Dynamic viscoelastic properties of collagen gels in the presence and absence of collagen fibrils

Heparinized collagen-based hydrogels for tissue engineering: physical, mechanical and biological properties

As the main protein forming the vascular extracellular matrix, collagen has a weak antigenicity, making it an attractive candidate for coatings of vascular grafts. In order to bring antithrombotic properties to collagen for obtaining suitable blood compatibility of surfaces and further bioactive molecule carrying capacity, heparinization appears as a method of choice. Thus, in this article, pH-driven self-assembly was used to form collagen-based hydrogels with physical incorporation of heparins, especially low molecular weight heparin or unfractionated heparin at 1 IU/mL and 6 IU/mL. These heparinized hydrogels were evaluated for their physicochemical properties including gelation kinetic, spreading, viscoelasticity, microstructure and heparin quantification, and their biocompatibility such as cytocompatibility and their capacity to release bioactive heparins with antithrombotic properties. The loading capacity of collagen-based hydrogels was higher for unfractionated heparin (60 to 80 %) than for low molecular weight heparin (20 %). Interestingly, the highest concentration (6 IU/mL) of heparin used to form collagen-based hydrogels resulted in the formation of a softer hydrogel owning a better spreadability compared to 1 IU/mL and non heparinized collagen-based hydrogel. The 3D structure observation showed a layered formation with visible pores or spaces between the layers in all types of collagen-based hydrogels. These layers are interconnected by fibrous structures, suggesting a networked architecture. Moreover, the amount of heparin released from collagen-based hydrogel prepared with 6 IU /mL was higher than from those heparinized with 1 IU/mL attested by a delay in blood coagulation (activated partial thromboplastin time and thrombin time) and the abolishment of thrombin generation. Those hydrogels were also biocompatible, with low albumin adsorption and no impact on cell viability. Finally, heparin is retained in these hydrogels for at least seven days after cell seeding, providing the possibility of long-term antithrombotic properties. Thus, we succeeded in develop/obtain coatings with antithrombogenic properties, biocompatible and able to retain heparins at least seven days, making them good candidates with a great potential for the development and application of efficient blood-contacting materials.

Interpenetrating gelatin/alginate mixed hydrogel: The simplest method to prepare an autoclavable scaffold

The choice of sterilization method for hydrogels used for cell culture influences the ease of preparing the gel. We prepared interpenetrating gelatin/calcium alginate hydrogels containing 1% (w/v) alginate and 1-16% (w/v) gelatin by molding with the mixture of gelatin/sodium alginate solution, followed by the addition of calcium ions by incubation in calcium chloride solution. It is the simplest method to prepare autoclavable gelatin/sodium hydrogel. We measured various properties of the hydrogels including volume, Young's modulus in the compression test, storage modulus, and loss modulus in the dynamic viscoelasticity measurement. The gelatin/alginate hydrogel can be easily fabricated into any shape by this method. After autoclave treatment, the hydrogel was shrunk to smaller than the original shape in similar figures. The shape of the gelatin/alginate hydrogel can be designed into any shape with the reduction ratio of the volume. Human osteosarcoma (HOS) cells adhered to the gelatin/alginate hydrogel and then proliferated. Gelatin/calcium alginate hydrogels with a high concentration are considered to be autoclavable culture substrates because of their low deformation and gelatin elution rate after autoclaving and the high amount of cells attached to the hydrogels.

Correlations on the Structure and Properties of Collagen Hydrogels Produced by E-Beam Crosslinking

In this study, a collagen hydrogel using collagen exclusively produced in Romania, was obtained by electron beam (e-beam) crosslinking. The purpose of our study is to obtain new experimental data on the crosslinking of collagen and to predict as faithfully as possible, its behavior at high irradiation doses and energies. To pursue this, the correlations between macromolecular structure and properties of collagen hydrogels were determined by rheological analysis, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Differential Scanning Calorimetry (DSC), respectively. The gel fraction, swelling degree, and network parameters of the collagen hydrogels were also investigated at different irradiation doses. Through experimental exploration, we concluded that irradiation with e-beam up to 25 kGy induces crosslinking processes in collagen structure without producing advanced degradation processes. E-beam technology is a great method to develop new materials for medical applications without adding other chemical reagents harmful to human health. The future aim is to develop new wound dressings for rapid healing based on collagen, through irradiation technologies.

