The effects of different crossing-linking conditions of genipin on type I collagen scaffolds: an in vitro evaluation

Computational and Experimental Characterization of Aligned Collagen across Varied Crosslinking Degrees

Collagen-based scaffolds have been widely used in tissue engineering. The alignment of collagen fibers and the degree of crosslinking in engineering tissue scaffolds significantly affect cell activity and scaffold stability. Changes in microarchitecture and crosslinking degree also impact the mechanical properties of collagen scaffolds. A clear understanding of the effects of collagen alignment and crosslinking degrees can help properly control these critical parameters for fabricating collagen scaffolds with desired mechanical properties. In this study, combined uniaxial mechanical testing and finite element method (FEM) were used to quantify the effects of fiber alignment and crosslinking degree on the mechanical properties of collagen threads. We have fabricated electrochemically aligned collagen (ELAC) and compared it with randomly distributed collagen at varying crosslinking degrees, which depend on genipin concentrations of 0.1% or 2% for crosslinking durations of 1, 4, and 24 h. Our results indicate that aligned collagen fibers and higher crosslinking degree contribute to a larger Young's modulus. Specifically, aligned fiber structure, compared to random collagen, significantly increases Young's modulus by 112.7% at a 25% crosslinking degree (0.1% (4 h), i.e., 0.1% genipin concentration with a crosslinking duration of 4 h). Moreover, the ELAC Young's modulus increased by 90.3% as the crosslinking degree doubled by changing the genipin concentration from 0.1% to 2% with the same 4 h crosslinking duration. Furthermore, verified computational models can predict mechanical properties based on specific crosslinking degrees and fiber alignments, which facilitate the controlled fabrication of collagen threads. This combined experimental and computational approach provides a systematic understanding of the interplay among fiber alignment, crosslinking parameters, and mechanical performance of collagen scaffolds. This work will enable the precise fabrication of collagen threads for desired tissue engineering performance, potentially advancing tissue engineering applications.

Effect of final irrigation protocols with chitosan nanoparticle and genipin on dentine against collagenase degradation: An ex-vivo study

AIM

Endodontic irrigants may affect the mechanical and chemical properties of dentine. This study evaluated the effects of various final irrigation protocols including the use of chitosan nanoparticle (CSnp) and cross-linking with genipin on the (1) mechanical and (2) chemical properties of dentine against enzymatic degradation.

METHODOLOGY

CSnp was synthesized and characterized considering physiochemical parameters and stability. The root canals of 90 single-rooted teeth were prepared and irrigated with NaOCl. Dentine discs were obtained and divided into groups according to the following irrigation protocols: Group NaOCl+EDTA, Group NaOCl+CSnp, Group NaOCl+EDTA+CSnp, Group NaOCl+CSnp+Genipin, Group NaOCl+EDTA+CSnp+Genipin and Group distilled water. (1) Mechanical changes were determined by microhardness analysis using Vickers-tester. (2) Chemical changes were determined by evaluating molecular and elemental compositions of dentine using Fourier transform infrared spectroscopy (FTIR) analysis and scanning electron microscope (SEM)/energy dispersive X-ray spectroscopy (EDS) analysis, respectively. All analyses were repeated after the discs were kept in collagenase for 24 h. Data were analysed with repeated measures analysis of variance and Bonferroni correction for microhardness analysis, and Kruskal-Wallis and Wilcoxon tests for FTIR and SEM/EDS analyses (p = .05).

RESULTS

(1) Collagenase application did not have a negative effect on microhardness only in Group NaOCl+EDTA+CSnp+Genipin when compared with the post-irrigation values (p > .05). Post-collagenase microhardness of Group NaOCl+EDTA+CSnp and Group NaOCl+CSnp+Genipin was similar to the initial microhardness (p > .05). (2) After collagenase, Amide III/PO 4 3 - ratio presented no change in Group NaOCl+EDTA+CSnp, Group NaOCl+CSnp+Genipin and Group NaOCl+EDTA+CSnp+Genipin (p > .05), while decreased in other groups (p < .05). Collagenase did not affectCO 3 2 - /PO 4 3 - ratio in the groups (p > .05). There were no changes in the groups in terms of elemental level before and after collagenase application (p > .05).

