Characterization of collagen gel solutions and collagen matrices for cell culture

Investigation of the Effects of Phenolic Extracts Obtained from Agro-Industrial Food Wastes on Gelatin Modification

In this study, modified bovine gelatin was produced using the alkaline technique with four different oxidized agro-industrial food waste (pomegranate peel (PP), grape pomace and seed (GP), black tea (BT), and green tea (GT)) phenolic extracts (AFWEs) at three different concentrations (1, 3, and 5% based on dry gelatin). The effect of waste type and concentration on the textural, rheological, emulsifying, foaming, swelling, and color properties of gelatin, as well as its total phenolic content and antioxidant activity, was investigated. Significant improvement in gel strength, thermal stability, and gelation rate of gelatin was achieved by modification with oxidized agro-industrial waste extracts. Compared to the control sample, 46.24% higher bloom strength in the GT5 sample, 5.29 and 6.01 °C higher gelling and melting temperatures in the PP5 sample, respectively, and 85.70% lower tmodel value in the GT3 sample were observed. Additionally, the total phenolic content, antioxidant activity, foam, and emulsion properties of the modified gels increased significantly. This study revealed that gelatins with improved technological and functional properties can be produced by using oxidized phenolic extracts obtained from agricultural industrial food wastes as cross-linking agents in the modification of gelatin.

Decellularized brain extracellular matrix based NGF-releasing cryogel for brain tissue engineering in traumatic brain injury

Traumatic brain injuries(TBI) pose significant challenges to human health, specifically neurological disorders and related motor activities. After TBI, the injured neuronal tissue is known for hardly regenerated and recovered to their normal neuron physiology and tissue compositions. For this reason, tissue engineering strategies that promote neuronal regeneration have gained increasing attention. This study explored the development of a novel neural tissue regeneration cryogel by combining brain-derived decellularized extracellular matrix (ECM) with heparin sulfate crosslinking that can perform nerve growth factor (NGF) release ability. Morphological and mechanical characterizations of the cryogels were performed to assess their suitability as a neural regeneration platform. After that, the heparin concnentration dependent effects of varying NGF concentrations on cryogel were investigated for their controlled release and impact on neuronal cell differentiation. The results revealed a direct correlation between the concentration of released NGF and the heparin sulfate ratio in cryogel, indicating that the cryogel can be tailored to carry higher loads of NGF with heparin concentration in cryogel that induced higher neuronal cell differentiation ratio. Furthermore, the study evaluated the NGF loaded cryogels on neuronal cell proliferation and brain tissue regeneration in vivo. The in vivo results suggested that the NGF loaded brain ECM derived cryogel significantly affects the regeneration of brain tissue. Overall, this research contributes to the development of advanced neural tissue engineering strategies and provides valuable insights into the design of regenerative cryogels that can be customized for specific therapeutic applications.

Engineering large and geometrically controlled vascularized nerve tissue in collagen hydrogels to restore large-sized volumetric muscle loss

Developing scalable vascularized and innervated tissue is a critical challenge for the successful clinical application of tissue-engineered constructs. Collagen hydrogels are extensively utilized in cell-mediated vascular network formation because of their naturally excellent biological properties. However, the substantial increase in hydrogel contraction induced by populated cells limits their long-term use. Previous studies attempted to mitigate this issue by concentrating collagen pre-polymer solutions or synthesizing covalently crosslinked collagen hydrogels. However, these methods only partially reduce hydrogel contraction while hindering blood vessel formation within the hydrogels. To address this challenge, we introduced additional support in the form of a supportive spacer to counteract the contraction forces of populated cells and prevent hydrogel contraction. This approach was found to promote cell spreading, resist hydrogel contraction, control hydrogel/tissue geometry, and even facilitate the engineering of functional blood vessels and host nerve growth in just one week. Subsequently, implanting these engineered tissues into muscle defect sites resulted in timely anastomosis with the host vasculature, leading to enhanced myogenesis, increased muscle innervation, and the restoration of injured muscle functionality. Overall, this innovative strategy expands the applicability of collagen hydrogels in fabricating large vascularized nerve tissue constructs for repairing volumetric muscle loss (∼63 %) and restoring muscle function.

Probing the alignment-dependent mechanical behaviors and time-evolutional aligning process of collagen scaffolds

Efficiently manipulating and reproducing collagen (COL) alignment in vitro remains challenging because many of the fundamental mechanisms underlying and guiding the alignment process are not known. We reconcile experiments and coarse-grained molecular dynamics simulations to investigate the mechanical behaviors of a growing COL scaffold and assay how changes in fiber alignment and various cross-linking densities impact their alignment dynamics under shear flow. We find higher cross-link densities and alignment levels significantly enhance the apparent tensile/shear moduli and strength of a bulk COL system, suggesting potential measures to facilitate the design of stronger COL based materials. Since fibril alignment plays a key factor in scaffold mechanics, we next investigate the molecular mechanism behind fibril alignment with Couette flow by computationally investigating the effects of COL's structural properties such as chain lengths, number of chains, tethering conditions, and initial COL conformations on the COL's final alignment level. Our computations suggest that longer chain lengths, more chains, greater amounts of tethering, and initial anisotropic COL conformations benefit the final alignment, but the effect of chain lengths may be more dominant over other factors. These results provide important parameters for consideration in manufacturing COL-based scaffolds where alignment and cross-linking are necessary for regulating performance.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

