Synthetic materials in the study of cell response to substrate rigidity

Cell self-patterning on uniform PDMS-surfaces with controlled mechanical cues

The exploitation of cell-instructive scaffolds with uniform physical/chemical surfaces and controlled stiffness will be greatly useful in tissue engineering applications to resemble the extracellular matrix (ECM) or topographical appearance of native tissues. We herein describe a versatile and straightforward method to assemble a polydimethylsiloxane (PDMS)-composite structure in which a uniformly laminin-coated membrane is placed on top of a micropatterned substrate that applies a stiffness gradient. This 'double-sheet' structure provides soft or stiff microdomains that guide the self-patterning of different cell types [e.g. chronic myeloid leukemia (KU812), cervix carcinoma (HeLa), NIH 3T3 and BJ], thereby stimulating their cytoskeletal remodeling. More interestingly, we used these uniform PDMS surfaces with patterned rigidity for obtaining co-cultures of tumor blood cells (KU812) and adherent fibroblasts (NIH 3T3) with spatially-controlled distribution. Thus, beyond single-cell stiffening and mechanosensing, these surfaces should also be used as simple and feasible co-culture systems for mimicking and dissecting the bidirectional interactions between blood cells and specific stromal elements of their in vivo microenvironment.

Scaffolds for dental pulp tissue engineering

For tissue engineering strategies, the choice of an appropriate scaffold is the first and certainly a crucial step. A vast variety of biomaterials is available: natural or synthetic polymers, extracellular matrix, self-assembling systems, hydrogels, or bioceramics. Each material offers a unique chemistry, composition and structure, degradation profile, and possibility for modification. The role of the scaffold has changed from passive carrier toward a bioactive matrix, which can induce a desired cellular behavior. Tailor-made materials for specific applications can be created. Recent approaches to generate dental pulp rely on established materials, such as collagen, polyester, chitosan, or hydroxyapatite. Results after transplantation show soft connective tissue formation and newly generated dentin. For dentin-pulp-complex engineering, aspects including vascularization, cell-matrix interactions, growth-factor incorporation, matrix degradation, mineralization, and contamination control should be considered. Self-assembling peptide hydrogels are an example of a smart material that can be modified to create customized matrices. Rational design of the peptide sequence allows for control of material stiffness, induction of mineral nucleation, or introduction of antibacterial activity. Cellular responses can be evoked by the incorporation of cell adhesion motifs, enzyme-cleavable sites, and suitable growth factors. The combination of inductive scaffold materials with stem cells might optimize the approaches for dentin-pulp complex regeneration.

Substrate stiffness regulates the proliferation, migration, and differentiation of epidermal cells

The aim of this study was to investigate the role of substrate stiffness on the proliferation, migration, and differentiation of epidermal cells. To investigate the effects of substrate stiffness on wound healing, epidermal cells were chosen and inoculated on silicone substrate with different values of Young's modulus of elasticity. The cell growth curve, MTT method, and cell cycle detection were used to investigate proliferation, and the scratch test was used to investigate cell migration. Fluorescence flow cytometry was used to study epidermal cell differentiation. The proliferation and migration of epidermal cells favoured stiffer surfaces. A highly stiff surface stimulated epidermal cell proliferation and migration and increased re-epithelialisation, but inhibited differentiation. The candidate pathways mediating epidermal cell proliferation and migration are linked to cell anchoring to substrates by integrin-mediated focal adhesion.

