%. The experimental optimization realizes further improvement of the yield, namely, shortening the exfoliation time. The present strategies can be applied to obtain a variety of 2D materials through exfoliation. Moreover, the present work suggests that combination of small experimental data, data‐scientific approach, and chemical perspective, i.e., experiment‐oriented materials informatics, has potentials for acceleration of materials design.

3D Printing of Highly Sensitive and Large-Measurement-Range Flexible Pressure Sensors with a Positive Piezoresistive Effect.

Piezoresistive composite-based flexible pressure sensors often suffer from a trade-off between the sensitivity and measurement range. Moreover, the sensitivity or measurement range is theoretically limited owing to the negative piezoresistive coefficient, resulting in resistance variation below 100%. Here, flexible pressure sensors were fabricated using the three-dimensional (3D) printing technique to improve both the sensitivity and sensing range through the positive piezoresistive effect. With the addition of carbon nanotubes (CNTs) and fumed silica nanoparticles (SiNPs) as a conductive filler and rheology modifier, respectively, the viscoelastic silicone rubber solution converted to a printable gel ink. Soft and porous composites (SPCs) were then directly printed in air at room temperature. The sensitivity and sensing range of the SPC-based pressure sensor can be simultaneously tuned by adjusting the conducting CNT and insulating SiNP contents. By optimizing the density of the CNT conductive network in the matrix, positive piezoresistive sensitivity (+0.096 kPa-1) and a large linear sensing range (0-175 kPa) were obtained. To demonstrate potential applications, the completely soft SPC-based sensor was successfully used in grasp sensing and gait monitoring systems. The 3D printed sensors were also assembled as a smart artificial sensory array to map the pressure distribution.

Modeling SMA-enabled soft deployable structures for kirigami/origami reflectors

The synergic combination of smart soft composites with kirigami/origami principles leads to self-deployable systems. To date, the development of soft deployable structures has largely been an empirical process. Focusing on the recently developed shape memory alloy (SMA)-based soft deployable structures, this paper describes an analytical model and a finite element (FE) numerical scheme to investigate deformation and deployment performance of this system. The study provides insights into the working principles of these soft deployable structures by understanding how the system deforms as a function of the SMA recovery strain for a number of geometrical parameters. The FE method was further used to simulate the fabrication and deployment process of three types of kirigami/origami reflectors. The realization of a two-step scheme, i.e., predeformation of the deployable frame and then the concurrent deformation of both frame and kirigami/origami films, is detailed herein. This makes our simulation capable of capturing the features of SMA-based soft deployable structures observed in experiments, and also renders the prediction versatile and manufacturing-oriented. We expect the work would enhance the understanding of soft deployable structures, and may open the door for design and fabrication of soft robotics and machines.

Programmable Liquid Metal Microstructures for Multifunctional Soft Thermal Composites

AbstractSoft, elastically deformable composites can enable new generations of multifunctional materials for electronics, robotics, and reconfigurable structures. Liquid metal (LM) droplets dispersed in elastomer matrices represent an emerging material architecture that has shown unique combinations of soft mechanical response with exceptional electrical and thermal functionalities. These properties are strongly dependent on the material composition and microstructure. However, approaches to control LM microdroplet morphology to program mechanical and functional properties are lacking. Here, this limitation is overcome by thermo‐mechanically shaping LM droplets in soft composites to create programmable microstructures in stress‐free materials. This enables LM loadings up to 70% by volume with prescribed particle aspect ratios and orientation, enabling control of microstructure throughout the bulk of the material. Through this microstructural control in soft composites, a material which simultaneously achieves a thermal conductivity as high as 13.0 W m−1 K−1 (&gt;70 × increase over polymer matrix) with low modulus (&lt;1.0 MPa) and high stretchability (&gt;750% strain) is demonstrated in stress‐free conditions. Such properties are required in applications that demand extreme mechanical flexibility with high thermal conductivity, which is demonstrated in soft electronics, wearable robotics, and electronics integrated into 3D printed materials.

Effect of the Elastomer Matrix on Thermoplastic Elastomer-Based Strain Sensor Fiber Composites.