Cell-Biomaterial Constructs for Wound Healing and Skin Regeneration

Over the years, conventional skin grafts, such as full-thickness, split-thickness, and pre-sterilized grafts from human or animal sources, have been at the forefront of skin wound care. However, these conventional grafts are associated with major challenges, including supply shortage, rejection by the immune system, and disease transmission following transplantation. Due to recent progress in nanotechnology and material sciences, advanced artificial skin grafts-based on the fundamental concepts of tissue engineering-are quickly evolving for wound healing and regeneration applications, mainly because they can be uniquely tailored to meet the requirements of specific injuries. Despite tremendous progress in tissue engineering, many challenges and uncertainties still face skin grafts in vivo, such as how to effectively coordinate the interaction between engineered biomaterials and the immune system to prevent graft rejection. Furthermore, in-depth studies on skin regeneration at the molecular level are lacking; as a consequence, the development of novel biomaterial-based systems that interact with the skin at the core level has also been slow. This review will discuss 1) the biological aspects of wound healing and skin regeneration, 2) important characteristics and functions of biomaterials for skin regeneration applications, and 3) synthesis and applications of common biomaterials for skin regeneration. Finally, the current challenges and future directions of biomaterial-based skin regeneration will be addressed.

A framework for developing community-focused medical physics outreach programs

PURPOSE

In STEM education and careers, underrepresented minorities (URMs) experience higher attrition than non-URM counterparts. Informal educational experiences, such as outreach, have been identified to increase URM awareness and enrollment in STEM. The objectives of this work were to (1) elucidate the current state of racial and ethnic diversity in medical physics and (2) provide a community-focused framework for building effective outreach programs geared toward K-12 URM students and their families.

METHODS

Self-reported racial and ethnic identity data from the American Association of Physicists in Medicine (AAPM) members were obtained to identify the percentage of URM members. Outreach programming was developed for home or away events. Home events occurred at the University of Wisconsin-Madison Department of Medical Physics; away events occurred at public community institutions that served URM and economically disadvantaged populations. Demonstrations, hands-on activities, and presentations covered radiation detection, radiotherapy, medical imaging, and medical physics career paths. High school students were asked about their awareness of medical physics prior to outreach events. Likert-scale surveys evaluated student level of agreement (1 = Strongly disagree to 5 = Strongly agree) that home events increased their career interests in medicine and physics and interest in pursuing STEM coursework.

RESULTS

Average percentage of AAPM URM members was 10.7% from 2014 to 2020. From 2016 to 2020, 42 outreach events occurred near or within the Madison metro area. Over 1900 individuals participated in outreach events, with 50 participants on average per event. The majority of home event participants indicated their interest in medical careers increased (65.4%) and were inspired to pursue more STEM courses (73.1%) after the program.

CONCLUSIONS

Our medical physics outreach program demonstrates a means of increasing awareness and interest around medical physics, particularly for underrepresented individuals. This article addresses gaps in the literature for how to create and implement effective, community-focused medical physics outreach programs.

Methods for studying MLO-Y4 osteocytes in collagen-hydroxyapatite scaffolds in the rotary cell culture system

Purpose: The rotary cell culture system (RCCS) is a common clinorotation device for cell culture. It is also used as a low-shear suspension culture bioreactor to form functionalized 3D tissue constructs and to model microgravity. We sought to develop a 3D scaffold composed of type I collagen and hydroxyapatite (collagen-HA) to characterize MLO-Y4 osteocytes following suspension culture or clinorotation.Materials and Methods: MLO-Y4 cells were embedded in collagen-HA. The scaffold was formed into droplets for suspension culture or wall-adhered to the RCCS for clinorotation. AFM, rheometry, immunofluorescence and qRT-PCR were employed to measure the scaffold stiffness, cell viability and gene expression of cells in collagen-HA scaffolds. Dendritic cells were visualized and quantified and gene expression after suspension culture and clinorotation was compared to static controls.Results: The optimized scaffold for the RCCS consisted of collagen with 6 mg/mL HA which had a stiffness of < 1 kPa. MLO-Y4 cell viability was higher in collagen-HA scaffolds, compared to scaffolds without HA. Collagen-HA scaffolds induced higher osteocyte-specific gene expression compared to cells cultured on 2D plastic. Cells in the scaffold downregulated DMP1, E11, IL-6, and RANKL, and had fewer dendritic cells following suspension culture whereas clinorotation downregulated DMP1 and E11 genes, compared to static controls.Conclusions: Suspension culture for 3 days in collagen-HA stimulates growth of osteocytes but may also desensitize them to mechanical cues. Clinorotation for 3 days in collagen-HA does not stimulate proliferation or expression of mechanosensitive genes, indicating that it may be an effective mechanical unloading environment.

