One of the defining properties of biological structural materials is self-healing, i.e., the ability to undergo long-term reparation after instantaneous damaging events, but also after microdamage due to repeated load cycling. To correctly model the fatigue life of such materials, self-healing must be included in fracture and fatigue laws, and related codes. Here, we adopt a numerical modelization of fatigue cycling of self-healing biological materials based on the hierarchical fiber bundle model and propose modifications in Griffith's and Paris' laws to account for the presence of self-healing. Simulations allow us to numerically verify these modified expressions and highlight the effect of the self-healing rate, in particular, for collagen-based materials such as human tendons and ligaments. The study highlights the effectiveness of the self healing process even for small healing rates and provides the possibility of improving the reliability of predictions of fatigue life in biomechanics, e.g., in sports medicine.

A facile method is developed to synthesize ion-based π-conjugated polymer nanoparticles. The polymer chosen is anionic poly(2-methoxy-5-propyloxysulfonate phenylene vinylene); MPS-PPV. The synthesis is based on nanoagglomeration via polyion association (termed as NAPA method) that includes polyion complex formation and subsequent globulization to fabricate nanoarchitectures in water-based liquid. Salient features include no spectral shift in the UV–vis absorption upon the nanoparticle formation, in contrast to a common observation that conjugated polymer nanoparticles made by a reprecipitation method exhibit blue shift in their peak. This behavior suggests the absence of further bending or kinking of the polymer backbone during nanoglobulization. Fluorescence peak energy is size dependent; the larger the particle size is, the lower is its fluorescence energy. This can be dominantly ascribed to the increased contribution of sole excitation of the chromophoric segments that have a long effective conjugation length. Because a small (large) particle has a large (small) surface-to-volume ratio, the blue-site (red-site) fluorescence is associated with the surface (inner) region of the polymer nanoparticles, respectively.

Predicting the impact of cross-links on the mechanics of carbon nanotube-based materials is a challenging endeavor, as the micro- and nanostructure is composed of continuous nanofibers, discontinuous interfaces, and covalent bridges. Here we demonstrate a new modeling solution in the context of the distinct element method (DEM). By representing nanotubes as bonded cylinder segments undergoing van der Waals adhesion, viscous friction, and contact bonding, we are able to simulate how cross-linking transforms a soft bundle into a strong one. We predict that the sp3-sp cross-links formed by interstitial carbon atoms can improve the tensile strength by an order of magnitude, in agreement with experiment and molecular dynamics simulations. The DEM methodology allows performing the multiscale simulation needed for developing strategies to further enhance the mechanical performance.

The mechanical behavior of human hair is determined by the interaction of trichocyte alpha keratin protein, matrix, and disulfide bonds crosslinking. Much effort has been spent to understand the link between the microscopic structure and the macroscopic fiber properties. Here we apply a mesoscopic coarse-grained model of the keratin macrofilament fibril combined with an analytical solution based on the concept of entropic hyperelasticity of the protein helix to investigate the link between the microscopic structure and the macroscopic properties of keratin fibers. The mesoscopic model provides good agreement with a wide range of experimental results. Based on the mesoscopic model, the predicted stress–strain curve of hair fibers agrees well with our own experimental measurements. The disulfide crosslink between the microfibril–matrix and matrix–matrix contributes to the initial modulus and provides stiffening at larger deformation of the trichocyte keratin fibers. The results show that the disulfide bonds reinforce the macrofilament and enhance the robustness of the macrofilament by facilitating the microfilaments to deform cooperatively. The availability of a mesoscopic model of this protein opens the possibility to further explore the relationship between microscopic chemical structure and macroscopic performance for a bottom-up description of soft materials.

The mechanical properties of nanoscale free-standing polymer thin films exhibit size dependence due to surface effects. However, it remains a challenge to determine the length scales at which differences are exhibited between film and bulk polymer properties. Here we use molecular dynamics simulations to uncover the dependence of elastic modulus (E) of free-standing films on film thickness and bulk properties. Comparison of the glass transition temperature (Tg) and E indicates that Tg converges to the bulk value slightly faster as the film thickness increases. The free-surface effects that give rise to a depression in E and Tg are observed to be stronger for polymers with weaker intermolecular interactions. The most intriguing aspect of our study is the finding that despite the observed decrease in the modulus of the film up to a thickness of over 100 nm, the local stress distribution reveals that the preserved length scale of perturbation of the free surface is only several nanometers.

The carbon foams prepared by the thermo-foaming of dispersions of activated carbon (AC) powder of various average particle sizes (4.9 to 15 μm) in an aqueous sucrose resin were characterized. The stability of the wet foams increased with the decrease in the AC particle size as finer particles preferentially adsorbed on the air–resin interface. The particle agglomeration leading to the foam collapse was observed at lower AC powder to sucrose weight ratios with the finer powders. The cell size (0.33–2.34 mm), foam density (0.1151–0.2281 g/cm3), and compressive strength (0.16–2.77 MPa) of the carbon foams depend on the AC particle size as well as the AC powder to sucrose weight ratio. The thermal conductivity of the carbon foams (0.036–0.049 W m−1 K−1) was much lower than that of the vitreous carbon foams of similar densities. The foams were fire resistant and amenable to machining with the conventional machines and tool.

