Mechanical and failure properties of extracellular matrix sheets as a function of structural protein composition

Exploiting Matrix Stiffness to Overcome Drug Resistance

Drug resistance is arguably one of the biggest challenges facing cancer research today. Understanding the underlying mechanisms of drug resistance in tumor progression and metastasis are essential in developing better treatment modalities. Given the matrix stiffness affecting the mechanotransduction capabilities of cancer cells, characterization of the related signal transduction pathways can provide a better understanding for developing novel therapeutic strategies. In this review, we aimed to summarize the recent advancements in tumor matrix biology in parallel to therapeutic approaches targeting matrix stiffness and its consequences in cellular processes in tumor progression and metastasis. The cellular processes governed by signal transduction pathways and their aberrant activation may result in activating the epithelial-to-mesenchymal transition, cancer stemness, and autophagy, which can be attributed to drug resistance. Developing therapeutic strategies to target these cellular processes in cancer biology will offer novel therapeutic approaches to tailor better personalized treatment modalities for clinical studies.

Emergence of an apparent yield phenomenon in the mechanics of stochastic networks with inter-fiber cohesion

In this work we investigate the contribution of inter-fiber cohesion to defining the mechanical behavior of stochastic crosslinked fiber networks. Fibers are athermal and store energy primarily in their bending and axial deformation modes. Cohesion between fibers is defined by an interaction potential. These structures are in equilibrium with the inter-fiber cohesive forces before external load is applied and their mechanical behavior is probed in uniaxial tension. Two types of configurations are considered: a state with high initial free volume in which contacts between fibers are scarce, and a state with low free volume and large number of fiber contacts. While in the absence of cohesion the response is hyperelastic, we observe that a yield point-like phenomenon develops as the strength of cohesion increases in both network types considered; we refer to this as an 'unlocking phenomenon'. The small strain stiffness increases as cohesion becomes more pronounced. The stiffness and unlocking stress are expressed in terms of network parameters and cohesion strength through a product of two functions, one dependent on network parameters only, and the other is a function of the cohesion strength. While the small strain response is controlled by cohesion, the large strain behavior is shown to be largely controlled by the network. Therefore, varying the strength of cohesion has no effect on strain stiffening. These observations provide a physical basis for the unlocking observed in both athermal and thermal network materials and are expected to facilitate the design of soft materials with novel properties.

An amino acid mixture, enriched with Krebs cycle intermediates, enhances extracellular matrix gene expression in cultured human fibroblasts

In the human body, the skin is one of the organs most affected by the aging process. Nutritional approaches aimed to counteract the age-induced decline of extracellular matrix (ECM) deposition could be a valuable tool to decrease the degenerative processes underlying skin aging. Here, we investigated the ability of a six-amino acid plus hyaluronic acid (6AAH) formulation enriched with tricarboxylic acid (TCA) intermediates to stimulate ECM gene expression. To this aim, human BJ fibroblasts were treated with 6AAH alone or plus succinate or malate alone or succinate plus malate (6AAHSM), and mRNA levels of several ECM markers were evaluated. 6AAHSM increased the expression of all the ECM markers significantly above 6AAH alone or plus only succinate or malate. Furthermore, in an in vitro oxidative damage model, 6AAHSM blunted the hydrogen peroxide-induced decline in ECM gene expression. Our data suggest that feeding cells with 6AAH enriched with TCAs could efficiently be employed as a non-pharmacological approach for counteracting skin aging.

ECM-Focused Proteomic Analysis of Ear Punch Regeneration in Acomys Cahirinus

In mammals, significant injury is generally followed by the formation of a fibrotic scar which provides structural integrity but fails to functionally restore damaged tissue. Spiny mice of the genus Acomys represent the first example of full skin autotomy in mammals. Acomys cahirinus has evolved extremely weak skin as a strategy to avoid predation and is able to repeatedly regenerate healthy tissue without scar after severe skin injury or full-thickness ear punches. Extracellular matrix (ECM) composition is a critical regulator of wound repair and scar formation and previous studies have suggested that alterations in its expression may be responsible for the differences in regenerative capacity observed between Mus musculus and A. cahirinus , yet analysis of this critical tissue component has been limited in previous studies by its insolubility and resistance to extraction. Here, we utilize a 2-step ECM-optimized extraction to perform proteomic analysis of tissue composition during wound repair after full-thickness ear punches in A. cahirinus and M. musculus from weeks 1 to 4 post-injury. We observe changes in a wide range of ECM proteins which have been previously implicated in wound regeneration and scar formation, including collagens, coagulation and provisional matrix proteins, and matricryptic signaling peptides. We additionally report differences in crosslinking enzyme activity and ECM protein solubility between Mus and Acomys. Furthermore, we observed rapid and sustained increases in CD206, a marker of pro-regenerative M2 macrophages, in Acomys, whereas little or no increase in CD206 was detected in Mus. Together, these findings contribute to a comprehensive understanding of tissue cues which drive the regenerative capacity of Acomys and identify a number of potential targets for future pro-regenerative therapies.

Low-power microwaves: a cell-compatible physical treatment to enhance the mechanical properties of self-assembling peptides

Biomaterials designed for tissue engineering applications should, among other requirements, mimic the native extracellular matrix (ECM) of the tissues to be regenerated, both in terms of biomimetic and mechanical properties. Ideally, the scaffold stiffness and stress resistance should be tuned for each specific implantation therapy. Self-assembling peptides (SAPs) are promising synthetic bionanomaterials prone to easy multi-functionalization, bestowing biomimetic properties. However, they usually yield soft and fragile hydrogels unsuited for the regeneration of medium-to-hard tissues. For this purpose, chemical cross-linking of SAPs is an option, but it often requires a moderately toxic and expensive chemical compound and/or the presence of specific residues/reactive sites, posing issues for its feasibility and translational potential. In this work, we introduced, characterized by rheology, atomic force microscopy (AFM), Thioflavin-T assay (ThT), and Fourier transform infrared (FT-IR) tests, and optimized (by tuning the power, temperature and treatment time) a novel fast, green and affordable methodology using mild microwave (MW) irradiation to increase the mechanical properties of diverse classes of SAPs. Low-power MWs increase stiffness, resilience, and β-structuration, while high-power MW treatments partially denature the tested SAPs. Our pure-physical methodology does not alter the SAP biomimetic properties (verified via in vitro tests of viability and differentiation of human neural stem cells), is compatible with already seeded cells, and is also synergic with genipin-based cross-linking of SAPs; therefore, it may become the next standard for SAP preparation in tissue engineering applications at hand of all research labs and in clinics.