In this study, a thermoplastic elastomer sensor fiber was embedded in an elastomer matrix. The effect of the matrix material on the sensor properties and the piezoresistive behavior of the single fiber-matrix composite system was investigated. For all composites, cycling test (dynamic test) and the relaxation behavior at different strains (quasi-static test) were investigated. In all cases, dynamic properties and quasi-static significantly changed after embedding, compared to the pure fiber. The composite with the silicone elastomer PDMS (Polydimethylsiloxane) as matrix material exhibited deviation from linear response of the resistivity at low strains and proved an unsuitable choice compared to natural rubber. The addition of a spring construct in the embedded sensor fiber natural rubber composite improved the linearity at low strains but increased the mechanical and electrical hysteresis of the soft matter sensor composite. Using pre-vulcanized natural rubber improved linearity at low strains and reduced significantly the stress and relative resistance relaxation as well as the resistance hysteresis, especially if the resistance remained low. In both cases of the pre-vulcanized rubber and the spring structure, the piezoresistive behavior was improved, and at the same time, the stiffness of the system was increased indicating that using a stiffer matrix can be a strategy for improving the sensor properties.

Microstructural evolution and failure in short fiber soft composites: Experiments and modeling

In this work, we systematically study the mechanics of short glass fiber composites with a soft, hyperelastic matrix (polydimethylsiloxane). The fiber orientation can be controlled by fabricating the materials with direct ink writing, an extrusion-based 3D printing method. We specifically characterize the damage evolution in these materials when subject to tensile and cyclic loading by developing a micromechanical model, a continuum model, and by performing experiments. The stress-stretch behavior and damage evolution are highly-dependent on both volume fraction and fiber orientation. We focus primarily on loading that is parallel with the fiber alignment, which shows rich complexities as a function of volume fraction, including variable softening and non-monotonic increase of the yield strength. Unlike most prior work, we use in situ optical measurements and digital image correlation during mechanical loading to quantify the highly-nonhomogeneous stretch field at macroscopic and microscopic length scales, and to simultaneously visualize the mechanisms underlying the damage evolution and complexities in the stress-stretch response, namely fiber-matrix debonding. The micromechanical model is based on modifications to the classic shear lag model, which include incorporation of fiber length distribution as a function of volume fraction and use of Weibull distributions to account for the probability of fiber-matrix debonding. The continuum model is based on hyperelastic strain energy with damage parameters for both matrix and fibers. The combination of the two models is able to capture the stress-stretch behavior and damage evolution well, and also the Mullins-like, history-dependent cyclic loading behavior we observe in experiments.

Instability-Induced Pattern Formations in Soft Magnetoactive Composites.

Elastic instabilities can trigger dramatic microstructure transformations giving rise to unusual behavior in soft matter. Motivated by this phenomenon, we study instability-induced pattern formations in soft magnetoactive elastomer (MAE) composites deforming in the presence of a magnetic field. We show that identical MAE composites with periodically distributed particles can switch to a variety of new patterns with different periodicity upon developments of instabilities. The newly formed patterns and postbuckling behavior of the MAEs are dictated by the magnitude of the applied magnetic field. We identify the particular levels of magnetic fields that give rise to strictly doubled or multiplied periodicity upon the onset of instabilities in the periodic particulate soft MAE. Thus, the predicted phenomenon can be potentially used for designing new reconfigurable soft materials with tunable material microstructures remotely controlled by a magnetic field.

Micromechanical Properties of Microstructured Elastomeric Hydrogels.

Local, micromechanical environment is known to influence cellular function in heterogeneous hydrogels, and knowledge gained in micromechanics will facilitate the improved design of biomaterials for tissue regeneration. In this study, a system comprising microstructured resilin-like polypeptide (RLP)-poly(ethylene glycol) (PEG) hydrogels is utilized. The micromechanical properties of RLP-PEG hydrogels are evaluated with oscillatory shear rheometry, compression dynamic mechanic analysis, small-strain microindentation, and large-strain indentation and puncture over a range of different deformation length scales. The measured elastic moduli are consistent with volume averaging models, indicating that volume fraction, not domain size, plays a dominant role in determining the low strain mechanical response. Large-strain indentation under a confocal microscope enables the visualization of the microstructured hydrogel micromechanical deformation, emphasizing the translation, rotation, and deformation of RLP-rich domains. The fracture initiation energy results demonstrate that failure of the composite hydrogels is controlled by the RLP-rich phase, and their independence with domain size suggested that failure initiation is controlled by multiple domains within the strained volume. This approach and findings provide new quantitative insight into the micromechanical response of soft hydrogel composites and highlight the opportunities in employing these methods to understand the physical origins of mechanical properties of soft synthetic and biological materials.