Matrix metalloproteinase-responsive collagen-oxidized hyaluronic acid injectable hydrogels for osteoarthritic therapy

Drug delivery system and intra-articular injection have been clinically applied to prolong drug residence time and reduce side effects in the treatment of osteoarthrosis. Herein, injectable hydrogels with sustained-dexamethasone sodium phosphate (DSP) release behavior in response to matrix metalloproteinase (MMP) were developed for osteoarthritic therapy. Hyaluronic acid undergoes specific oxidation in the present of sodium periodate to prepare oxidized hyaluronic acid (OHA). Then the DSP-loaded collagen-based hydrogels (Col-OHA) were developed by the Schiff's base crosslinking between OHA and Type I collagen besides the self-assembly of collagen induced by OHA. The results indicate that the collagen self-assembly into collagen fibrils makes great contribution for shortening gelation time of Col-OHA hydrogels. Col-OHA hydrogels possess interconnected porous microstructure, good injectability, excellent self-healing performance, strong mechanical property, low swelling ability, good blood compatibility and no cytotoxicity. Significantly, Col-OHA hydrogels show highly sensitive and significantly substantially sustained release of DSP in response to MMP. DSP-loaded Col-OHA hydrogel possesses significant inhibition for the production of inflammatory cytokines in the joint synovium, which can effectively relieve the symptoms of osteoarthritis continuously. Col-OHA hydrogel has no obvious effect on liver and kidney functions. Overall, the Col-OHA hydrogels with excellent biocompatibility are the promising drug-loading system for the intra-articular injection therapy of osteoarthrosis.

Dual-Cross-linked Liquid Crystal Hydrogels with Controllable Viscoelasticity for Regulating Cell Behaviors

The liquid crystal properties and viscoelasticity of the natural bone extracellular matrix (ECM) play a decisive role in guiding cell behavior, conducting cell signals, and regulating mineralization. Here, we develop a facile approach for preparing a novel polysaccharide hydrogel with liquid crystal properties and viscoelasticity similar to those of natural bone ECM. First, a series of chitin whisker/chitosan (CHW/CS) hydrogels were prepared by chemical cross-linking with genipin, in which CHW can self-assemble to form cholesteric liquid crystals under ultrasonic treatment and CS chains can enter into the gaps between the helical layers of the CHW cholesteric liquid crystal phase to endow morphological stability and good mechanical properties. Subsequently, the obtained chemically cross-linked liquid crystal hydrogels were immersed into the desired concentration of the NaCl solution to form physical cross-linking. Due to the Hofmeister effect, the as-prepared dual-cross-linked liquid crystal hydrogels showed an enhanced modulus, viscoelasticity similar to that of natural ECM with relatively fast stress relaxation behavior, and fold surface morphology. Compared to both CHW/CS hydrogels without liquid crystal properties and CHW/CS liquid crystal hydrogels without further physical cross-linking, the dual-cross-linked CHW/CS liquid crystal hydrogels are more favorable for the adhesion, proliferation, and osteogenic differentiation of bone marrow mesenchymal stem cells. This approach could inspire the design of hydrogels mimicking the liquid crystal properties and viscoelasticity of natural bone ECM for bone repair.

Coordination with zirconium: A facile approach to improve the mechanical properties and thermostability of gelatin hydrogel

The poor mechanical property and thermostability restricted applications of gelatin hydrogel. Herein, a facile and inexpensive approach of immerging cooling induced gelatin hydrogels into Zr(SO₄)₂ dilute solution was applied to overcome these shortages. After this treatment, the micropores in hydrogel decreased to tens of microns while the water content slightly decreased. XPS results revealed that the coordination bonds formed between amino or carboxyl groups of gelatins and Zr⁴⁺. After immerging in 0.06 M Zr⁴⁺ solution, mechanical tests showed that the elastic modulus, compressive modulus and compressive strength of hydrogel were about 400, 1192 and 476 kPa, respectively, which were approximate 100, 11 and 5 times larger than those of pure gelatin. The DSC data indicated that the thermoreversible temperature of triple helix structure in gelatin was improved from about 30 °C to 55 °C. More importantly, the rheological temperature sweep test revealed that hydrogels with 0.06 M Zr⁴⁺ treatment can maintain the hydrogel state without melting even at 80 °C. CCK-8 tests and Calcein-AM/PI double-stain experiments demonstrated Zr⁴⁺ coordination was non-cytotoxic. These promising data indicated this nontoxic method was efficient and had potential to fabricate gelatin related materials for further application.