Preclinical and clinical orthotopic transplantation of decellularized/engineered tracheal scaffolds: A systematic literature review

Severe tracheal injuries that cannot be managed by mobilization and end-to-end anastomosis represent an unmet clinical need and an urgent challenge to face in surgical practice; within this scenario, decellularized scaffolds (eventually bioengineered) are currently a tempting option among tissue engineered substitutes. The success of a decellularized trachea is expression of a balanced approach in cells removal while preserving the extracellular matrix (ECM) architecture/mechanical properties. Revising the literature, many Authors report about different methods for acellular tracheal ECMs development; however, only few of them verified the devices effectiveness by an orthotopic implant in animal models of disease. To support translational medicine in this field, here we provide a systematic review on studies recurring to decellularized/bioengineered tracheas implantation. After describing the specific methodological aspects, orthotopic implant results are verified. Furtherly, the only three clinical cases of compassionate use of tissue engineered tracheas are reported with a focus on outcomes.

Physical, Mechanical, and Biological Properties of Fibrin Scaffolds for Cartilage Repair

Articular cartilage is a highly organized tissue that provides remarkable load-bearing and low friction properties, allowing for smooth movement of diarthrodial joints; however, due to the avascular, aneural, and non-lymphatic characteristics of cartilage, joint cartilage has self-regeneration and repair limitations. Cartilage tissue engineering is a promising alternative for chondral defect repair. It proposes models that mimic natural tissue structure through the use of cells, scaffolds, and signaling factors to repair, replace, maintain, or improve the specific function of the tissue. In chondral tissue engineering, fibrin is a biocompatible biomaterial suitable for cell growth and differentiation with adequate properties to regenerate damaged cartilage. Additionally, its mechanical, biological, and physical properties can be enhanced by combining it with other materials or biological components. This review addresses the biological, physical, and mechanical properties of fibrin as a biomaterial for cartilage tissue engineering and as an element to enhance the regeneration or repair of chondral lesions.

Designer DNA Hydrogels Stimulate 3D Cell Invasion by Enhanced Receptor Expression and Membrane Endocytosis

DNA has emerged as one of the smartest biopolymers to bridge the gap between chemical science and biology to design scaffolds like hydrogels by physical entanglement or chemical bonding with remarkable properties. We present here a completely new application of DNA-based hydrogels in terms of their capacity to stimulate membrane endocytosis, leading to enhanced cell spreading and invasion for cells in ex vivo 3D spheroids models. Multiscale simulation studies along with DLS data showed that the hydrogel formation was enhanced at lower temperature and it converts to liquid with increase in temperature. DNA hydrogels induced cell spreading as observed by the increase in cellular area by almost two-fold followed by an increase in the receptor expression, the endocytosis, and the 3D invasion potential of migrating cells. Our first results lay the foundation for upcoming diverse applications of hydrogels to probe and program various cellular and physiological processes that can have lasting applications in stem cell programming and regenerative therapeutics.

The ECM: To Scaffold, or Not to Scaffold, That Is the Question

The extracellular matrix (ECM) has pleiotropic effects, ranging from cell adhesion to cell survival. In tissue engineering, the use of ECM and ECM-like scaffolds has separated the field into two distinct areas-scaffold-based and scaffold-free. Scaffold-free techniques are used in creating reproducible cell aggregates which have massive potential for high-throughput, reproducible drug screening and disease modeling. Though, the lack of ECM prevents certain cells from surviving and proliferating. Thus, tissue engineers use scaffolds to mimic the native ECM and produce organotypic models which show more reliability in disease modeling. However, scaffold-based techniques come at a trade-off of reproducibility and throughput. To bridge the tissue engineering dichotomy, we posit that finding novel ways to incorporate the ECM in scaffold-free cultures can synergize these two disparate techniques.

Mechanical properties of cell sheets and spheroids: the link between single cells and complex tissues

Cell aggregates, including sheets and spheroids, represent a simple yet powerful model system to study both biochemical and biophysical intercellular interactions. However, it is becoming evident that, although the mechanical properties and behavior of multicellular structures share some similarities with individual cells, yet distinct differences are observed in some principal aspects. The description of mechanical phenomena at the level of multicellular model systems is a necessary step for understanding tissue mechanics and its fundamental principles in health and disease. Both cell sheets and spheroids are used in tissue engineering, and the modulation of mechanical properties of cell constructs is a promising tool for regenerative medicine. Here, we review the data on mechanical characterization of cell sheets and spheroids, focusing both on advances in the measurement techniques and current understanding of the subject. The reviewed material suggest that interplay between the ECM, intercellular junctions, and cellular contractility determines the behavior and mechanical properties of the cell aggregates.

Anisotropy in mechanical unfolding of protein upon partner-assisted pulling and handle-assisted pulling

Proteins as force-sensors respond to mechanical cues and regulate signaling in physiology. Proteins commonly connect the source and response points of mechanical cues in two conformations, independent proteins in end-to-end geometry and protein complexes in handshake geometry. The force-responsive property of independent proteins in end-to-end geometry is studied extensively using single-molecule force spectroscopy (SMFS). The physiological significance of the complex conformations in force-sensing is often disregarded as mere surge protectors. However, with the potential of force-steering, protein complexes possess a distinct mechano-responsive property over individual force-sensors. To decipher, we choose a force-sensing protein, cadherin-23, from tip-link complex and perform SMFS using end-to-end geometry and handshake complex geometry. We measure higher force-resilience of cadherin-23 with preferential shorter extensions in handshake mode of pulling over the direct mode. The handshake geometry drives the force-response of cadherin-23 through different potential-energy landscapes than direct pulling. Analysis of the dynamic network structure of cadherin-23 under tension indicates narrow force-distributions among residues in cadherin-23 in direct pulling, resulting in low force-dissipation paths and low resilience to force. Overall, the distinct and superior mechanical responses of cadherin-23 in handshake geometry than single protein geometry highlight a probable evolutionary drive of protein-protein complexes as force-conveyors over independent ones.