Tuning the Mechanical and Geometric Properties of Electrochemically Aligned Collagen Threads Toward Applications in Biohybrid Robotics

Electrochemically aligned collagen (ELAC) threads fabricated by the isoelectric focusing (IF) of collagen have previously shown potential in tissue engineering and more recently in the fabrication of biohybrid robot structures. For applications in biohybrid robotics, ELAC structures are needed that are both robust and compliant enough to facilitate muscle actuation. However, studies on the effects of IF parameters, and the interactions of such fabrication parameters, on the mechanical and geometric properties of resulting ELAC threads have not been previously found in literature. Understanding the impact of these manufacturing parameters on the material properties is critical to facilitate biohybrid robot design. In this study, the effects of IF duration, IF voltage, and collagen solution concentration were investigated and showed statistically significant effects on adjusting ELAC properties via single-factor experiments. The interactions between parameters exhibited significant joint effects on ELAC property tuning through two-factor experiments. Scanning electron microscopy and 2,4,6-trinitrobenzenesulfonic (TNBS) assays revealed the correlation between high mechanical properties and a combination of low porosity and high degree of crosslinking. By simply tuning IF parameters without changing other fabrication steps, such as crosslinker concentration, ELAC threads with a wide range of mechanical and geometric properties were fabricated. The average tensile modulus of the resulting ELAC threads ranged from 198 ± 90 to 758 ± 138 MPa. The average cross-sectional area ranged from 7756 ± 1000 to 1775 ± 457 μm2. The resultant mapping between IF parameters and ELAC thread properties enabled the production of strong and flexible threads with customizable properties.

Engineered cell-laden alginate microparticles for 3D culture

Advanced microfabrication technologies and biocompatible hydrogel materials facilitate the modeling of 3D tissue microenvironment. Encapsulation of cells in hydrogel microparticles offers an excellent high-throughput platform for investigating multicellular interaction with their surrounding microenvironment. Compartmentalized microparticles support formation of various unique cellular structures. Alginate has emerged as one of the most dominant hydrogel materials for cell encapsulation owing to its cytocompatibility, ease of gelation, and biocompatibility. Alginate hydrogel provides a permeable physical boundary to the encapsulated cells and develops an easily manageable 3D cellular structure. The interior structure of alginate hydrogel can further regulate the spatiotemporal distribution of the embedded cells. This review provides a specific overview of the representative engineering approaches to generate various structures of cell-laden alginate microparticles in a uniform and reproducible manner. Capillary nozzle systems, microfluidic droplet systems, and non-chip based high-throughput microfluidic systems are highlighted for developing well-regulated cellular structure in alginate microparticles to realize potential drug screening platform and cell-based therapy. We conclude with the discussion of current limitations and future directions for realizing the translation of this technology to the clinic.

Natural bio-based monomers for biomedical applications: a review

In recent years, synthetic and semi-synthetic polymer materials have been widely used in various applications. Especially concerning biomedical applications, their biocompatibility, biodegradability, and non-toxicity have increased the interest of researchers to discover and develop new products for the well-being of humanity. Among the synthetic and semi-synthetic materials, the use of natural bio-based monomeric materials presents a possible novel avenue for the development of new biocompatible, biodegradable, and non-toxic products. The purpose of this article is to review the information on the role of natural bio-based monomers in biomedical applications. Increased eco-friendliness, biocompatibility, biodegradability, non-toxicity, and intrinsic biological activity are some of the attributes which make itaconic, succinic, citric, hyaluronic, and glutamic acids suitable potential materials for biomedical applications. Herein, we summarize the most recent advances in the field over the past ten years and specifically highlight new and interesting discoveries in biomedical applications. Natural origin acid-based bio-monomers for biomedical applications.

Crosslinking Collagen Constructs: Achieving Cellular Selectivity Through Modifications of Physical and Chemical Properties

Collagen-based constructs have emerged in recent years as ideal candidates for tissue engineering implants. For many biomedical applications, collagen is crosslinked in order to improve the strength, stiffness and stability of the construct. However, the crosslinking process may also result in unintended changes to cell viability, adhesion or proliferation on the treated structures. This review provides a brief overview of some of both the most commonly used and novel crosslinkers used with collagen, and suggests a framework by which crosslinking methods can be compared and selected for a given tissue engineering application.

Overcoming the Design Challenge in 3D Biomimetic Hybrid Scaffolds for Bone and Osteochondral Regeneration by Factorial Design