Lose the stress: Viscoelastic materials for cell engineering

Biomaterials are widely used to study and control a variety of cell behaviors, including stem cell differentiation, organogenesis, and tumor invasion. While considerable attention has historically been paid to biomaterial elastic (storage) properties, it has recently become clear that viscous (loss) properties can also powerfully influence cell behavior. Here we review advances in viscoelastic materials for cell engineering. We begin by discussing collagen, an abundant naturally occurring biomaterial that derives its viscoelastic properties from its fibrillar architecture, which enables dissipation of applied stresses. We then turn to two other naturally occurring biomaterials that are more frequently modified for engineering applications, alginate and hyaluronic acid, whose viscoelastic properties may be tuned by modulating network composition and crosslinking. We also discuss the potential of exploiting engineered fibrous materials, particularly electrospun fiber-based materials, to control viscoelastic properties. Finally, we review mechanisms through which cells process viscous and viscoelastic cues as they move along and within these materials. The ability of viscoelastic materials to relax cell-imposed stresses can dramatically alter migration on two-dimensional surfaces and confinement-imposed barriers to engraftment and infiltration in three-dimensional scaffolds. STATEMENT OF SIGNIFICANCE: Most tissues and many biomaterials exhibit some viscous character, a property that is increasingly understood to influence cell behavior in profound ways. This review discusses the origin and significance of viscoelastic properties of common biomaterials, as well as how these cues are processed by cells to influence migration. A deeper understanding of the mechanisms of viscoelastic behavior in biomaterials and how cells interpret these inputs should aid the design and selection of biomaterials for specific applications.

A viscoelastic model for human myocardium

Understanding the biomechanics of the heart in health and disease plays an important role in the diagnosis and treatment of heart failure. The use of computational biomechanical models for therapy assessment is paving the way for personalized treatment, and relies on accurate constitutive equations mapping strain to stress. Current state-of-the art constitutive equations account for the nonlinear anisotropic stress-strain response of cardiac muscle using hyperelasticity theory. While providing a solid foundation for understanding the biomechanics of heart tissue, most current laws neglect viscoelastic phenomena observed experimentally. Utilizing experimental data from human myocardium and knowledge of the hierarchical structure of heart muscle, we present a fractional nonlinear anisotropic viscoelastic constitutive model. The model is shown to replicate biaxial stretch, triaxial cyclic shear and triaxial stress relaxation experiments (mean error ∼7.68%), showing improvements compared to its hyperelastic (mean error ∼24%) counterparts. Model sensitivity, fidelity and parameter uniqueness are demonstrated. The model is also compared to rate-dependent biaxial stretch as well as different modes of biaxial stretch, illustrating extensibility of the model to a range of loading phenomena. STATEMENT OF SIGNIFICANCE: The viscoelastic response of human heart tissues has yet to be integrated into common constitutive models describing cardiac mechanics. In this work, a fractional viscoelastic modeling approach is introduced based on the hierarchical structure of heart tissue. From these foundations, the current state-of-the-art biomechanical models of the heart muscle are transformed using fractional viscoelasticity, replicating passive muscle function across multiple experimental tests. Comparisons are drawn with current models to highlight the improvements of this approach and predictive responses show strong qualitative agreement with experimental data. The proposed model presents the first constitutive model aimed at capturing viscoelastic nonlinear response across multiple testing regimes, providing a platform for better understanding the biomechanics of myocardial tissue in health and disease.

UV irradiation of Type I collagen gels changed the morphology of the interconnected brain capillary endothelial cells on them

We cultured mouse brain capillary endothelial cell line bEnd.3 on the UV-irradiated Type I collagen gel. Morphology of bEnd.3 cells on the Type I collagen gel was drastically changed if the gel was crosslinked by UV irradiation. The interconnecting network of bEnd.3 cells which have cord-like morphology on the soft collagen gels was converted to the monolayer of the flat cells, tightly-bound each other covering the gel surface, in a confluent state. The collagen gels were mechanically stiffened by UV irradiation for 15 min with UV light at 254 nm showing approximately two times higher value of Young's modulus E (1.51 ± 0.58 kPa) than the control gel (3.17 ± 1.17 kPa). AFM images of the collagen fibrils were not severely changed after irradiation. Collagen subunit proteins were crosslinked and degraded simultaneously under UV irradiation proved by results of SDS-PAGE and separation by centrifugation. Expression of Integrin gene was measured by quantitative real-time PCR. Expression of the integrin α2 gene, tight junction protein 1 gene, and claudin 5 gene were down-regulated in cells on the UV irradiated collagen gel in comparison with the unirradiated one while expression of the integrin β1 gene and Integrin α1 gene did not significantly change. Thick actin filaments were more clearly observed in the cells on the UV-irradiated collagen gel than the unirradiated one by fluorescent microscopy. We conclude that UV irradiation made the collagen gel stiffened and changed the physiological state of bEnd.3 cells including their adhesion, extension, and proliferation.