Precise Tuning of Polymeric Fiber Dimensions to Enhance the Mechanical Properties of Alginate Hydrogel Matrices

Hydrogels based on biopolymers, such as alginate, are commonly used as scaffolds in tissue engineering applications as they mimic the features of the native extracellular matrix (ECM). However, in their native state, they suffer from drawbacks including poor mechanical performance and a lack of biological functionalities. Herein, we have exploited a crystallization-driven self-assembly (CDSA) methodology to prepare well-defined one-dimensional micellar structures with controlled lengths to act as a mimic of fibrillar collagen in native ECM and improve the mechanical strength of alginate-based hydrogels. Poly(ε-caprolactone)-b-poly(methyl methacrylate)-b-poly(N, N-dimethyl acrylamide) triblock copolymers were self-assembled into 1D cylindrical micelles with precise lengths using CDSA epitaxial growth and subsequently combined with calcium alginate hydrogel networks to obtain nanocomposites. Rheological characterization determined that the inclusion of the cylindrical structures within the hydrogel network increased the strength of the hydrogel under shear. Furthermore, the strain at flow point of the alginate-based hydrogel was found to increase with nanoparticle content, reaching an improvement of 37% when loaded with 500 nm cylindrical micelles. Overall, this study has demonstrated that one-dimensional cylindrical nanoparticles with controlled lengths formed through CDSA are promising fibrillar collagen mimics to build ECM scaffold models, allowing exploration of the relationship between collagen fiber size and matrix mechanical properties.

Naturally Formed Chitinous Skeleton Isolated from the Marine Demosponge Aplysina fistularis as a 3D Scaffold for Tissue Engineering

Tissue engineering (TE) is a field of regenerative medicine that has been experiencing a special boom in recent years. Among various materials used as components of 3D scaffolds, naturally formed chitinous materials seem to be especially attractive because of their abundance, non-toxic and eco-friendly character. In this study, chitinous skeleton isolated from the marine sponge Aplysina fistularis (phylum: Porifera) was used for the first time as a support for the cultivation of murine fibroblasts (Balb/3T3), human dermal fibroblasts (NHDF), human keratinocyte (HaCaT), and human neuronal (SH-SY5Y) cells. Characterization techniques such as ATR FTIR, TGA, and μCT, clearly indicate that an interconnected macro-porous, thermostable, pure α-chitin scaffold was obtained after alkali–acid treatment of air-dried marine sponge. The biocompatibility of the naturally formed chitin scaffolds was confirmed by cell attachment and proliferation determined by various microscopic methods (e.g., SEM, TEM, digital microscopy) and specific staining. Our observations show that fibroblasts and keratinocytes form clusters on scaffolds that resemble a skin structure, including the occurrence of desmosomes in keratinocyte cells. The results obtained here suggest that the chitinous scaffold from the marine sponge A. fistularis is a promising biomaterial for future research about tissues regeneration.

Nonlinear Mechanical Properties of Prestressed Branched Fibrous Networks

Random fiber networks constitute the solid skeleton of many biological materials such as the cytoskeleton of cells and extracellular matrix of soft tissues. These random networks show unique mechanical properties such as nonlinear shear strain-stiffening and strain softening when subjected to preextension and precompression, respectively. In this study, we perform numerical simulations to characterize the influence of axial prestress on the nonlinear mechanical response of random network structures as a function of their micromechanical and geometrical properties. We build our numerical network models using the microstructure of disordered hexagonal lattices and quantify their nonlinear shear response as a function of uniaxial prestress strain. We consider three different material models for individual fibers and fully characterize their influence on the mechanical response of prestressed networks. Moreover, we investigate both the influence of geometric disorder keeping the network connectivity constant and the influence of the randomness in the stiffness of individual fibers keeping their mean stiffness constant. The effects of network connectivity and bending rigidity of fibers are also determined. Several important conclusions are made, including that the tensile and compressive prestress strains, respectively, increase and decrease the initial network shear stiffness but have no effect on the maximal shear modulus. We discuss the findings in terms of microstructural properties such as the local strain energy distribution.

A nanomechanical model enables comprehensive characterization of biological tissues in ultrasound imaging

Sonography, elastography, sonoelastography are ultrasound imaging techniques commonly used in the clinical practice for the diagnosis of many pathological conditions. These highly reliable, non-invasive methods use high frequency, elastic pressure waves (ultrasounds) to interrogate the internal structure of biological tissues and organs, and the continuum mechanics hypothesis to reconstruct, from the output of the system, the biophysical characteristics of the samples. Nevertheless, continuum mechanics disregards the discrete nature of tissues and organs, resulting in an inability for the model to describe some important tissue biophysical characteristics such as the cell size and their spatial layout. Here, we used the theory of doublet mechanics - a discrete nano-mechanical field theory - to model the propagation of ultrasounds in a multilayered biological tissue. We found that the output of the model exhibits a very high sensitivity to the macro and micro characteristics of the tissue, including cell size. We used results from the model to correlate the internal structure of the samples to the reflection coefficient, i.e. the continuum level response of the system. This model, and its more sophisticated evolutions that will be developed over time, can complement traditional ultrasound imaging, and provide ways to analyze non-invasively living tissues with a resolution inaccessible to conventional techniques of analysis, including positron emission tomography, computer tomography, and magnetic resonance.

Integrated lung tissue mechanics one piece at a time: Computational modeling across the scales of biology

The lung is a delicately balanced and highly integrated mechanical system. Lung tissue is continuously exposed to the environment via the air we breathe, making it susceptible to damage. As a consequence, respiratory diseases present a huge burden on society and their prevalence continues to rise. Emergent function is produced not only by the sum of the function of its individual components but also by the complex feedback and interactions occurring across the biological scales - from genes to proteins, cells, tissue and whole organ - and back again. Computational modeling provides the necessary framework for pulling apart and putting back together the pieces of the body and organ systems so that we can fully understand how they function in both health and disease. In this review, we discuss models of lung tissue mechanics spanning from the protein level (the extracellular matrix) through to the level of cells, tissue and whole organ, many of which have been developed in isolation. This is a vital step in the process but to understand the emergent behavior of the lung, we must work towards integrating these component parts and accounting for feedback across the scales, such as mechanotransduction. These interactions will be key to unlocking the mechanisms occurring in disease and in seeking new pharmacological targets and improving personalized healthcare.