Pristine multiwalled carbon nanotubes (P-MWCNTs) were functionalized with carboxylic groups (MWCNT-COOH) through oxidation reactions and then reduced to produce hydroxyl groups (MWCNT-OH). Pristine and functionalized MWCNTs were used to produce poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) nanocomposites with 0.5 wt% of MWCNTs. MWCNT functionalization was verified by visual stability in water, infrared and Raman spectroscopy, and zeta potential measurements. Pristine and functionalized MWCNTs acted as the nucleating agent in a PHBV matrix, as verified by differential scanning calorimetry (DSC). However, the dispersion of filler into the matrix, thermal stability, and direct current (DC) conductivity were affected by MWCNT functionalization. Scanning electron microscopy (SEM) showed that filler dispersion into the PHBV matrix was improved with MWCNT functionalization. The surface roughness was reduced with the addition and functionalization of MWCNT. The thermal stability of PHBV/MWCNT-COOH, PHBV/P-MWCNT, and PHBV/MWCNT-OH nanocomposites were 20, 30, and 30 °C higher than neat PHBV, respectively, as verified by thermogravimetry analysis (TGA). Addition of pristine and functionalized MWCNTs provided electrical conductivity in nanocomposite, which was higher for PHBV/P-MWCNTs (1.2 × 10−5 S cm−1).

The dispersion of carbon nanotubes is one of the problems in the application of polymer nanocomposites. In this study, the effect of chemical functionalization of the carbon nanotube surface on the dispersion of the tubes within a polymer is reported. The effect of carbon nanotube weight loading on the properties of polymer membrane was also studied. Multiwalled carbon nanotubes were dispersed in Nafion® matrix by melt processing techniques to form nanocomposite membranes. The morphology, dc electrical conductivity, thermal stability, mechanical properties, and proton conductivity of these nanocomposites were investigated. Nitric acid functionalized carbon nanotubes were evenly dispersed with Nafion as observed by scanning electron microscopy. The measurements of mechanical properties indicate that this processing method and carbon nanotube loading can improve the modulus of the nanocomposites.

In recent years wearable devices have attracted significant attention. Flexibility and stretchability are required for comfortable wear of such devices. In this paper, we report flexible and stretchable touch sensors with two different patterns (interdigitated and diamond-shaped capacitors). The touch sensors were made of screen-printed silver nanowire electrodes embedded in polydimethylsiloxane. For each pattern, the simulation-based design was conducted to choose optimal dimensions for the highest touch sensitivity. The sensor performances were characterized as-fabricated and under deformation (e.g., bending and stretching). While the interdigitated touch sensors were easier to fabricate, the diamond-shaped ones showed higher touch sensitivity under as-fabricated, stretching or even bending conditions. For both types of sensors, the touch sensitivity remained nearly constant under stretching up to 15%, but varied under bending. They also showed robust performances under cyclic loading and against oxidation.

Polymethylmethacrylate (PMMA) and polyimide (PI) samples are implanted by 40 keV Cu+ ions with high fluences to synthesize copper nanoparticles in shallow polymer layers. The produced metal/polymer nanocomposites are studied using atomic force and scanning electron microscopies as well as optical transmission spectroscopy. It is found that nucleation and growth of copper nanoparticles are strongly fluence-dependent as well as they are affected by the polymer properties, in particular, by radiation stability yielding different nanostructures for the implanted PI and PMMA. Shallow synthesized nanoparticles are observed to partly tower above the sample surface due to a side effect of high-fluence irradiation leading to considerable sputtering of polymers. Implantation and particle formation significantly change optical properties of both polymers reducing transmittance in the UV–visible range due to structural and compositional change as well as causing an absorption band related to localized surface plasmon resonance (LSPR) of the nanoparticles. The role of polymer type and its degradation under the implantation on LSPR is studied to optimize conditions for the formation of nanoplasmonic materials.

Piezoelectricity and magnetoelectricity are contradictory properties with a rather limited set of natural (often hard) materials that exhibit both. Composite materials – almost always restricted to hard ones – provide a limited recourse with the attendant limitations of small strains, fabrication challenges among others. In this article, using the concept of electrets, we propose a simple scheme to design soft, highly deformable materials that simultaneously exhibit piezoelectricity and magnetoelectricity. We demonstrate that merely by embedding charges and ensuring elastic heterogeneity, the geometrically nonlinear behavior of soft materials leads to an emergent piezoelectric and magnetoelectric behavior. We find that an electret configuration made of sufficiently soft (nonpiezoelectric and nonmagnetic) polymer foams can exhibit simultaneous magnetoelectricity and piezoelectricity with large coupling constants that exceed the best-known ceramic composites. Moreover, we show that these properties can be tuned with the action of an external field.