CONCLUSIONS

CSnp and genipin positively affected the microhardness and molecular composition of dentine. This effect was more pronounced when CSnp was used after EDTA.

Ultra-Rapid and Specific Gelation of Collagen Molecules for Transparent and Tough Gels by Transition Metal Complexation

Collagen is the most abundant protein in the human body and one of the main components of stromal tissues in tumors which have a high elastic modulus of over 50 kPa. Although collagen has been widely used as a cell culture scaffold for cancer cells, there have been limitations when attempting to fabricate a tough collagen gel with cells like a cancer stroma. Here, rapid gelation of a collagen solution within a few minutes by transition metal complexation is demonstrated. Type I collagen solution at neutral pH shows rapid gelation with a transparency of 81% and a high modulus of 1,781 kPa by mixing with K2 PtCl4 solution within 3 min. Other transition metal ions also show the same rapid gelation, but not basic metal ions. Interestingly, although type I to IV collagen molecules show rapid gelation, other extracellular matrices  do not exhibit this phenomenon. Live imaging of colon cancer organoids in 3D culture indicates a collective migration property with modulating high elastic modulus, suggesting activation for metastasis progress. This technology will be useful as a new class of 3D culture for cells and organoids due to its facility for deep-live observation and mechanical stiffness adjustment.

Removal of collagen three-dimensional scaffold bubbles utilizing a vacuum suction technique

The process of generating type I/II collagen scaffolds is fraught with bubble formation, which can interfere with the three-dimensional structure of the scaffold. Herein, we applied low-temperature vacuum freeze-drying to remove mixed air bubbles under negative pressure. Type I and II rubber sponges were acid-solubilized via acid lysis and enzymolysis. Thereafter, vacuum negative pressure was applied to remove bubbles, and the cover glass press method was applied to shape the type I/II original scaffold. Vacuum negative pressure was applied for a second time to remove any residual bubbles. Subsequent application of carbamide/N-hydroxysuccinimide cross-linked the scaffold. The traditional method was used as the control group. The structure and number of residual bubbles and pore sizes of the two scaffolds were compared. Based on the relationship between the pressure and the number of residual bubbles, a curve was created, and the time of ice formation was calculated. The bubble content of the experimental group was significantly lower than that of the control group (P < 0.05). The pore diameter of the type I/II collagen scaffold was higher in the experimental group than in the control group. The time of icing effect of type I and II collagen solution was 136.54 ± 5.26 and 144.40 ± 6.45 s, respectively. The experimental scaffold had a more regular structure with actively proliferating chondrocytes that possessed adherent pseudopodia. The findings indicated that the vacuum negative pressure method did not affect the physical or chemical properties of collagen, and these scaffolds exhibited good biocompatibility with chondrocytes.

Collagen-Based Biomimetic Systems to Study the Biophysical Tumour Microenvironment

The extracellular matrix (ECM) is a pericellular network of proteins and other molecules that provides mechanical support to organs and tissues. ECM biophysical properties such as topography, elasticity and porosity strongly influence cell proliferation, differentiation and migration. The cell's perception of the biophysical microenvironment (mechanosensing) leads to altered gene expression or contractility status (mechanotransduction). Mechanosensing and mechanotransduction have profound implications in both tissue homeostasis and cancer. Many solid tumours are surrounded by a dense and aberrant ECM that disturbs normal cell functions and makes certain areas of the tumour inaccessible to therapeutic drugs. Understanding the cell-ECM interplay may therefore lead to novel and more effective therapies. Controllable and reproducible cell culturing systems mimicking the ECM enable detailed investigation of mechanosensing and mechanotransduction pathways. Here, we discuss ECM biomimetic systems. Mainly focusing on collagen, we compare and contrast structural and molecular complexity as well as biophysical properties of simple 2D substrates, 3D fibrillar collagen gels, cell-derived matrices and complex decellularized organs. Finally, we emphasize how the integration of advanced methodologies and computational methods with collagen-based biomimetics will improve the design of novel therapies aimed at targeting the biophysical and mechanical features of the tumour ECM to increase therapy efficacy.