Cross-platform mechanical characterization of lung tissue

Published data on the mechanical strength and elasticity of lung tissue is widely variable, primarily due to differences in how testing was conducted across individual studies. This makes it extremely difficult to find a benchmark modulus of lung tissue when designing synthetic extracellular matrices (ECMs). To address this issue, we tested tissues from various areas of the lung using multiple characterization techniques, including micro-indentation, small amplitude oscillatory shear (SAOS), uniaxial tension, and cavitation rheology. We report the sample preparation required and data obtainable across these unique but complimentary methods to quantify the modulus of lung tissue. We highlight cavitation rheology as a new method, which can measure the modulus of intact tissue with precise spatial control, and reports a modulus on the length scale of typical tissue heterogeneities. Shear rheology, uniaxial, and indentation testing require heavy sample manipulation and destruction; however, cavitation rheology can be performed in situ across nearly all areas of the lung with minimal preparation. The Young's modulus of bulk lung tissue using micro-indentation (1.4±0.4 kPa), SAOS (3.3±0.5 kPa), uniaxial testing (3.4±0.4 kPa), and cavitation rheology (6.1±1.6 kPa) were within the same order of magnitude, with higher values consistently reported from cavitation, likely due to our ability to keep the tissue intact. Although cavitation rheology does not capture the non-linear strains revealed by uniaxial testing and SAOS, it provides an opportunity to measure mechanical characteristics of lung tissue on a microscale level on intact tissues. Overall, our study demonstrates that each technique has independent benefits, and each technique revealed unique mechanical features of lung tissue that can contribute to a deeper understanding of lung tissue mechanics.

A microfluidic chamber-based approach to map the shear moduli of vascular cells and other soft materials

There is growing interest in quantifying vascular cell and tissue stiffness. Most measurement approaches, however, are incapable of assessing stiffness in the presence of physiological flows. We developed a microfluidic approach which allows measurement of shear modulus (G) during flow. The design included a chamber with glass windows allowing imaging with upright or inverted microscopes. Flow was controlled gravitationally to push culture media through the chamber. Fluorescent beads were conjugated to the sample surface and imaged before and during flow. Bead displacements were calculated from images and G was computed as the ratio of imposed shear stress to measured shear strain. Fluid-structure simulations showed that shear stress on the surface did not depend on sample stiffness. Our approach was verified by measuring the moduli of polyacrylamide gels of known stiffness. In human pulmonary microvascular endothelial cells, G was 20.4 ± 12 Pa and decreased by 20% and 22% with increasing shear stress and inhibition of non-muscle myosin II motors, respectively. The G showed a larger intra- than inter-cellular variability and it was mostly determined by the cytosol. Our shear modulus microscopy can thus map the spatial distribution of G of soft materials including gels, cells and tissues while allowing the visualization of microscopic structures such as the cytoskeleleton.

Extracellular matrix-derived hydrogels for dental stem cell delivery

Decellularized mammalian extracellular matrices (ECM) have been widely accepted as an ideal substrate for repair and remodelling of numerous tissues in clinical and pre-clinical studies. Recent studies have demonstrated the ability of ECM scaffolds derived from site-specific homologous tissues to direct cell differentiation. The present study investigated the suitability of hydrogels derived from different source tissues: bone, spinal cord and dentine, as suitable carriers to deliver human apical papilla derived mesenchymal stem cells (SCAP) for spinal cord regeneration. Bone, spinal cord, and dentine ECM hydrogels exhibited distinct structural, mechanical, and biological characteristics. All three hydrogels supported SCAP viability and proliferation. However, only spinal cord and bone derived hydrogels promoted the expression of neural lineage markers. The specific environment of ECM scaffolds significantly affected the differentiation of SCAP to a neural lineage, with stronger responses observed with spinal cord ECM hydrogels, suggesting that site-specific tissues are more likely to facilitate optimal stem cell behavior for constructive spinal cord regeneration. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 319-328, 2017.

Biomechanical relationships between the corneal endothelium and Descemet's membrane

The posterior face of the cornea consists of the corneal endothelium, a monolayer of cuboidal cells that secrete and attach to Descemet's membrane, an exaggerated basement membrane. Dysfunction of the endothelium compromises the barrier and pump functions of this layer that maintain corneal deturgesence. A large number of corneal endothelial dystrophies feature irregularities in Descemet's membrane, suggesting that cells create and respond to the biophysical signals offered by their underlying matrix. This review provides an overview of the bidirectional relationship between Descemet's membrane and the corneal endothelium. Several experimental methods have characterized a richly topographic and compliant biophysical microenvironment presented by the posterior surface of Descemet's membrane, as well as the ultrastructure and composition of the membrane as it builds during a lifetime. We highlight the signaling pathways involved in the mechanotransduction of biophysical cues that influence cell behavior. We present the specific example of Fuchs' corneal endothelial dystrophy as a condition in which a dysregulated Descemet's membrane may influence the progression of disease. Finally, we discuss some disease models and regenerative strategies that may facilitate improved treatments for corneal dystrophies.