Scaffolds for bone regeneration have been engineered by a plethora of manufacturing technologies and biomaterials. However, the performance of these systems is often limited by lack of robustness in the process design, that hampers their scalability to clinical application. In the present study, Design of Experiment (DoE) was used as statistical tool to design the biofabrication of hybrid hydroxyapatite (HA)/collagen scaffolds for bone regeneration and optimize their integration in a multilayer osteochondral device. The scaffolds were synthesized via a multi-step bioinspired process consisting in HA nano-crystals nucleation on the collagen self-assembling fibers and ribose glycation was used as collagen cross-linking method to modulate the mechanical and physical properties. The process design was performed by selecting hydrogel concentration, HA/collagen ratio and cross-linker content as key variables and the fabrication was carried out basing on a full factorial design. Scaffold performances were tested by evaluating porosity, swelling ratio, degradation rate and mechanical behavior as model output responses while physicochemical properties of the constructs were evaluated by TGA, ICP, FT-IR spectroscopy, and XRD analysis. Physicochemical characterizations confirmed the nucleation of a biomimetic inorganic phase and the interaction of the HA and collagenic components. The DoE model revealed a significant interaction between HA content and collagen cross-linking in determining porosity, swelling and mechanical properties of the scaffolds. The combined effect of hydrogel concentration and mineral phase played a key role on porosity and swelling while degradation resulted to be mainly affected by the HA loading and ribose content. The model was then used to determine the suitable input parameters for the synthesis of multi-layer scaffolds with graded mineralization rate, that can be used to mimic the whole cartilage-bone interface. This work proved that experimental design applied to complex biofabrication processes represents an effective and reliable way to design hybrid constructs with standardized and tunable properties for osteochondral tissue engineering.

Matrix degradation regulates osteoblast protrusion dynamics and individual migration

Protrusions are one of the structures that cells use to sense their surrounding environment in a probing and exploratory manner as well as to communicate with other cells. In particular, osteoblasts embedded within a 3D matrix tend to originate a large number of protrusions compared to other type of cells. In this work, we study the role that mechanochemical properties of the extracellular matrix (ECM) play on the dynamics of these protrusions, namely, the regulation of the size and number of emanating structures. In addition, we also determine how the dynamics of the protrusions may lead the 3D movement of the osteoblasts. Significant differences were found in protrusion size and cell velocity, when degradation activity due to metalloproteases was blocked by means of an artificial broad-spectrum matrix metalloproteinase inhibitor, whereas stiffening of the matrix by introducing transglutaminase crosslinking, only induced slight changes in both protrusion size and cell velocity, suggesting that the ability of cells to create a path through the matrix is more critical than the matrix mechanical properties themselves. To confirm this, we developed a cell migration computational model in 3D including both the mechanical and chemical properties of the ECM as well as the protrusion mechanics, obtaining good agreement with experimental results.

Differentiation of Induced Pluripotent Stem Cells towards Mesenchymal Stromal Cells is Hampered by Culture in 3D Hydrogels

Directed differentiation of induced pluripotent stem cells (iPSCs) towards specific lineages remains a major challenge in regenerative medicine, while there is a growing perception that this process can be influenced by the three-dimensional environment. In this study, we investigated whether iPSCs can differentiate towards mesenchymal stromal cells (MSCs) when embedded into fibrin hydrogels to enable a one-step differentiation procedure within a scaffold. Differentiation of iPSCs on tissue culture plastic or on top of fibrin hydrogels resulted in a typical MSC-like phenotype. In contrast, iPSCs embedded into fibrin gel gave rise to much smaller cells with heterogeneous growth patterns, absence of fibronectin, faint expression of CD73 and CD105, and reduced differentiation potential towards osteogenic and adipogenic lineage. Transcriptomic analysis demonstrated that characteristic genes for MSCs and extracellular matrix were upregulated on flat substrates, whereas genes of neural development were upregulated in 3D culture. Furthermore, the 3D culture had major effects on DNA methylation profiles, particularly within genes for neuronal and cardiovascular development, while there was no evidence for epigenetic maturation towards MSCs. Taken together, iPSCs could be differentiated towards MSCs on tissue culture plastic or on a flat fibrin hydrogel. In contrast, the differentiation process was heterogeneous and not directed towards MSCs when iPSCs were embedded into the hydrogel.

Brain-stiffness-mimicking tilapia collagen gel promotes the induction of dorsal cortical neurons from human pluripotent stem cells

The mechanical properties of the extracellular microenvironment, including its stiffness, play a crucial role in stem cell fate determination. Although previous studies have demonstrated that the developing brain exhibits spatiotemporal diversity in stiffness, it remains unclear how stiffness regulates stem cell fate towards specific neural lineages. Here, we established a culture substrate that reproduces the stiffness of brain tissue using tilapia collagen for in vitro reconstitution assays. By adding crosslinkers, we obtained gels that are similar in stiffness to living brain tissue (150-1500 Pa). We further examined the capability of the gels serving as a substrate for stem cell culture and the effect of stiffness on neural lineage differentiation using human iPS cells. Surprisingly, exposure to gels with a stiffness of approximately 1500 Pa during the early period of neural induction promoted the production of dorsal cortical neurons. These findings suggest that brain-stiffness-mimicking gel has the potential to determine the terminal neural subtype. Taken together, the crosslinked tilapia collagen gel is expected to be useful in various reconstitution assays that can be used to explore the role of stiffness in neurogenesis and neural functions. The enhanced production of dorsal cortical neurons may also provide considerable advantages for neural regenerative applications.