Three-dimensional tailor-made collagen-like proteins hydrogel for tissue engineering applications

Despite the potential tunable properties of blank slate collagen-like proteins (CLP), an alternative to animal-originated collagen, assembling them into a stable 3D hydrogel to mimic extracellular matrix is a challenge. To address this constraint, the CLP (without hydroxyproline, CLPpro) and its variants encoding functional unnatural amino acids such as hydroxyproline (CLPhyp) and 3,4-dihydroxyphenylalanine (CLPdopa) were generated through genetic code engineering for 3D hydrogel development. The CLPhyp and CLPdopa were chosen to enhance the intermolecular hydrogen bond interaction through additional hydroxyl moiety and thereby facilitate the self-assembly into a fibrillar network of the hydrogel. Hydrogelation was induced through genipin as a cross-linker, enabling intermolecular cross-linking to form a hydrogel. Spectroscopic and rheological analyses confirmed that CLPpro and its variants maintained native triple-helical structure, which is necessary for its function, and viscoelastic nature of the hydrogels, respectively. Unlike CLPpro, the varients (CLPhyp and CLPdopa) increased pore size formation in the hydrogel scaffold, facilitating 3T3 fibroblast cell interactions. DSC analysis indicated that the stability of the hydrogels got increased upon the genetic incorporation of hydroxyproline (CLPhyp) and dopa (CLPdopa) in CLPpro. In addition, CLPdopa hydrogel was found to be relatively stable against collagenase enzyme compared to CLPpro and CLPhyp. It is the first report on 3D biocompatible hydrogel preparation by tailoring CLP sequence with non-natural amino acids. These next-generation tunable CLP hydrogels open a new venue to design synthetic protein-based biocompatible 3D biomaterials for tissue engineering applications.

Extracellular Matrix Stiffness in Lung Health and Disease

The extracellular matrix (ECM) provides structural support and imparts a wide variety of environmental cues to cells. In the past decade, a growing body of work revealed that the mechanical properties of the ECM, commonly known as matrix stiffness, regulate the fundamental cellular processes of the lung. There is growing appreciation that mechanical interplays between cells and associated ECM are essential to maintain lung homeostasis. Dysregulation of ECM-derived mechanical signaling via altered mechanosensing and mechanotransduction pathways is associated with many common lung diseases. Matrix stiffening is a hallmark of lung fibrosis. The stiffened ECM is not merely a sequelae of lung fibrosis but can actively drive the progression of fibrotic lung disease. In this article, we provide a comprehensive view on the role of matrix stiffness in lung health and disease. We begin by summarizing the effects of matrix stiffness on the function and behavior of various lung cell types and on regulation of biomolecule activity and key physiological processes, including host immune response and cellular metabolism. We discuss the potential mechanisms by which cells probe matrix stiffness and convert mechanical signals to regulate gene expression. We highlight the factors that govern matrix stiffness and outline the role of matrix stiffness in lung development and the pathogenesis of pulmonary fibrosis, pulmonary hypertension, asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. We envision targeting of deleterious matrix mechanical cues for treatment of fibrotic lung disease. Advances in technologies for matrix stiffness measurements and design of stiffness-tunable matrix substrates are also explored. © 2022 American Physiological Society. Compr Physiol 12:3523-3558, 2022.

Mechanically Tunable Extracellular Matrix of Genipin Crosslinked Collagen and Its Effect on Endothelial Function

Mechanical rigidity of a matrix, to which cells adhere, plays a significant role in regulating phenotypic cellular behaviors such as spreading and junction formation because vascular cells sense and respond to changes in their mechanical environment. Controlling mechanical properties of extracellular matrix by using a crosslinker is important for cell and tissue mechanobiology. In this paper, we explored genipin, a natural plant extract, to crosslink collagen-I in order to enhance mechanical properties with low cytotoxicity. We characterized the effects of genipin concentration on the mechanical properties, color change, degradation, structure, cell viability, and endothelial function such as transendothelial electrical resistance (TEER). Through the analysis of both material properties and endothelial response, it was found that genipin-based glycation caused an increase in viscoelastic moduli in collagen hydrogels, as well as increased fiber density in their structural morphology. Endothelial cells were found to form better barriers, express higher levels of tight junction proteins, and exhibit better adhesion on stiffer matrices.