The influence of matrix elasticity on chondrocyte behavior in 3D

Cells actively probe the stiffness of their surrounding and respond to it. The authors recently found that maintenance of the chondrogenic phenotype was directly influenced by this property in 2D. Since studies about this process in 3D are still largely absent, this study aimed to transfer this knowledge into a 3D environment. Agarose was modified with RGD to allow active stiffness sensing or RGE as a control. Hydrogels with different mechanical properties were produced by using different concentrations of agarose. Primary chondrocytes were incorporated into the gel, cultured for up to two weeks, and then constructs were analyzed. Cells were surrounded by their own ECM from an early stage and maintained their chondrogenic phenotype, independent of substrate composition, as indicated by a high collagen type II and a lack of collagen type I production. However, softer gels showed higher DNA and GAG content and larger cell clusters than stiff gels in both RGD- and RGE-modified agarose. The authors hypothesize that matrix elasticity in the tested range does not influence the maintenance of the chondrogenic phenotype in 3D but rather the size of the formed cell ECM clusters. The deviation of these findings from previous results in 2D stresses the importance of moving towards 3D systems that more closely mimic in vivo conditions.

A material's point of view on recent developments of polymeric biomaterials: control of mechanical and biochemical properties

Cells respond to a variety of stimuli, including biochemical, topographical and mechanical signals originating from their micro-environment. Cell responses to the mechanical properties of their substrates have been increasingly studied for about 14 years. To this end, several types of materials based on synthetic and natural polymers have been developed. Presentation of biochemical ligands to the cells is also important to provide additional functionalities or more selectivity in the details of cell/material interaction. In this review article, we will emphasize the development of synthetic and natural polymeric materials with well-characterized and tunable mechanical properties. We will also highlight how biochemical signals can be presented to the cells by combining them with these biomaterials. Such developments in materials science are not only important for fundamental biophysical studies on cell/material interactions but also for the design of a new generation of advanced and highly functional biomaterials.

Measurements of elastic moduli of silicone gel substrates with a microfluidic device

Thin layers of gels with mechanical properties mimicking animal tissues are widely used to study the rigidity sensing of adherent animal cells and to measure forces applied by cells to their substrate with traction force microscopy. The gels are usually based on polyacrylamide and their elastic modulus is measured with an atomic force microscope (AFM). Here we present a simple microfluidic device that generates high shear stresses in a laminar flow above a gel-coated substrate and apply the device to gels with elastic moduli in a range from 0.4 to 300 kPa that are all prepared by mixing two components of a transparent commercial silicone Sylgard 184. The elastic modulus is measured by tracking beads on the gel surface under a wide-field fluorescence microscope without any other specialized equipment. The measurements have small and simple to estimate errors and their results are confirmed by conventional tensile tests. A master curve is obtained relating the mixing ratios of the two components of Sylgard 184 with the resulting elastic moduli of the gels. The rigidity of the silicone gels is less susceptible to effects from drying, swelling, and aging than polyacrylamide gels and can be easily coated with fluorescent tracer particles and with molecules promoting cellular adhesion. This work can lead to broader use of silicone gels in the cell biology laboratory and to improved repeatability and accuracy of cell traction force microscopy and rigidity sensing experiments.

High refractive index silicone gels for simultaneous total internal reflection fluorescence and traction force microscopy of adherent cells

Substrate rigidity profoundly impacts cellular behaviors such as migration, gene expression, and cell fate. Total Internal Reflection Fluorescence (TIRF) microscopy enables selective visualization of the dynamics of substrate adhesions, vesicle trafficking, and biochemical signaling at the cell-substrate interface. Here we apply high-refractive-index silicone gels to perform TIRF microscopy on substrates with a wide range of physiological elastic moduli and simultaneously measure traction forces exerted by cells on the substrate.

Quantitative analysis of the combined effect of substrate rigidity and topographic guidance on cell morphology

Live cells are exquisitely sensitive to both the substratum rigidity and texture. To explore cell responses to both these types of inputs in a precisely controlled fashion, we analyzed the responses of Chinese hamster ovary (CHO) cells to nanotopographically defined substrata of different rigidities, ranging from 1.8 MPa to 1.1 GPa. Parallel arrays of nanogrooves (800-nm width, 800-nm space, and 800-nm depth) on polyurethane (PU)-based material surfaces were fabricated by UV-assisted capillary force lithography (CFL) over an area of 5 mm × 3 mm. We observed dramatic morphological responses of CHO cells, evident in their elongation and polarization along the nanogrooves direction. The cells were progressively more spread and elongated as the substratum rigidity increased, in an integrin β1 dependent manner. However, the degree of orientation was independent of substratum rigidity, suggesting that the cell shape is primarily determined by the topographical cues.