3D bioprinting of skin tissue: From pre-processing to final product evaluation

Bioprinted skin tissue has the potential for aiding drug screening, formulation development, clinical transplantation, chemical and cosmetic testing, as well as basic research. Limitations of conventional skin tissue engineering approaches have driven the development of biomimetic skin equivalent via 3D bioprinting. A key hope for bioprinting skin is the improved tissue authenticity over conventional skin equivalent construction, enabling the precise localization of multiple cell types and appendages within a construct. The printing of skin faces challenges broadly associated with general 3D bioprinting, including the selection of cell types and biomaterials, and additionally requires in vitro culture formats that allow for growth at an air-liquid interface. This paper provides a thorough review of current 3D bioprinting technologies used to engineer human skin constructs and presents the overall pipelines of designing a biomimetic artificial skin via 3D bioprinting from the design phase (i.e. pre-processing phase) through the tissue maturation phase (i.e. post-processing) and into final product evaluation for drug screening, development, and drug delivery applications.

Cross-Linked Collagen Gels Using Gold Nanoparticles

Cross-linking collagen results in the formation of a collagen matrix that is one of the most widely used biomaterials for applications in drug delivery, tissue engineering, molecular scaffolds, and materials for wound dressing. The properties of the collagen matrix depend on the cross-linking strategy employed. Here we describe the preparation and characterization of a nonfibrillar collagen matrix stabilized by cross-linking with tiopronin-protected, 3-4 nm polyvalent gold nanoparticles.

Combined experimental and computational characterization of crosslinked collagen-based hydrogels

Collagen hydrogels are widely used for in-vitro experiments and tissue engineering applications. Their use has been extended due to their biocompatibility with cells and their capacity to mimic biological tissues; nevertheless their mechanical properties are not always optimal for these purposes. Hydrogels are formed by a network of polymer filaments embedded on an aqueous substrate and their mechanical properties are mainly defined by the filament network architecture and the individual filament properties. To increase properties of native collagen, such as stiffness or strain-stiffening, these networks can be modified by adding crosslinking agents that alter the network architecture, increasing the unions between filaments. In this work, we have investigated the effect of one crosslinking agent, transglutaminase, in collagen hydrogels with varying collagen concentration. We have observed a linear dependency of the gel rigidity on the collagen concentration. Moreover, the addition of transglutaminase has induced an earlier strain-stiffening of the collagen gels. In addition, to better understand the mechanical implications of collagen concentration and crosslinkers inclusion, we have adapted an existing computational model, based on the worm-like chain model (WLC), to reproduce the mechanical behavior of the collagen gels. With this model we can estimate the parameters of the biopolymer networks without more sophisticated techniques, such as image processing or network reconstruction, or, inversely, predict the mechanical properties of a defined collagen network.

Nondestructive assessment of collagen hydrogel cross-linking using time-resolved autofluorescence imaging

We investigate the use of a fiber-based, multispectral fluorescence lifetime imaging (FLIm) system to nondestructively monitor changes in mechanical properties of collagen hydrogels caused by controlled application of widely used cross-linking agents, glutaraldehyde (GTA) and ribose. Postcross-linking, fluorescence lifetime images are acquired prior to the hydrogels being processed by rheological or tensile testing to directly probe gel mechanical properties. To preserve the sterility of the ribose-treated gels, FLIm is performed inside a biosafety cabinet (BSC). A pairwise correlation analysis is used to quantify the relationship between mean hydrogel fluorescence lifetimes and the storage or Young's moduli of the gels. In the GTA study, we observe strong and specific correlations between fluorescence lifetime and the storage and Young's moduli. Similar correlations are not observed in the ribose study and we postulate a reason for this. Finally, we demonstrate the ability of FLIm to longitudinally monitor dynamic cross-link formation. The strength of the GTA correlations and deployment of our fiber-based FLIm system inside the aseptic environment of a BSC suggests that this technique may be a valuable tool for the tissue engineering community where longitudinal assessment of tissue construct maturation in vitro is highly desirable.

Stress-induced plasticity of dynamic collagen networks

The structure and mechanics of tissues is constantly perturbed by endogenous forces originated from cells, and at the same time regulate many important cellular functions such as migration, differentiation, and growth. Here we show that 3D collagen gels, major components of connective tissues and extracellular matrix (ECM), are significantly and irreversibly remodeled by cellular traction forces, as well as by macroscopic strains. To understand this ECM plasticity, we develop a computational model that takes into account the sliding and merging of ECM fibers. We have confirmed the model predictions with experiment. Our results suggest the profound impacts of cellular traction forces on their host ECM during development and cancer progression, and suggest indirect mechanical channels of cell-cell communications in 3D fibrous matrices.The structure and mechanics of tissues is constantly perturbed by endogenous forces originated from cells. Here the authors show that 3D collagen gels, major components of connective tissues and extracellular matrix, are significantly and irreversibly remodelled by cellular traction forces and by macroscopic strains.