Structural and Biochemical Changes in Pericardium upon Genipin Cross-Linking Investigated Using Nondestructive and Label-Free Imaging Techniques

Tissue cross-linking represents an important and often used technique to enhance the mechanical properties of biomaterials. For the first time, we investigated biochemical and structural properties of genipin (GE) cross-linked equine pericardium (EP) using optical imaging techniques in tandem with quantitative atomic force microscopy (AFM). EP was cross-linked with GE at 37 °C, and its biochemical and biomechanical properties were observed at various time points up to 24 h. GE cross-linked EP was monitored by the normalized ratio between its second-harmonic generation (SHG) and two-photon autofluorescence emissions and remained unchanged for untreated EP; however, a decreasing ratio due to depleted SHG and elevated autofluorescence and a fluorescence band at 625 nm were found for GE cross-linked EP. The mean autofluorescence lifetime of GE cross-linked EP also decreased. The biochemical signature of GE cross-linker and shift in collagen bands were detected and quantified using shifted excitation Raman difference spectroscopy as an innovative approach for tackling artifacts with high fluorescence backgrounds. AFM images indicated a higher and increasing Young's modulus correlated with cross-linking, as well as collagen structural changes in GE cross-linked EP, qualitatively explaining the observed decrease in the second-harmonic signal. In conclusion, we obtained detailed information about the biochemical, structural, and biomechanical effects of GE cross-linked EP using a unique combination of optical and force microscopy techniques in a nondestructive and label-free manner.

Gelation Methods to Assemble Fibrous Proteins

Gelation is an efficient way to fabricate fibrous protein materials. Briefly, it is an aggregation process where protein molecules assembly from a random structure into an organized structure such as nanofibrillar networks. According to their mechanisms, the fibrous proteins gelation can be classified into physical gelation and chemical gelation. The physical gelation is formed by the conformational transformation of fibroin proteins, which can be triggered by temperature, concentration, pH, or shear force. On the other hand, the chemical gelation is to cross-link fibrous proteins through chemical and/or enzymatic reactions. In this chapter, we summarize the protocols for preparing fibrous protein hydrogels, including both physical and chemical methods. The mechanisms of these gelation methods are also highlighted.

Translation complex stabilization on messenger RNA and footprint profiling to study the RNA responses and dynamics of protein biosynthesis in the cells

During protein biosynthesis, ribosomes bind to messenger (m)RNA, locate its protein-coding information, and translate the nucleotide triplets sequentially as codons into the corresponding sequence of amino acids, forming proteins. Non-coding mRNA features, such as 5' and 3' untranslated regions (UTRs), start sites or stop codons of different efficiency, stretches of slower or faster code and nascent polypeptide interactions can alter the translation rates transcript-wise. Most of the homeostatic and signal response pathways of the cells converge on individual mRNA control, as well as alter the global translation output. Among the multitude of approaches to study translational control, one of the most powerful is to infer the locations of translational complexes on mRNA based on the mRNA fragments protected by these complexes from endonucleolytic hydrolysis, or footprints. Translation complex profiling by high-throughput sequencing of the footprints allows to quantify the transcript-wise, as well as global, alterations of translation, and uncover the underlying control mechanisms by attributing footprint locations and sizes to different configurations of the translational complexes. The accuracy of all footprint profiling approaches critically depends on the fidelity of footprint generation and many methods have emerged to preserve certain or multiple configurations of the translational complexes, often in challenging biological material. In this review, a systematic summary of approaches to stabilize translational complexes on mRNA for footprinting is presented and major findings are discussed. Future directions of translation footprint profiling are outlined, focusing on the fidelity and accuracy of inference of the native in vivo translation complex distribution on mRNA.

Mimicking the Hierarchical Organization of Natural Collagen: Toward the Development of Ideal Scaffolding Material for Tissue Regeneration