Construction of collagen gel with high viscoelasticity and thermal stability via combining cross-linking and dehydration

Collagen gel is widely used in tissue engineering due to excellent biological properties and swollen three-dimensional network structure. To improve viscoelasticity and thermal stability, collagen gels consisting of fibrils were cross-linked with glutaraldehyde and sequentially dehydrated via ethanol or heating (named as EGC or HGC, respectively). For EGC, ethanol replaced free and loosely bound water and then combined with tightly bound water, inducing the increase in hydrogen bonds and molecular interactions. Therefore, the thermal transition temperature (T_t ) and storage modulus (G') obviously increased from 47.3 ± 0.5°C and 0.1 kPa to 92.7 ± 0.8°C and 7.8 kPa, respectively. Unfortunately, the high deformation (γ > 60%) and low recovery percentage (R < 15%) reflected the poor anti-deformation of gels due to the volatility of ethanol. For HGC, the entanglement and rigidity of fibrils increased owing to the contraction of cross-linked fibrils and cohesive action of denatured collagen. As a result, HGC were more resistant to deformation and exhibited more elasticity than native collagen gel, accompanied by the fact that G' and R increased to 28.8 kPa and 90.0% ± 0.7%. Additionally, HGC exhibited higher T_t (121.4 ± 0.5°C) due to lower water content and higher collagen concentration.

Combined numerical and experimental biomechanical characterization of soft collagen hydrogel substrate

This work presents a combined experimental-numerical framework for the biomechanical characterization of highly hydrated collagen hydrogels, namely with 0.20, 0.30 and 0.40% (by weight) of collagen concentration. Collagen is the most abundant protein in the extracellular matrix of animals and humans. Its intrinsic biocompatibility makes collagen a promising substrate for embedding cells within a highly hydrated environment mimicking natural soft tissues. Cell behaviour is greatly influenced by the mechanical properties of the surrounding matrix, but the biomechanical characterization of collagen hydrogels has been challenging up to now, since they present non-linear poro-viscoelastic properties. Combining the stiffness outcomes from rheological experiments with relevant literature data on collagen permeability, poroelastic finite element (FE) models were developed. Comparison between experimental confined compression tests available in the literature and analogous FE stress relaxation curves showed a close agreement throughout the tests. This framework allowed establishing that the dynamic shear modulus of the collagen hydrogels is between 0.0097 ± 0.018 kPa for the 0.20% concentration and 0.0601 ± 0.044 kPa for the 0.40% concentration. The Poisson's ratio values for such conditions lie within the range of 0.495-0.485 for 0.20% and 0.480-0.470 for 0.40%, respectively, showing that rheology is sensitive enough to detect these small changes in collagen concentration and thus allowing to link rheology results with the confined compression tests. In conclusion, this integrated approach allows for accurate constitutive modelling of collagen hydrogels. This framework sets the grounds for the characterization of related hydrogels and to the use of this collagen parameterization in more complex multiscale models.

In situ cross-linked electrospun fiber scaffold of collagen for fabricating cell-dense muscle tissue

Engineered muscle tissues used as transplant tissues in regenerative medicine should have a three-dimensional and cell-dense structure like native tissue. For fabricating a 3D cell-dense muscle tissue from myoblasts, we proposed the electrospun type I collagen microfiber scaffold of the string-shape like a harp. The microfibers were oriented in the same direction to allow the myoblasts to align, and were strung at low density with micrometer intervals to create space for the cells to occupy. To realize this shape of the scaffold, we employed in situ cross-linking during electrospinning process for the first time to collagen fibers. The collagen microfibers in situ cross-linked with glutaraldehyde stably existed in the aqueous media and completely retained the original shape to save the spaces between the fibers for over 14 days. On the contrary, the conventional cross-linking method by exposure to a glutaraldehyde aqueous solution vapor partially dissolved and damaged the fiber to lose a low-density shape of the scaffold. Myoblasts could penetrate into the interior of the in situ cross-linked string-shaped scaffold and form the cell-dense muscle tissues. Histochemical analysis showed the total area occupied by the cells in the cross section of the tissue was approximately 73 %. Furthermore, the resulting muscle tissue fabricated from primary myoblasts showed typical sarcomeric cross-striations and the entire tissue continuously pulsated by autonomous contraction. Together with the in situ cross-linking, the string-shaped scaffold provides an efficient methodology to fabricate a cell-dense 3D muscle tissue, which could be applied in regenerative medicine in future.