Effects of Type I Collagen Concentration in Hydrogel on the Growth and Phenotypic Expression of Rat Chondrocytes

It is controversial whether type I collagen itself can maintain and improve chondrogenic phenotype of chondrocytes in a three-dimensional (3D) environment. In this study, we examined the effect of type I collagen concentration in hydrogel (0.5, 1, and 2 mg/ml) on the growth and phenotype expression of rat chondrocytes in vitro. All collagen hydrogels showed substantial contractions during culture, in a concentration-dependent manner, which was due to the cell proliferation. The cell viability was shown to be the highest in 2 mg/ml collagen gel. The mRNA expression of chondrogenic phenotypes, including SOX9, type II collagen, and aggrecan, was significantly up-regulated, particularly in 1 mg/ml collagen gel. Furthermore, the production of type II collagen and glycosaminoglycan (GAG) content was also enhanced. The results suggest that type I collagen hydrogel is not detrimental to, but may be useful for, the chondrocyte culture for cartilage tissue engineering.

Microfibrous silver-coated polymeric scaffolds with tunable mechanical properties

Electrospun scaffolds of poly(glycerol sebacate)/poly(ε-caprolactone) (PGS/PCL) have been used for engineered tissues due to their desirable thermal and mechanical properties as well as their tunable degradability.

Characterization of Collagen Type I and II Blended Hydrogels for Articular Cartilage Tissue Engineering

Biomaterials that provide signals present in the native extracellular matrix have been proposed as scaffolds to support improved cartilage regeneration. This study harnesses the biological activity of collagen type II and the superior mechanical properties of collagen type I by characterizing gels made of collagen type I and II blends. The collagen blend hydrogels were able to incorporate both types of collagen and retained chondroitin sulfate and hyaluronic acid. Cryo-scanning electron microscopy images showed that the 3:1 ratio of collagen type I to type II gels had a lower void space percentage (36.4%) than the 1:1 gels (46.5%). The complex modulus was larger for the 3:1 gels (G* = 5.0 Pa) compared to the 1:1 gels (G* = 1.2 Pa). The 3:1 blend consistently formed gels with superior mechanical properties compared to the other blends and has the potential to be implemented as a scaffold for articular cartilage engineering.

A Novel Poly(L-lactide) (PLLA)/Fibroin Hybrid Scaffold to Promote Hepatocyte Viability and Decrease Macrophage Responses

The purpose of this study was to evaluate the hepatocellular compatibility and assess the inflammatory response of a novel hybrid scaffold of poly(L-lactide) (PLLA) and fibroin. The hybrid scaffold was obtained by freezing and lyophilizing a blend of fibroin microspheres and PLLA solution. FTIR and scanning electron microscopy (SEM) analysis indicated that fibroin microspheres were on the surface of the hybrid scaffold. Compared to the PLLA scaffold, SEM and laser scanning confocal microscope (LSCM) analyses showed that the human hepatocellular carcinoma HepG2 cells had spread and proliferated much more in the hybrid scaffold. The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assays indicated a greater number of cells in this hybrid scaffold. Furthermore, a mouse RAW264.7 macrophages cell line was utilized to characterize and compare the mRNA profiles of TNF-alpha using real time-polymerase chain reaction (RT-PCR). The inflammatory response of the macrophages grown in the PLLA/fibroin scaffold rapidly declined compared to those in the PLLA scaffold and reached the level of cells grown in Dulbecco's Modified Eagle Medium (DMEM). The hepatocellular compatibility and lower level of inflammatory response makes the PLLA/fibroin scaffold a promising candidate for hepatic tissue engineering.

Controlling Biostability of Collagen Films for Fibroblast Cytocompatibility

The influence of cross-linking reagent, glutaraldehyde, on the biostability of collagen films was examined. With greater cross-linking, water absorption ability, swelling degree and degradation rate were all decreased. Based on scanning electron microscopy observations, the collagen films changed from a compact structure to a fiber-dominated morphology after cross-linking. Cross-linking provides a more integrated fiber-like structure that impeded degradation. To assess the influence of the cross-linking on cytocompatibility, in vitro cell culture was performed on the cross-linked collagen films using human fibroblasts. Good cytocompatibility of the initial collagen was largely retained after cross-linking, while biostability was greatly improved.

Three-Dimensional Porous Sponges from Collagen Biowastes

Three-dimensional, functional, and porous scaffolds can find applications in a variety of fields. Here we report the synthesis of hierarchical and interconnected porous sponges using a simple freeze-drying technique, employing collagen extracted from animal skin wastes and superparamagnetic iron oxide nanoparticles. The ultralightweight, high-surface-area sponges exhibit excellent mechanical stability and enhanced absorption of organic contaminants such as oils and dye molecules. Additionally, these biocomposite sponges display significant cellular biocompatibility, which opens new prospects in biomedical uses. The approach highlights innovative ways of transforming biowastes into advanced hybrid materials using simple and scalable synthesis techniques.