The influence of elasticity and surface roughness on myogenic and osteogenic-differentiation of cells on silk-elastin biomaterials

The interactions of C2C12 myoblasts and human bone marrow stem cells (hMSCs) with silk-tropoelastin biomaterials, and the capacity of each to promote attachment, proliferation, and either myogenic- or osteogenic-differentiation were investigated. Temperature-controlled water vapor annealing was used to control beta-sheet crystal formation to generate insoluble silk-tropoelastin biomaterial matrices at defined ratios of the two proteins. These ratios controlled surface roughness and micro/nano-scale topological patterns, and elastic modulus, stiffness, yield stress, and tensile strength. A combination of low surface roughness and high stiffness in the silk-tropoelastin materials promoted proliferation and myogenic-differentiation of C2C12 cells. In contrast, high surface roughness with micro/nano-scale surface patterns was favored by hMSCs. Increasing the content of human tropoelastin in the silk-tropoelastin materials enhanced the proliferation and osteogenic-differentiation of hMSCs. We conclude that the silk-tropoelastin composition facilitates fine tuning of the growth and differentiation of these cells.

Combining dynamic stretch and tunable stiffness to probe cell mechanobiology in vitro

Cells have the ability to actively sense their mechanical environment and respond to both substrate stiffness and stretch by altering their adhesion, proliferation, locomotion, morphology, and synthetic profile. In order to elucidate the interrelated effects of different mechanical stimuli on cell phenotype in vitro, we have developed a method for culturing mammalian cells in a two-dimensional environment at a wide range of combined levels of substrate stiffness and dynamic stretch. Polyacrylamide gels were covalently bonded to flexible silicone culture plates and coated with monomeric collagen for cell adhesion. Substrate stiffness was adjusted from relatively soft (G′ = 0.3 kPa) to stiff (G′ = 50 kPa) by altering the ratio of acrylamide to bis-acrylamide, and the silicone membranes were stretched over circular loading posts by applying vacuum pressure to impart near-uniform stretch, as confirmed by strain field analysis. As a demonstration of the system, porcine aortic valve interstitial cells (VIC) and human mesenchymal stem cells (hMSC) were plated on soft and stiff substrates either statically cultured or exposed to 10% equibiaxial or pure uniaxial stretch at 1Hz for 6 hours. In all cases, cell attachment and cell viability were high. On soft substrates, VICs cultured statically exhibit a small rounded morphology, significantly smaller than on stiff substrates (p<0.05). Following equibiaxial cyclic stretch, VICs spread to the extent of cells cultured on stiff substrates, but did not reorient in response to uniaxial stretch to the extent of cells stretched on stiff substrates. hMSCs exhibited a less pronounced response than VICs, likely due to a lower stiffness threshold for spreading on static gels. These preliminary data demonstrate that inhibition of spreading due to a lack of matrix stiffness surrounding a cell may be overcome by externally applied stretch suggesting similar mechanotransduction mechanisms for sensing stiffness and stretch.

Sensing and modulation of invadopodia across a wide range of rigidities

Recent studies have suggested that extracellular matrix rigidity regulates cancer invasiveness, including the formation of cellular invadopodial protrusions; however, the relevant mechanical range is unclear. Here, we used a combined analysis of tissue-derived model basement membrane (BM) and stromal matrices and synthetic materials to understand how substrate rigidity regulates invadopodia. Urinary bladder matrix-BM (UBM-BM) was found to be a rigid material with elastic moduli of 3-8 MPa, as measured by atomic force microscopy and low-strain tensile testing. Stromal elastic moduli were ∼6-fold lower, indicating a more compliant material. Using synthetic substrates that span kPa-GPa moduli, we found a peak of invadopodia-associated extracellular matrix degradation centered around 30 kPa, which also corresponded to a peak in invadopodia/cell. Surprisingly, we observed another peak in invadopodia numbers at 2 GPa as well as gene expression changes that indicate cellular sensing of very high moduli. Based on the measured elastic moduli of model stroma and BM, we expected to find more invadopodia formation on the stroma, and this was verified on the stromal versus BM side of UBM-BM. These data suggest that cells can sense a wide range of rigidities, up into the GPa range. Furthermore, there is an optimal rigidity range for invadopodia activity that may be limited by BM rigidity.