Mimicking Form and Function of Native Small Diameter Vascular Conduits Using Mulberry and Non-mulberry Patterned Silk Films

Autologous graft replacement as a strategy to treat diseased peripheral small diameter (≤6 mm) blood vessel is often challenged by prior vein harvesting. To address this issue, we fabricated native-tissue mimicking multilayered small diameter vascular graft (SDVG) using mulberry (Bombyx mori) and Indian endemic non-mulberry (Antheraea assama and Philosamia ricini) silk. Patterned silk films were fabricated on microgrooved PDMS mold, casted by soft lithography. The biodegradable patterned film templates with aligned cell sheets were rolled onto an inert mandrel to mimic vascular conduit. The hemocompatible and mechanically strong non-mulberry films with RGD motif supported ∼1.2 folds greater proliferation of vascular cells with aligned anchorage. Elicitation of minimal immune response on subcutaneous implantation of the films in mice was complemented by ∼45% lower TNF α secretion by in vitro macrophage culture post 7 days. Pattern-induced alignment favored the functional contractile phenotype of smooth muscle cells (SMCs), expressing the signature markers-calponin, α-smooth muscle actin (α-SMA), and smooth muscle myosin heavy chain (SM-MHC). Endothelial cells (ECs) exhibited a typical punctuated pattern of von Willebrand factor (vWF). Deposition of collagen and elastin by the SMCs substantiated the aptness of the graft with desired biomechanical attributes. Furthermore, the burst strength of the fabricated conduit was in the range of ∼915-1260 mmHg, a prerequisite to withstand physiological pressure. This novel fabrication approach may eliminate the need of maturation in a pulsatile bioreactor for obtaining functional cellular phenotype. This work is thereby an attestation to the immense prospects of exploring non-mulberry silk for bioengineering a multilayered vascular conduit similar to a native vessel in "form and function", befitting for in vivo transplantation.

Cardiac extracellular matrix-fibrin hybrid scaffolds with tunable properties for cardiovascular tissue engineering

Solubilized cardiac extracellular matrix (ECM) is being developed as an injectable therapeutic that offers promise for promoting cardiac repair. However, the ECM alone forms a hydrogel that is very soft compared to the native myocardium. As both the stiffness and composition of the ECM are important in regulating cell behavior and can have complex synergistic effects, we sought to develop an ECM-based scaffold with tunable biochemical and mechanical properties. We used solubilized rat cardiac ECM from two developmental stages (neonatal, adult) combined with fibrin hydrogels that were cross-linked with transglutaminase. We show that ECM was retained within the gels and that the Young's modulus could be tuned to span the range of the developing and mature heart. C-kit+ cardiovascular progenitor cells from pediatric patients with congenital heart defects were seeded into the hybrid gels. Both the elastic modulus and composition of the scaffolds impacted the expression of endothelial and smooth muscle cell genes. Furthermore, we demonstrate that the hybrid gels are injectable, and thus have potential for minimally invasive therapies. ECM-fibrin hybrid scaffolds offer new opportunities for exploiting the effects of both composition and mechanical properties in directing cell behavior for tissue engineering.

Highly Aligned Nanofibrous Scaffold Derived from Decellularized Human Fibroblasts

Native tissues are endowed with a highly organized nanofibrous extracellular matrix (ECM) that directs cellular distribution and function. The objective of this study is to create a purely natural, uniform, and highly aligned nanofibrous ECM scaffold for potential tissue engineering applications. Synthetic nanogratings (130 nm in depth) were used to direct the growth of human dermal fibroblasts for up to 8 weeks, resulting in a uniform 70 μm-thick fibroblast cell sheet with highly aligned cells and ECM nanofibers. A natural ECM scaffold with uniformly aligned nanofibers of 78 ± 9 nm in diameter was generated after removing the cellular components from the detached fibroblast sheet. The elastic modulus of the scaffold was well maintained after the decellularization process because of the preservation of elastin fibers. Reseeding human mesenchymal stem cells (hMSCs) showed the excellent capacity of the scaffold in directing and supporting cell alignment and proliferation along the underlying fibers. The scaffold's biocompatibility was further examined by an in vitro inflammation assay with seeded macrophages. The aligned ECM scaffold induced a significantly lower immune response compared to its unaligned counterpart, as detected by the pro-inflammatory cytokines secreted from macrophages. The aligned nanofibrous ECM scaffold holds great potential in engineering organized tissues.

Complexity and emergent phenomena

Complex biological systems operate under non-equilibrium conditions and exhibit emergent properties associated with correlated spatial and temporal structures. These properties may be individually unpredictable, but tend to be governed by power-law probability distributions and/or correlation. This article reviews the concepts that are invoked in the treatment of complex systems through a wide range of respiratory-related examples. Following a brief historical overview, some of the tools to characterize structural variabilities and temporal fluctuations associated with complex systems are introduced. By invoking the concept of percolation, the notion of multiscale behavior and related modeling issues are discussed. Spatial complexity is then examined in the airway and parenchymal structures with implications for gas exchange followed by a short glimpse of complexity at the cellular and subcellular network levels. Variability and complexity in the time domain are then reviewed in relation to temporal fluctuations in airway function. Next, an attempt is given to link spatial and temporal complexities through examples of airway opening and lung tissue viscoelasticity. Specific examples of possible and more direct clinical implications are also offered through examples of optimal future treatment of fibrosis, exacerbation risk prediction in asthma, and a novel method in mechanical ventilation. Finally, the potential role of the science of complexity in the future of physiology, biology, and medicine is discussed.

Multiscale model predicts tissue-level failure from collagen fiber-level damage

Excessive tissue-level forces communicated to the microstructure and extracellular matrix of soft tissues can lead to damage and failure through poorly understood physical processes that are multiscale in nature. In this work, we propose a multiscale mechanical model for the failure of collagenous soft tissues that incorporates spatial heterogeneity in the microstructure and links the failure of discrete collagen fibers to the material response of the tissue. The model, which is based on experimental failure data derived from different collagen gel geometries, was able to predict the mechanical response and failure of type I collagen gels, and it demonstrated that a fiber-based rule (at the micrometer scale) for discrete failure can strongly shape the macroscale failure response of the gel (at the millimeter scale). The model may be a useful tool in predicting the macroscale failure conditions for soft tissues and engineered tissue analogs. In addition, the multiscale model provides a framework for the study of failure in complex fiber-based mechanical systems in general.