Evaluation of epigallocatechin-3-gallate (EGCG) cross-linked collagen membranes and concerns on osteoblasts

Collagen membranes have ideal biological and mechanical properties for supporting infiltration and proliferation of osteoblasts and play a vital role in guided bone regeneration (GBR). However, pure collagen can lead to inflammation, resulting in progressive bone resorption. Therefore, a method for regulating the level of inflammatory cytokines at surgical sites is paramount for the healing process. Epigallocatechin-3-gallate (EGCG) is a component extracted from green tea with numerous biological activities including an anti-inflammatory effect. Herein, we present a novel cross-linked collagen membrane containing different concentrations of EGCG (0.0064%, 0.064%, and 0.64%) to regulate the level of inflammatory factors secreted by pre-osteoblast cells; improve cell proliferation; and increase the tensile strength, wettability, and thermal stability of collagen membranes. Scanning electron microscope images show that the surfaces of collagen membranes became smoother and the collagen fiber diameters became larger with EGCG treatment. Measurement of the water contact angle demonstrated that introducing EGCG improved membrane wettability. Fourier transform infrared spectroscopy analyses indicated that the backbone of collagen was intact, and the thermal stability was significant improved in differential scanning calorimetry. The mechanical properties of 0.064% and 0.64% EGCG-treated collagen membranes were 1.5-fold greater than those of the control. The extent of cross-linking was significantly increased, as determined by a 2,4,6-trinitrobenzenesulfonic acid solution assay. The Cell Counting Kit-8 (CCK-8) and live/dead assays revealed that collagen membrane cross-linked by 0.0064% EGCG induced greater cell proliferation than pure collagen membranes. Additionally, real-time polymerase chain reaction and enzyme-linked immunosorbent assay results showed that EGCG significantly affected the production of inflammatory factors secreted by MC3T3-E1 cells. Taken together, our results indicate that treatment of collagen membranes with appropriate concentrations of EGCG has an anti-inflammatory effect and shows promise for GBR applications.

Modulating In Vivo Degradation Rate of Injectable Extracellular Matrix Hydrogels

Extracellular matrix (ECM) derived hydrogels are increasingly used as scaffolds to stimulate endogenous repair. However, few studies have examined how altering the degradation rates of these materials affect cellular interaction in vivo. This study sought to examine how crosslinking or matrix metalloproteinase (MMP) inhibition by doxycycline could be employed to modulate the degradation rate of an injectable hydrogel derived from decellularized porcine ventricular myocardium. While both approaches were effective in reducing degradation in vitro, only doxycycline significantly prolonged hydrogel degradation in vivo without affecting material biocompatibility. In addition, unlike crosslinking, incorporation of doxycycline into the hydrogel did not affect mechanical properties. Lastly, the results of this study highlighted the need for development of novel crosslinkers for in situ modification of injectable ECM-derived hydrogels, as none of the crosslinking agents investigated in this study were both biocompatible and effective.

Microstructure and Mechanical Property of Glutaraldehyde-Treated Porcine Pulmonary Ligament

There is a significant need for fixed biological tissues with desired structural and material constituents for tissue engineering applications. Here, we introduce the lung ligament as a fixed biological material that may have clinical utility for tissue engineering. To characterize the lung tissue for potential clinical applications, we studied glutaraldehyde-treated porcine pulmonary ligament (n = 11) with multiphoton microscopy (MPM) and conducted biaxial planar experiments to characterize the mechanical property of the tissue. The MPM imaging revealed that there are generally two families of collagen fibers distributed in two distinct layers: The first family largely aligns along the longitudinal direction with a mean angle of θ = 10.7 ± 9.3 deg, while the second one exhibits a random distribution with a mean θ = 36.6 ± 27.4. Elastin fibers appear in some intermediate sublayers with a random orientation distribution with a mean θ = 39.6 ± 23 deg. Based on the microstructural observation, a microstructure-based constitutive law was proposed to model the elastic property of the tissue. The material parameters were identified by fitting the model to the biaxial stress-strain data of specimens, and good fitting quality was achieved. The parameter e0 (which denotes the strain beyond which the collagen can withstand tension) of glutaraldehyde-treated tissues demonstrated low variability implying a relatively consistent collagen undulation in different samples, while the stiffness parameters for elastin and collagen fibers showed relatively greater variability. The fixed tissues presented a smaller e0 than that of fresh specimen, confirming that glutaraldehyde crosslinking increases the mechanical strength of collagen-based biomaterials. The present study sheds light on the biomechanics of glutaraldehyde-treated porcine pulmonary ligament that may be a candidate for tissue engineering.

Is dialdehyde starch a valuable cross-linking agent for collagen/elastin based materials?

Collagen and elastin are the main structural proteins in mammal bodies. They provide mechanical support, strength, and elasticity to various organs and tissues, e.g. skin, tendons, arteries, and bones. They are readily available, biodegradable, biocompatible and they stimulate cell growth. The physicochemical properties of collagen and elastin-based materials can be modified by cross-linking. Glutaraldehyde is one of the most efficient cross-linking agents. However, the unreacted molecules can be released from the material and cause cytotoxic reactions. Thus, the aim of our work was to investigate the influence of a safer, macromolecular cross-linking agent--dialdehyde starch (DAS). The properties of hydrogels based on collagen/elastin mixtures (95/5, 90/10) containing 5 and 10% of DAS and neutralized via dialysis against deionized water were tested. The homogenous, transparent, stiff hydrogels were obtained. The DAS addition causes the formation of intermolecular cross-linking bonds but does not affect the secondary structure of the proteins. As a result, the thermal stability, mechanical strength, and, surprisingly, swelling ability increased. At the same time, the surface properties test and in vitro study show that the materials are attractive for 3T3 cells. Moreover, the materials containing 10% of DAS are more resistant to enzymatic degradation.