In this work, the electrical characterization of nanocomposites made of epoxy resins with multiwalled carbon nanotubes is presented. As the filler, two different types of multiwalled carbon nanotubes with different aspect ratios (280, 1250) and defectiveness were selected. The production procedure, the morphological characterization, the I–V DC characteristics, and the low frequency complex permittivity (in the range 100 kHz–12 MHz) of these nanocomposites are discussed. To investigate the dispersion of the solution, a study which linked the mixing time to the zeta potential was performed. The experimental results show that with the same matrix and by using the same measurement techniques, the two nanocomposites give different results and can be correlated with the characteristics of nanotubes. The dc conductivity of the nanocomposites was measured by means of a two-point probe technique. The conductivity in the frequency range 100 KHz–12 MHz was evaluated using a circular disk capacitor and measuring the impedance. The measured conductivity follows a percolation scaling law of the form σ ∝ (p − pc)t. A best fit to the measured conductivity data was obtained and the values of the exponent t compared to those in the literature.

The overarching aim of biomimetic approaches to materials synthesis is to mimic simultaneously the structure and function of a natural material, in such a way that these functional properties can be systematically tailored and optimized. In the case of synthetic spider silk fibers, to date functionalities have largely focused on mechanical properties. A rapidly expanding body of literature documents this work, building on the emerging knowledge of structure–function relationships in native spider silks, and the spinning processes used to create them. Here, we describe some of the benchmark achievements reported until now, with a focus on the last five years. Progress in protein synthesis, notably the expression on full-size spidroins, has driven substantial improvements in synthetic spider silk performance. Spinning technology, however, lags behind and is a major limiting factor in biomimetic production. We also discuss applications for synthetic silk that primarily capitalize on its nonmechanical attributes, and that exploit the remarkable range of structures that can be formed from a synthetic silk feedstock.

Soft, sensitive, and conformable strain sensors can provide tactile sensation to prosthetic limbs and can be used for epidermal and wearable health monitoring. High strain sensitivity is often achieved by using piezoelectric ceramics, such as lead zirconate titanate (PZT), with known issues for large-area scalability, rigidity, and biocompatibility. Here, we report a nature-inspired, piezoresistive, soft, and benign core–shell nanofibrous sensor that exhibits an unprecedented gauge factor in excess of 60, arising from a reversible disjointing/jointing of a large number of interfiber junctions, consequently changing the current path and resistance in response to both tensile and compressive strains. Nanofiber textile sensor arrays are demonstrated with fast, low-voltage, accurate, and repeatable sensing over 1000 cycles for epidermal monitoring of limb and musculoskeletal movements and radial pulse waveform, for real-time monitoring of simulated intermittent Parkinson's tremors, and for biaxial tactile sensing and localization of point of touch.

Arrangement and conformational interactions of carbon nanotubes (CNTs) and matrix upon electrospinning have been examined by surface-sensitive helium ion microscopy (He-IM). The enhanced surficial information is mostly a consequence of convoluted topographic sensitivity and reduced electrostatic charging, resulting from He ion–matter interactions. In addition, we have explored the correlation of findings by He-IM imaging with secondary electron microscopy (SEM), transmission electron microscopy (TEM), and near edge x-ray absorption fine structure (NEXAFS) spectroscopy; the latter encouraged by similar sampling depth profiles in both techniques. This study provides further evidence of strong conformational relations between filler and matrix (which we have reported recently) and of the presence of a tightly bound polymeric phase interaction between the CNT and the bulk matrix. We conclude that both the absence of electrostatic charging and enhanced surface sensitivity in He-IM offer a remarkable opportunity to study electrospinning dynamics in nanocomposites.

Through implicit solvent coarse-grained molecular dynamics simulations, we investigate the equilibrium morphologies resulting from the self-assembly of building blocks composed of anisotropically functionalized icosahedral viral capsid nanoparticles (NPs). We investigate the self-assembled aggregate morphologies for variations in the functional group chain length and solvent quality. We observe specific building block architectures to favor the formation of n-mers, chain- and network-like structures. Our work is in agreement with the earlier simulation studies on icosahedral gold nanocrystals that generate self-assembled chain-like structures. [G. Bilalbegovic, Comput. Mater. Sci.31, 181 (2004).] In addition, our results agree with those by Finn et al., who have shown small predominantly chain-like aggregates with mannose-decorated cowpea mosaic virus (CPMV) [K.S. Raja, Q. Wang, and M.G. Finn, ChemBioChem4, 1348–1351 (2003)] and small aggregates with oligonucleotide functionalization on the CPMV capsid. [E. Strable, J.E. Johnson, and M.G. Finn, Nano Lett.4, 1385–1389 (2004).] Visual inspection suggests that our results most likely span the low temperature limits explored by Finn et al. and show a good degree of agreement with the experimental results at an annealing temperature of 4 °C. [E. Strable, J.E. Johnson, and M.G. Finn, Nano Lett.4, 1385–1389 (2004).] Our investigations reveal the possibility of novel n-mer type aggregates that could be synthesized using icosahedral NPs with appropriate surface functionalization and solvent conditions.