Using growth factor conditioning to modify the properties of human cell derived extracellular matrix

We have recently reported on a bench-top approach for isolating extracellular matrix (ECM) from pure populations of cells grown in culture using sacrificial, open-celled foams to concentrate and capture the ECM. To increase both the accumulation and the strength of the ECM harvested, cell-seeded polyurethane (PU) foams were cultured in media supplemented with either transforming growth factor β-1 (TGFβ1) or hepatocyte growth factor (HGF). At the end of a 3-week culture period, ECM yield was significantly increased for samples conditioned in supplemented media. Control foams yielded 48 ± 12 mg of material for every gram of PU foam seeded. Yield values increased to 102 ± 21 and 243 ± 25 mg for HGF and TGFβ1-treated samples, respectively. HGF supplementation increased the modulus by 59%, while TGFβ1 treatment increased the elastic modulus by 204%. TGFβ1-stimulated material was organized into a network that was markedly denser than control material, with HGF-stimulated network density intermediate to TGFβ1 and controls. Our study showed that TGFβ1-treated samples were collagen enriched while HGF samples had an increased gylcosaminoglycan concentration. The results demonstrate that growth factor supplementation, particularly with TGFβ1, can significantly alter the biomechanical properties of cell-derived ECM that may be used for therapeutic applications.

The major transitions of life from a network perspective

Many attempts have been made to understand the origin of life and biological complexity both at the experimental and theoretical levels but neither is fully explained. In an influential work, Maynard Smith and Szathmáry (1995) argued that the majority of the increase in complexity is not gradual, but it is associated with a few so-called major transitions along the way of the evolution of life. For each major transition, they identified specific mechanisms that could account for the change in complexity related to information transmission across generations. In this work, I propose that the sudden and unexpected improvement in the functionality of an organism that followed a major transition was enabled by a phase transition in the network structure associated with that function. The increase in complexity following a major transition is therefore directly linked to the emergence of a novel structure-function relation which altered the course of evolution. As a consequence, emergent phenomena arising from these network phase transitions can serve as a common organizing principle for understanding the major transitions. As specific examples, I analyze the emergence of life, the emergence of the genetic apparatus, the rise of the eukaryotic cells, the evolution of movement and mechanosensitivity, and the emergence of consciousness. Finally, I discuss the implications of network associated phase transitions to issues that bear relevance to the history, the immediate present and perhaps the future, of life.

New ureteral scaffold constructed with composite poly(L-lactic acid)-collagen and urothelial cells by new centrifugal seeding system

A tissue-engineered ureteral scaffold was constructed with composited poly L-lactic acid (PLLA)-collagen endoluminal stent and uroepithelial cells (UECs) using a new seeding system. The electrospun PLLA-collagen nanofibrous mesh was seeded efficiently with human ureteral epithelial cells using a modified centrifugal seeding device. The cellular nanofibrous mesh was then wound around a spiral endoluminal stent to form a cellular composited PLLA-collagen ureteral scaffold. The cellular ureteral scaffold was subcutaneously implanted into nude mice. Cell attachment, distribution, and viability in vitro were investigated along with the cell fate in vivo. (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay showed that scaffolds seeded with centrifugal method had higher cellular activity than scaffolds seeded with static method (p < 0.05), and the metabolic activity per cell had no significant differences between the two methods (p > 0.05). Histologic analysis showed that the entrapped UECs remained in the scaffolds after 2 wk of implantation. The results of the study indicated that the composited PLLA-collagen endoluminal stent could serve as alternative cell carrier for tissue engineering ureter. In addition, the new modified centrifugal seeding system allowed rapid homogeneous distribution of cells onto the nanofibrous mesh, which will be useful to ureteral reconstruction.

Micropatterned cell sheets with defined cell and extracellular matrix orientation exhibit anisotropic mechanical properties

For an arterial replacement graft to be effective, it must possess the appropriate strength in order to withstand long-term hemodynamic stress without failure, yet be compliant enough that the mismatch between the stiffness of the graft and the native vessel wall is minimized. The native vessel wall is a structurally complex tissue characterized by circumferentially oriented collagen fibers/cells and lamellar elastin. Besides the biochemical composition, the functional properties of the wall, including stiffness, depend critically on the structural organization. Therefore, it will be crucial to develop methods of producing tissues with defined structures in order to more closely mimic the properties of a native vessel. To this end, we sought to generate cell sheets that have specific ECM/cell organization using micropatterned polydimethylsiloxane (PDMS) substrates to guide cell organization and tissue growth. The patterns consisted of large arrays of alternating grooves and ridges. Adult bovine aortic smooth muscle cells cultured on these substrates in the presence of ascorbic acid produced ECM-rich sheets several cell layers thick in which both the cells and ECM exhibited strong alignment in the direction of the micropattern. Moreover, mechanical testing revealed that the sheets exhibited mechanical anisotropy similar to that of native vessels with both the stiffness and strength being significantly larger in the direction of alignment, demonstrating that the microscale control of ECM organization results in functional changes in macroscale material behavior.

Impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin on cutaneous wound healing

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a representative of a large group of polyhalogenated aromatic hydrocarbons that are widespread environmental contaminants. Administration of TCDD to laboratory animals or cultured cells results in a number of adverse effects that are well documented. For example, the effects of TCDD observed in developing organisms indicate that exposure to this class of environmental contaminants significantly alters embryo morphogenesis. However, it is not clear whether tissue regeneration in adult animals may be similarly affected. With this in mind, we examined the impact of TCDD exposure on wound healing using a murine cutaneous wound healing model. Our results indicate that TCDD exposure did not significantly alter the time needed for wound closure. However, in the TCDD-treated mice, a significant decrease in tensile strength in the healed wounds was observed which is indicative of an aberrantly healed wound. Immunostaining revealed that exposure to TCDD increased the population of macrophages detected within the wounded tissue at the latter stages of wound healing. Our findings support the idea that exposure to environmental contaminants such as TCDD is proinflammatory in the wounded tissue, disrupts normal healing and ultimately produces in a poorly healed wound.

Lung parenchymal mechanics

The lung parenchyma comprises a large number of thin-walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed.

Stress delocalization in crack tolerant electrospun nanofiber networks

The fracture toughness of a noncontinuum fibrous network produced by electrospinning polyamide 6 nanofibers is investigated. The mechanical properties of the nanofiber network is observed to be independent of various incorporated macroscopic crack lengths, resulting in an apparent increase in fracture toughness with increasing crack length as evaluated using conventional fracture mechanics. Strain mapping of the nanofiber network indicates stress delocalization mechanisms operating around these macroscopic cracks in the network. The deformation behavior of the nanofiber network will therefore depend on the volume of fibers being loaded in the network and not the number of fibers in the cross-sectional width defining continuum sample mechanics. These results indicate a propensity for both the synthetic electrospun nanofibrous network in this work and potentially other nanofibrous networks to resist failure from macroscopic cracks incorporated within the material.