Graphene oxide decorated electrospun gelatin nanofibers: Fabrication, properties and applications

Gelatin nanofiber fabricated by electrospinning process is found to mimic the complex structural and functional properties of natural extracellular matrix for tissue regeneration. In order to improve the physico-chemical and biological properties of the nanofibers, graphene oxide is incorporated in the gelatin to form graphene oxide decorated gelatin nanofibers. The current research effort is focussed on the fabrication and evaluation of physico-chemical and biological properties of graphene oxide-gelatin composite nanofibers. The presence of graphene oxide in the nanofibers was established by transmission electron microscopy (TEM). We report the effect of incorporation of graphene oxide on the mechanical, thermal and biological performance of the gelatin nanofibers. The tensile strength of gelatin nanofibers was increased from 8.29±0.53MPa to 21±2.03MPa after the incorporation of GO. In order to improve the water resistance of nanofibers, natural based cross-linking agent, namely, dextran aldehyde was employed. The cross-linked composite nanofibers showed further increase in the tensile strength up to 56.4±2.03MPa. Graphene oxide incorporated gelatin nanofibers are evaluated for bacterial activity against gram positive (Staphylococcus aureus) and gram negative (Escherichia coli) bacteria and cyto compatibility using mouse fibroblast cells (L-929 cells). The results indicate that the graphene oxide incorporated gelatin nanofibers do not prevent bacterial growth, nevertheless support the L-929 cell adhesion and proliferation.

An acetal-based polymeric crosslinker with controlled pH-sensitivity

.An acetal based polymeric cross-linker with controlled pH-sensitivity was used for the synthesis of collagen hydrogels and sponges. The novel cross-linker was synthesized using DE-ATRP and was more biocompatible compared to the commercial 4-star PEG.

Collagen/elastin hydrogels cross-linked by squaric acid

Hydrogels based on collagen and elastin are very valuable materials for medicine and tissue engineering. They are biocompatible; however their mechanical properties and resistance for enzymatic degradation need to be improved by cross-linking. Up to this point many reagents have been tested but more secure reactants are still sought. Squaric acid (SqAc), 3,4-dihydroxy 3-cyclobutene 1,2-dione, is a strong, cyclic acid, which reacts easily with amine groups. The properties of hydrogels based on collagen/elastin mixtures (95/5, 90/10) containing 5%, 10% and 20% of SqAc and neutralized via dialysis against deionized water were tested. Cross-linked, 3-D, transparent hydrogels were created. The cross-linked materials are stiffer and more resistant to enzymatic degradation than those that are unmodified. The pore size, swelling ability and surface polarity are reduced due to 5% and 10% of SqAc addition. At the same time, the cellular response is not significantly affected by the cross-linking. Therefore, squaric acid would be regarded as a safe, effective cross-linking agent.

Incorporation of fibrin into a collagen-glycosaminoglycan matrix results in a scaffold with improved mechanical properties and enhanced capacity to resist cell-mediated contraction

Fibrin has many uses as a tissue engineering scaffold, however many in vivo studies have shown a reduction in function resulting from the susceptibility of fibrin to cell-mediated contraction. The overall aim of the present study was to develop and characterise a reinforced natural scaffold using fibrin, collagen and glycosaminoglycan (FCG), and to examine the cell-mediated contraction of this scaffold in comparison to fibrin gels. Through the use of an injection loading technique, a homogenous FCG scaffold was developed. Mechanical testing showed a sixfold increase in compressive modulus and a thirtyfold increase in tensile modulus of fibrin when reinforced with a collagen-glycosaminoglycan backbone structure. Human vascular smooth muscle cells (vSMCs) were successfully incorporated into the FCG scaffold and demonstrated excellent viability over 7 days, while proliferation of these cells also increased significantly. VSMCs were seeded into both FCG and fibrin-only gels at the same seeding density for 7 days and while FCG scaffolds did not demonstrate a reduction in size, fibrin-only gels contracted to 10% of their original diameter. The FCG scaffold, which is composed of natural biomaterials, shows potential for use in applications where dimensional stability is crucial to the functionality of the tissue.

STATEMENT OF SIGNIFICANCE

Fibrin is a versatile scaffold for tissue engineering applications, but its weak mechanical properties leave it susceptible to cell-mediated contraction, meaning the dimensions of the fibrin construct will change over time. We have reinforced fibrin with a collagen glycosaminoglycan matrix and characterised the mechanical properties and bioactivity of the reinforced fibrin (FCG). This is the first scaffold manufactured from all naturally derived materials that resists cell-mediated contraction. In fact, over 7 days, the FCG scaffold fully resisted cell-mediated contraction of vascular smooth muscle cells. This FCG scaffold has many potential applications where natural scaffold materials can encourage regeneration.