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Self-sensing materials with deformation-dependent electrical conductivity have been studied in diverse applications. A limitation of self-sensing materials is that they do not directly report on the underlying condition of the material. That is, it would be much more useful to know the actual stresses, strains or damages that give rise to observed conductivity changes. The process of deducing material condition from electrical data is called theself-sensing inverse problem(SSIP). Previous work has required electrical impedance tomography (EIT) as an intermediate step to estimate the conductivity distribution prior to estimating the spatially varying mechanics. But this is undesirable because EIT is an ill-posed inverse problem and is highly dependent on regularization, which renders the SSIP subject to the same assumptions and limitations of EIT. The contribution of this manuscript is the development of adirectSSIP formulation that omits the EIT step such that mechanics are predicted directly from voltage–current data. The effects of regularization type, regularization norm and formulation (difference versus absolute imaging) are also explored. The direct SSIP formulation is experimentally validated on a soft carbon nanofibre-modified polyurethane pressure sensor and compared to ANSYS-solved mechanics with good agreement. 

Abstract Materials that are self-sensing via the piezoresistive effect have been widely explored for embedded sensing in aerospace structural composites, pressure sensing in touch pads, diagnostics in biomedical implant technology, and many other applications. In this approach, changes in electrical transport of the material are used as an indicator of stress/strain, pressure, or damage. However, engineers and other users of self-sensing materials are typically not directly interested in the electrical state of the material. Rather, they want to know the underlying mechanical state of the material that gives rise to an observed electrical change. Recovering material condition from electrical observations is referred to as the self-sensing inverse problem (SSIP). Prior work by the author has shown that the SSIP can be solved using an electrical impedance tomography (EIT)-generated conductivity map as an input, but this is undesirable because it requires solving a second inverse problem (i.e., the SSIP) on top of the EIT inverse problem. To that end, a direct formulation for the SSIP is herein presented. In this approach, voltage-current data is directly inverted to find the displacement field without using EIT as an intermediate step. Additionally, the direct SSIP is solved within the primal-dual interior point (PDIPM) framework such that an ℓ1-norm can be used on the regularization term, which promotes sparsity in the solution space. This approach is applied to a representative piezoresistive nanocomposite using the Laplace matrix as regularization to promote a spatially smooth solution. From the displacement field, strains and stresses are calculated and compared to a commercial finite element solution with good accuracy. 

Abstract As organisms age, the fitness of the offspring they produce can decline, which is often attributed to parental senescence. However, few studies have tested for effects of parental age on offspring fitness in wild populations or in short-lived vertebrates, and only recently have studies begun to examine such effects in male and female offspring independently. Here, we use five generations of mark-recapture and genetic parentage data from an island population of a short-lived lizard, the brown anole (Anolis sagrei), to test for effects of maternal and paternal age on the survival to adulthood, first-year reproductive success, longevity, and lifetime fitness of their offspring. When comparing parents of different ages within the same offspring cohort, survival to adulthood increased with paternal age in sons, but we found no effects of maternal or paternal age on any component of fitness in daughters and no evidence that parental age effects differed based on the sex of the parent or the offspring. When considering repeated measures of individual parents sampled at multiple ages, we found that first-year reproductive success of sons decreased with paternal age, but longevity of sons increased with maternal age. However, when pooling sons and daughters, we found no overall effects of parental age on any component of offspring fitness, and little evidence that parental age effects differed between sons and daughters. Our study adds to the growing literature suggesting that negative effects of parental age on offspring fitness may not be as prevalent as once thought, particularly in wild populations. 

Abstract Dispersal is a costly, though potentially rewarding, behavior with important fitness consequences for juveniles. When the costs or benefits of juvenile dispersal differ between sexes, sex-biased dispersal should be favored, though such sex-specific consequences are rarely measured for multiple components of lifetime fitness in the wild. Here, we use detailed mark–recapture data from 4 annual cohorts of juveniles with associated estimates of juvenile survival and adult reproductive success from genetic parentage to measure natural selection on 2 dispersal phenotypes (binary dispersal propensity and continuous dispersal distance) in an island population of lizards (Anolis sagrei). Juvenile dispersal was consistently male-biased, with males exhibiting a higher propensity to disperse and dispersing twice as far as females. Males that dispersed had higher survival to adulthood and total fitness than males that remained philopatric, whereas fitness components did not differ between dispersing and philopatric females. Although this result indicates that the fitness benefits of dispersal are sex-specific, we found no difference in fitness components between dispersing and philopatric males after accounting for body size, which was positively correlated with dispersal. Likewise, we did not consistently detect direct selection on dispersal distance in either sex when including body size in multivariate selection analyses, nor did we find consistent correlational selection on combinations of body size and dispersal distance in either sex. We conclude that selection on juvenile dispersal propensity is sex-specific, but likely indirect and mediated through its positive association with body size in males. 