Visualization of mouse pancreas architecture using MR microscopy

Pancreatic diseases, which include diabetes, pancreatitis, and pancreatic cancer, are often difficult to detect and/or stage, contributing to a reduced quality of life and lifespan for patients. Thus, there is need for a technology that can visualize tissue changes in the pancreas, improve understanding of disease progression, and facilitate earlier detection in the human population. Because of low spatial resolution, current clinical magnetic resonance imaging (MRI) at low field strength has yet to fully visualize the exocrine, endocrine, vascular, and stromal components of the pancreas. We used high field strength magnetic resonance microscopy (μMRI) to image mouse pancreas ex vivo without contrast agents at high spatial resolution. We analyzed the resulting high-resolution images using volume rendering to resolve components in the pancreas, including acini, islets, blood vessels, and extracellular matrix. Locations and dimensions of pancreatic components as seen in three-dimensional μMRI were compared with histological images, and good correspondence was found. Future longitudinal studies could expand on the use of in vivo μMRI in mouse models of pancreatic diseases. Capturing three-dimensional structural changes through μMRI could help to identify early cellular and tissue changes associated with pancreatic disease, serving as a mode of improved detection in the clinic for endocrine and exocrine pathologies.

Stacking of aligned cell sheets for layer-by-layer control of complex tissue structure

Children suffering from congenital heart defects (CHD) often require vascular reconstruction. Pediatric patients would greatly benefit from a cell-based tissue engineered vascular patch (TEVP) that has potential for growth. As artery structure and function are intimately linked, mimicking native tissue organization is an important design consideration. In this study, we cultured human mesenchymal stem cell on patterned thermo-responsive substrates. Cell alignment improved over time up to 2 wk in culture when sheets were ready for harvest. We then used cell sheets as "functional units" to build complex tissue structures that mimic native vascular smooth muscle cell organization in the medial layer of the artery. Cell sheets could be stacked using a gelatin stamp such that individual sheets in the construct were well aligned with each other (mimic of circumferential orientation) or at angles with respect to each other (mimic of herringbone structure). Controlling tissue organization layer-by-layer will be a powerful approach to building tissues with well defined and complex structure.

Mechanical failure, stress redistribution, elastase activity and binding site availability on elastin during the progression of emphysema

Emphysema is a disease of the lung parenchyma with progressive alveolar tissue destruction that leads to peripheral airspace enlargement. In this review, we discuss how mechanical forces can contribute to disease progression at various length scales. Airspace enlargement requires mechanical failure of alveolar walls. Because the lung tissue is under a pre-existing tensile stress, called prestress, the failure of a single wall results in a redistribution of the local prestress. During this process, the prestress increases on neighboring alveolar walls which in turn increases the probability that these walls also undergo mechanical failure. There are several mechanisms that can contribute to this increased probability: exceeding the failure threshold of the ECM, triggering local mechanotransduction to release enzymes, altering enzymatic reactions on ECM molecules. Next, we specifically discuss recent findings that stretching of elastin induces an increase in the binding off rate of elastase to elastin as well as unfolds hidden binding sites along the fiber. We argue that these events can initiate a positive feedback loop which generates slow avalanches of breakdown that eventually give rise to the relentless progression of emphysema. We propose that combining modeling at various length scales with corresponding biological assays, imaging and mechanics data will provide new insight into the progressive nature of emphysema. Such approaches will have the potential to contribute to resolving many of the outstanding issues which in turn may lead to the amelioration or perhaps the treatment of emphysema in the future.

Mechanical forces regulate elastase activity and binding site availability in lung elastin

Many fundamental cellular and extracellular processes in the body are mediated by enzymes. At the single molecule level, enzyme activity is influenced by mechanical forces. However, the effects of mechanical forces on the kinetics of enzymatic reactions in complex tissues with intact extracellular matrix (ECM) have not been identified. Here we report that physiologically relevant macroscopic mechanical forces modify enzyme activity at the molecular level in the ECM of the lung parenchyma. Porcine pancreatic elastase (PPE), which binds to and digests elastin, was fluorescently conjugated (f-PPE) and fluorescent recovery after photobleach was used to evaluate the binding kinetics of f-PPE in the alveolar walls of normal mouse lungs. Fluorescent recovery after photobleach indicated that the dissociation rate constant (k(off)) for f-PPE was significantly larger in stretched than in relaxed alveolar walls with a linear relation between k(off) and macroscopic strain. Using a network model of the parenchyma, a linear relation was also found between k(off) and microscopic strain on elastin fibers. Further, the binding pattern of f-PPE suggested that binding sites on elastin unfold with strain. The increased overall reaction rate also resulted in stronger structural breakdown at the level of alveolar walls, as well as accelerated decay of stiffness and decreased failure stress of the ECM at the macroscopic scale. These results suggest an important role for the coupling between mechanical forces and enzyme activity in ECM breakdown and remodeling in development, and during diseases such as pulmonary emphysema or vascular aneurysm. Our findings may also have broader implications because in vivo, enzyme activity in nearly all cellular and extracellular processes takes place in the presence of mechanical forces.

Semiflexible filamentous composites

Inspired by the ubiquity of composite filamentous networks in nature, we investigate models of biopolymer networks that consist of interconnected floppy and stiff filaments. Numerical simulations carried out in three dimensions allow us to explore the microscopic partitioning of stresses and strains between the stiff and floppy fractions cs and cf and reveal a nontrivial relationship between the mechanical behavior and the relative fraction of stiff polymer: when there are few stiff polymers, nonpercolated stiff "inclusions" are protected from large deformations by an encompassing floppy matrix, while at higher fractions of stiff material the stiff network is independently percolated and dominates the mechanical response.