Influence on the physicochemical properties of fish collagen gels using self-assembly and simultaneous cross-linking with the N-hydroxysuccinimide adipic acid derivative

Collagen gels from Southern catfish (Silurus meridionalis Chen) skins were prepared via the self-assembly of collagen molecules and simultaneous cross-linking with the N-hydroxysuccinimide adipic acid derivative (NHS-AA). The doses of NHS-AA were converted to [NHS-AA]/[NH2] ratios (0.025-1.6, calculated by the [active ester group] of NHS-AA and [ε-NH2] of lysine and hydroxylysine residues of collagen). When the ratio < 0.05, collagen gels were formed by collagen molecule self-assembly, resulting in the opalescent appearance of collagen gels and the characteristic D-periodicity of partial collagen fibrils, the collagen gel ([NHS-AA]/[NH2] = 0.05) displayed a small increase in denaturation temperature (Td, 42.8 °C), remaining weight (12.59%), specific water content (SWC 233.7) and elastic modulus (G' 128.4 Pa) compared with uncross-linked collagen gel (39.1 °C, 9.12%, 222.4 and 85.4 Pa, respectively). As the ratio > 0.05, disappearance of D-periodicity and a gradual change in appearance from opalescent to transparent suggested that the inhibition of NHS-AA in the self-assembly of collagen molecules was more obvious. As a result, the collagen gel ([NHS-AA]/[NH2] = 0.2) had the lowest Td (35.8 °C), remaining weight (7.96%), SWC (130.9) and G' (31.9 Pa). When the ratio was 1.6, the collagen molecule self-assembly was markedly suppressed and the formation of collagen gel was predominantly via the covalent cross-linking bonds which led to the transparent appearance, and the maximum values of Td (47.0 °C), remaining weight (45.92%) and G' (420.7 Pa) of collagen gel. These results indicated that collagen gels with different properties can be prepared using different NHS-AA doses.

Gamma-cross-linked nonfibrillar collagen gel as a scaffold for osteogenic differentiation of mesenchymal stem cells

We fabricated a transparent nonfibrillar collagen gel using gamma irradiation (5 kGy) and cultured rat mesenchymal stem cells (MSCs) on both the gamma-irradiated collagen gel and on unirradiated fibrillar collagen gel. Cells attached well and proliferated with high viability on the surface of both gels. The cells cultured on the gamma-irradiated nonfibrillar gel had a unique elongated shape and adhered to each other in culture. After 21 days of culture in dexamethasone-containing culture medium, the contents of bone-specific osteocalcin and calcium on the gamma-irradiated nonfibrillar gel were 1.4 and 1.9 times higher than those on fibrillar collagen gel, respectively. These data show that osteogenic differentiation of MSCs was promoted more efficiently on the gamma-cross-linked nonfibrillar gel than on the fibrillar gel and demonstrate the potential of the gamma-irradiated collagen gel for use in bone tissue engineering.

Dynamic viscoelastic properties of collagen gels with high mechanical strength

We developed a new method for the preparation of mechanically strong collagen gels by combining successively basic gel formation, followed by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) cross-linking and lyophilization. Gels cross-linked three times with this method showed stronger mechanical properties (G': 3730±2060 Pa, G″: 288±35 Pa) than a conventional gel that was sequentially cross-linked with EDC once (G': 226±70 Pa, G″: 21±4.4 Pa), but not as strong as the same gel with heating for 30 min at 80°C (G': 7010±830 Pa, G″: 288±35 Pa) reported in our previous paper. The conventional collagen gel was cross-linked with EDC once, heated once, and then subjected twice to a lyophilization-gel formation-cross-linking cycle to give three-cycled gel 2. This gel had the strongest mechanical properties (G': 40,200±18,000 Pa, G″: 3090±1400 Pa, Young's modulus: 0.197±0.069 MPa) of the gels tested. These promising results suggest possible applications of the gels as scaffolds in tissue engineering research.