Substrates for cardiovascular tissue engineering

Cardiovascular tissue engineering aims to find solutions for the suboptimal regeneration of heart valves, arteries and myocardium by creating 'living' tissue replacements outside (in vitro) or inside (in situ) the human body. A combination of cells, biomaterials and environmental cues of tissue development is employed to obtain tissues with targeted structure and functional properties that can survive and develop within the harsh hemodynamic environment of the cardiovascular system. This paper reviews the up-to-date status of cardiovascular tissue engineering with special emphasis on the development and use of biomaterial substrates. Key requirements and properties of these substrates, as well as methods and readout parameters to test their efficacy in the human body, are described in detail and discussed in the light of current trends toward designing biologically inspired microenviroments for in situ tissue engineering purposes.

Lessons from (patho)physiological tissue stiffness and their implications for drug screening, drug delivery and regenerative medicine

Diseased tissues are noted for their compromised mechanical properties, which contribute to organ failure; regeneration entails restoration of tissue structure and thereby functions. Thus, the physical signature of a tissue is closely associated with its biological function. In this review, we consider a mechanics-centric view of disease and regeneration by drawing parallels between in vivo tissue-level observations and corroborative cellular evidence in vitro to demonstrate the importance of the mechanical stiffness of the extracellular matrix in these processes. This is not intended to devalue the importance of biochemical signaling; in fact, as we discuss, many mechanical stiffness-driven processes not only require cooperation with biochemical cues, but they ultimately converge at common signaling cascades to influence cell and tissue function in an integrative manner. The study of how physical and biochemical signals collectively modulate cell function not only brings forth a more holistic understanding of cell (patho)biology, but it also creates opportunities to control material properties to improve culture platforms for research and drug screening and aid in the rationale design of biomaterials for molecular therapy and tissue engineering applications.

A numerical model for durotaxis

Cell migration is a phenomenon that is involved in several physiological processes. In the absence of external guiding factors it shares analogies with Brownian motion. The presence of biochemical or biophysical cues, on the other hand, can influence cell migration transforming it in a biased random movement. Recent studies have shown that different cell types are able to recognise the mechanical properties of the substratum over which they move and that these properties direct the motion through a process called durotaxis. In this work a 2D mathematical model for the description of this phenomenon is presented. The model is based on the Langevin equation that has been modified to take into account the local mechanical properties of the substratum perceived by the cells. Numerical simulations of the model provide individual cell tracks, whose characteristics can be compared with experimental observations directly. The present model is solved for two important cases: an isotropic substratum, to check that random motility is recovered as a subcase, and a biphasic substratum, to investigate durotaxis. The degree of agreement is satisfactory in both cases. The model can be a useful tool for quantifying relevant parameters of cell migration as a function of the substratum mechanical properties.

Mitral valvular interstitial cell responses to substrate stiffness depend on age and anatomic region