Engineering hydrogels as extracellular matrix mimics

Extracellular matrix (ECM) is a complex cellular environment consisting of proteins, proteoglycans, and other soluble molecules. ECM provides structural support to mammalian cells and a regulatory milieu with a variety of important cell functions, including assembling cells into various tissues and organs, regulating growth and cell-cell communication. Developing a tailored in vitro cell culture environment that mimics the intricate and organized nanoscale meshwork of native ECM is desirable. Recent studies have shown the potential of hydrogels to mimic native ECM. Such an engineered native-like ECM is more likely to provide cells with rational cues for diagnostic and therapeutic studies. The research for novel biomaterials has led to an extension of the scope and techniques used to fabricate biomimetic hydrogel scaffolds for tissue engineering and regenerative medicine applications. In this article, we detail the progress of the current state-of-the-art engineering methods to create cell-encapsulating hydrogel tissue constructs as well as their applications in in vitro models in biomedicine.

Effect of collagen digestion on the passive elastic properties of diaphragm muscle in rat

Effects of collagen digestion have been defined up to the fibril level. However, the question remains as to whether the alteration of skeletal muscle extracellular matrix (ECM) affects a muscle's passive elastic response. Various elastography methods have been applied as tools for evaluating the mechanical properties and ECM content of skeletal muscle. In an effort to develop an ECM altered skeletal muscle model, this study determined the effect of collagen digestion on the passive elastic properties of skeletal muscle. Passive mechanical properties of rat diaphragms were evaluated in various degrees of collagen digestion. Between cyclic loading tests, muscle strips were immersed in various concentrations of clostridium histolyticum derived bacterial collagenase. All samples were later viewed via light microscopy. Cyclic testing revealed linear relationships between passive muscle stiffness and digestion time at multiple concentrations. These results demonstrate that collagenase digestion of the ECM in skeletal muscle could be used as a simple and reliable model of mechanically altered in vitro tissue samples.

Tough supersoft sponge fibers with tunable stiffness from a DNA self-assembly technique

Tough and soft: Highly porous, spongelike materials self-assemble by calcium ion condensation of DNA-wrapped carbon nanotubes (SWNTs-DNA; see picture, IL = ionic liquid). The toughness, modulus, and swellability of the electrically conductive sponges can be tuned by controlling the density and strength of interfiber junctions. The sponges have compliances similar to the softest natural tissue, while robust interfiber junctions give high toughness.

Biomechanical behaviour of oesophageal tissues: material and structural configuration, experimental data and constitutive analysis

The aim of the present work is to propose an approach to the biomechanical analysis of oesophagus by defining an appropriate constitutive model and the associated constitutive parameters. The configuration of the different tissues and layers that compose the oesophagus shows very complicated internal anatomy, geometry and mechanical properties. The coupling of these tissues adds to the complexity. The constitutive models must be capable of interpreting the highly non-linear mechanical response. This is done adopting a specific hyperelastic anisotropic formulation. Experimental data are essential for the investigation of the tissues' biomechanical behaviour and also represent the basis for the definition of constitutive parameters to be adopted within the constitutive formulation developed. This action is provided by using a specific stochastic optimization procedure, addressed to the minimization of a cost function that interprets the discrepancy between experimental data and results from the analytical models developed. Unfortunately, experimental data at disposal do not satisfy all requested information and a particular solution must be provided with regard to definition of the lateral contraction of soft tissues. The anisotropic properties of the tissues are investigated considering the configuration of embedded fibres, according to their mechanical characteristics, quantity and distribution. Collagen and muscular fibres must be considered. The formulation provided on the basis of axiomatic theory of constitutive relationships and the procedure for constitutive parameters identification are described. The evaluation of constitutive parameters requires the analysis of data from experimental tests, that are extracted from the literature. Result validation is performed by comparing the experimental data and model results. In this way a valid basis is provided for the investigation of biomechanical behaviour of oesophagus, looking at deeper information from the experimental point of view that should offer data to be implemented in the procedure for a more detailed and accurate problem definition.

An experimental and theoretical study on the anisotropy of elastin network

The mechanical properties of elastin network from bovine thoracic aorta under biaxial tensile loading were studied both experimentally and theoretically. Histology and scanning electron microscopy were performed to verify the removal of cells, collagen, and other extracellular matrix components. Equi- and nonequi-biaxial tests were performed to study the effect of different loading conditions on the stress-strain responses of the elastin network. The mechanical properties of different elastin sections along the thoracic aorta were examined and studied to understand the anisotropy of elastin along the whole artery. Biaxial tensile test data comparing elastin vs. intact aorta showed that elastin is mainly responsible for the linear elastic response of the arterial wall at lower strains. Experimental results revealed that elastin network possesses significant anisotropic mechanical properties with the circumferential direction being stiffer than the longitudinal direction. The mechanical properties of elastin vary significantly along the thoracic aorta, with the thin section appearing to have the highest tangent modulus. Biological assay results indicate that elastin content is about the same along the thoracic aorta. The mechanical behavior of elastin network was well captured by the eight-chain statistical mechanics based microstructural model. Material parameters obtained from the equi-biaxial test were able to predict the stress-strain responses of elastin network under arbitrary nonequi-biaxial loading conditions. Also, by varying material parameters in the model, the changes in microstructure such as elastin fiber orientation and cross-linking density on the macroscopic mechanical properties of elastin network were discussed.

Effect of cell shape and packing density on granulosa cell proliferation and formation of multiple layers during early follicle development in the ovary

The postnatal mouse ovary is rich in quiescent and early-growing oocytes, each one surrounded by a layer of somatic granulosa cells (GCs) on a basal lamina. As oocytes start to grow the GCs change shape from flattened to cuboidal, increase their proliferation and form multiple layers, providing a unique model for studying the relationship between cell shape, proliferation and multilayering within the context of two different intercommunicating cell types: somatic and germ cells. Proliferation of GCs was quantified using immunohistochemistry for Ki67 and demonstrated that, unusually, cuboidal cells divided more than flat cells. As a second layer of GCs started to appear, cells on the basal lamina reached maximum packing density and the axes of their mitoses became perpendicular to the basal lamina, resulting in cells dividing inwards to form second and subsequent layers. Proliferation of basal GCs was less than that of inner cells. Ultrastructurally, collagen fibrils outside the basal lamina became more numerous as follicles developed. We propose that the basement membrane and/or theca cells that surround the follicle provide an important confinement for rapidly dividing columnar cells so that they attain maximum packing density, which restricts lateral mitosis and promotes inwardly oriented cell divisions and subsequent multilayering.