Polymers are a primary building block in many biomaterials, often interacting with anisotropic backgrounds. While previous studies have considered polymer dynamics within nematic solvents, rarely are the effects of anisotropic viscosity and polymer elongation differentiated. Here, we study polymers embedded in nematic liquid crystals with isotropic viscosity via numerical simulations to explicitly investigate the effect of nematicity on macromolecular conformation and how conformation alone can produce anisotropic dynamics. We employ a hybrid multi-particle collision dynamics and molecular dynamics technique that captures nematic orientation, thermal fluctuations and hydrodynamic interactions. The coupling of the polymer segments to the director field of the surrounding nematic elongates the polymer, producing anisotropic diffusion even in nematic solvents with isotropic viscosity. For intermediate coupling, the competition between background anisotropy and macromolecular entropy leads to hairpins – sudden kinks along the backbone of the polymer. Experiments of DNA embedded in a solution of rod-like fd viruses qualitatively support the role of hairpins in establishing characteristic conformational features that govern polymer dynamics. Hairpin diffusion along the backbone exponentially slows as coupling increases. Better understanding two-way coupling between polymers and their surroundings could allow the creation of more biomimetic composite materials. 

Abstract Active processes drive biological dynamics across various scales and include subcellular cytoskeletal remodelling, tissue development in embryogenesis and the population-level expansion of bacterial colonies. In each of these, biological functionality requires collective flows to occur while self-organised structures are protected. However, the mechanisms by which active flows can spontaneously constrain their dynamics to preserve structure are not known. Here, by studying collective flows and defect dynamics in active nematic films, we demonstrate the existence of a self-constraint, namely a two-way, spontaneously arising relationship between activity-driven isosurfaces of flow boundaries and mesoscale nematic structures. We show that self-motile defects are tightly constrained to viscometric surfaces, which are contours along which the vorticity and the strain rate are balanced. This in turn reveals that self-motile defects break mirror symmetry when they move along a single viscometric surface. This is explained by an interdependence between viscometric surfaces and bend walls, which are elongated narrow kinks in the orientation field. These findings indicate that defects cannot be treated as solitary points. Instead, their associated mesoscale deformations are key to the steady-state coupling to hydrodynamic flows. This mesoscale cross-field self-constraint offers a framework for tackling complex three-dimensional active turbulence, designing dynamic control into biomimetic materials and understanding how biological systems can employ active stress for dynamic self-organisation. 

Abstract Phenotypic sexual dimorphism often involves the hormonal regulation of sex-biased expression for underlying genes. However, it is generally unknown whether the evolution of hormonally mediated sexual dimorphism occurs through upstream changes in tissue sensitivity to hormone signals, downstream changes in responsiveness of target genes, or both. Here, we use comparative transcriptomics to explore these possibilities in 2 species of Sceloporus lizards exhibiting different patterns of sexual dichromatism. Sexually dimorphic S. undulatus develops blue and black ventral coloration in response to testosterone, while sexually monomorphic S. virgatus does not, despite exhibiting similar sex differences in circulating testosterone levels. We administered testosterone implants to juveniles of each species and used RNAseq to quantify gene expression in ventral skin. Transcriptome-wide responses to testosterone were stronger in S. undulatus than in S. virgatus, suggesting species differences in tissue sensitivity to this hormone signal. Species differences in the expression of genes for androgen metabolism and sex hormone-binding globulin were consistent with this idea, but expression of the androgen receptor gene was higher in S. virgatus, complicating this interpretation. Downstream of androgen signaling, we found clear species differences in hormonal responsiveness of genes related to melanin synthesis, which were upregulated by testosterone in S. undulatus, but not in S. virgatus. Collectively, our results indicate that hormonal regulation of melanin synthesis pathways contributes to the development of sexual dimorphism in S. undulatus, and that changes in the hormonal responsiveness of these genes in S. virgatus contribute to the evolutionary loss of ventral coloration.