The material properties of heart valves depend on the subject's age, the state of the disease and the complex valvular microarchitecture. Furthermore, valvular interstitial cells (VICs) are mechanosensitive, and their synthesis of extracellular matrix not only determines the valve's material properties but also provides an adhesive substrate for VICs. However, the interrelationship between substrate stiffness and VIC phenotype and synthetic properties is poorly understood. Given that the local mechanical environment (substrate stiffness) surrounding VICs differs among different age groups and different anatomic regions of the valve, it was hypothesized that there may be an age- and valve-region-specific response of VICs to substrate stiffness. Therefore, 6-week-, 6-month- and 6-year-old porcine VICs from the center of the mitral valve anterior leaflet (MVAC) and posterior leaflet (PML) were seeded onto poly(ethylene) glycol hydrogels of different stiffnesses and stained for markers of VIC activation (smooth muscle alpha-actin (SMaA)) and collagen synthesis (heat shock protein-47 (HSP47), prolyl 4-hydroxylase (P4H)). Six-week-old MVAC demonstrated decreased SMaA, P4H and HSP47 on stiffer gels, while 6-week-old PML only demonstrated decreased HSP47. Six-month-old MVAC demonstrated no difference between substrates, while 6-month-old PML demonstrated decreased SMaA, P4H and HSP47. Six-year-old MVAC demonstrated decreased P4H and HSP47, while 6-year-old PML demonstrated decreased P4H and increased HSP47. In conclusion, the age-specific and valve-region-specific responses of VICs to substrate stiffness link VIC phenotype to the leaflet regional matrix in which the VICs reside. These data provide further rationale for investigating the role of substrate stiffness in VIC remodeling within diseased and tissue engineered valves.

Enzymatic cross-linking of a nanofibrous peptide hydrogel

The rheological properties of the environment in which a cell lives play a key role in how the cells will respond to that environment and may modify cell proliferation, morphology and differentiation. Effective means of modifying these properties are needed, particularly for peptide hydrogels which are generally relatively weak and soft. In this report we describe the enzymatic cross-linking of a nanofibrous multidomain peptide hydrogel. When this method was used, the storage modulus, G', could be increased to over 4000 Pa without changes in hydrogel concentration and without dramatic changes in nanostructural architecture. Enzymatic cross-linking represents a mild and simple method for increasing the mechanical strength of peptide hydrogels in applications for which the robustness of the gel is essential. This method should be suitable for a broad array of peptide hydrogels containing lysine such as those currently under study by many different groups.

Modified Gellan Gum hydrogels with tunable physical and mechanical properties

Gellan Gum (GG) has been recently proposed for tissue engineering applications. GG hydrogels are produced by physical crosslinking methods induced by temperature variation or by the presence of divalent cations. However, physical crosslinking methods may yield hydrogels that become weaker in physiological conditions due to the exchange of divalent cations by monovalent ones. Hence, this work presents a new class of GG hydrogels crosslinkable by both physical and chemical mechanisms. Methacrylate groups were incorporated in the GG chain, leading to the production of a methacrylated Gellan Gum (MeGG) hydrogel with highly tunable physical and mechanical properties. The chemical modification was confirmed by proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FTIR-ATR). The mechanical properties of the developed hydrogel networks, with Young's modulus values between 0.15 and 148 kPa, showed to be tuned by the different crosslinking mechanisms used. The in vitro swelling kinetics and hydrolytic degradation rate were dependent on the crosslinking mechanisms used to form the hydrogels. Three-dimensional (3D) encapsulation of NIH-3T3 fibroblast cells in MeGG networks demonstrated in vitro biocompatibility confirmed by high cell survival. Given the highly tunable mechanical and degradation properties of MeGG, it may be applicable for a wide range of tissue engineering approaches.

Polymers for Fabricating Nerve Conduits

Peripheral nerve regeneration is a complicated and long-term medical challenge that requires suitable guides for bridging nerve injury gaps and restoring nerve functions. Many natural and synthetic polymers have been used to fabricate nerve conduits as well as luminal fillers for achieving desired nerve regenerative functions. It is important to understand the intrinsic properties of these polymers and techniques that have been used for fabricating nerve conduits. Previously extensive reviews have been focused on the biological functions and in vivo performance of polymeric nerve conduits. In this paper, we emphasize on the structures, thermal and mechanical properties of these naturally derived synthetic polymers, and their fabrication methods. These aspects are critical for the performance of fabricated nerve conduits. By learning from the existing candidates, we can advance the strategies for designing novel polymeric systems with better properties for nerve regeneration.