Biological materials found in living organisms, many of which are proteins, feature a complex hierarchical organization. Type I collagen, a fibrous structural protein ubiquitous in the mammalian body, provides a striking example of such a hierarchical material, with peculiar architectural features ranging from the amino acid sequence at the nanoscale (primary structure) up to the assembly of fibrils (quaternary structure) and fibers, with lengths of the order of microns. Collagen plays a dominant role in maintaining the biological and structural integrity of various tissues and organs, such as bone, skin, tendons, blood vessels, and cartilage. Thus, "artificial" collagen-based fibrous assemblies, endowed with appropriate structural properties, represent ideal substrates for the development of devices for tissue engineering applications. In recent years, with the ultimate goal of developing three-dimensional scaffolds with optimal bioactivity able to promote both regeneration and functional recovery of a damaged tissue, numerous studies focused on the capability to finely modulate the scaffold architecture at the microscale and the nanoscale in order to closely mimic the hierarchical features of the extracellular matrix and, in particular, the natural patterning of collagen. All of these studies clearly show that the accurate characterization of the collagen structure at the submolecular and supramolecular levels is pivotal to the understanding of the relationships between the nanostructural/microstructural properties of the fabricated scaffold and its macroscopic performance. Several studies also demonstrate that the selected processing, including any crosslinking and/or sterilization treatments, can strongly affect the architecture of collagen at various length scales. The aim of this review is to highlight the most recent findings on the development of collagen-based scaffolds with optimized properties for tissue engineering. The optimization of the scaffolds is particularly related to the modulation of the collagen architecture, which, in turn, impacts on the achieved bioactivity.

Histological, Biomechanical, and Biological Properties of Genipin-Crosslinked Decellularized Peripheral Nerves

Acellular nerve allografts (ANGs) represent a promising alternative in nerve repair. Our aim is to improve the structural and biomechanical properties of biocompatible Sondell (SD) and Roosens (RS) based ANGs using genipin (GP) as a crosslinker agent ex vivo. The impact of two concentrations of GP (0.10% and 0.25%) on Wistar rat sciatic nerve-derived ANGs was assessed at the histological, biomechanical, and biocompatibility levels. Histology confirmed the differences between SD and RS procedures, but not remarkable changes were induced by GP, which helped to preserve the nerve histological pattern. Tensile test revealed that GP enhanced the biomechanical properties of SD and RS ANGs, being the crosslinked RS ANGs more comparable to the native nerves used as control. The evaluation of the ANGs biocompatibility conducted with adipose-derived mesenchymal stem cells cultured within the ANGs confirmed a high degree of biocompatibility in all ANGs, especially in RS and RS-GP 0.10% ANGs. Finally, this study demonstrates that the use of GP could be an efficient alternative to improve the biomechanical properties of ANGs with a slight impact on the biocompatibility and histological pattern. For these reasons, we hypothesize that our novel crosslinked ANGs could be a suitable alternative for future in vivo preclinical studies.

Best of Both Hydrogel Worlds: Harnessing Bioactivity and Tunability by Incorporating Glycosaminoglycans in Collagen Hydrogels

Collagen, the most abundant protein in mammals, has garnered the interest of scientists for over 50 years. Its ubiquitous presence in all body tissues combined with its excellent biocompatibility has led scientists to study its potential as a biomaterial for a wide variety of biomedical applications with a high degree of success and widespread clinical approval. More recently, in order to increase their tunability and applicability, collagen hydrogels have frequently been co-polymerized with other natural and synthetic polymers. Of special significance is the use of bioactive glycosaminoglycans-the carbohydrate-rich polymers of the ECM responsible for regulating tissue homeostasis and cell signaling. This review covers the recent advances in the development of collagen-based hydrogels and collagen-glycosaminoglycan blend hydrogels for biomedical research. We discuss the formulations and shortcomings of using collagen in isolation, and the advantages of incorporating glycosaminoglycans (GAGs) in the hydrogels. We further elaborate on modifications used on these biopolymers for tunability and discuss tissue specific applications. The information presented herein will demonstrate the versatility and highly translational value of using collagen blended with GAGs as hydrogels for biomedical engineering 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.

Development prospects of curable osteoplastic materials in dentistry and maxillofacial surgery

The article presents classification of the thermosetting materials for bone augmentation. The physical, mechanical, biological, and clinical properties of such materials are reviewed. There are two main types of curable osteoplastic materials: bone cements and hydrogels. Compared to hydrogels, bone cements have high strength features, but their biological properties are not ideal and must be improved. Hydrogels are biocompatible and closely mimic the extracellular matrix. They can be used as cytocompatible scaffolds for tissue engineering, as can protein- and nucleic acid-activated structures. Hydrogels may be impregnated with osteoinductors such as proteins and genetic vectors without conformational changes. However, the mechanical properties of hydrogels limit their use for load-bearing bone defects. Thus, improving the strength properties of hydrogels is one of the possible strategies to achieve the basis for an ideal osteoplastic material.

Development and physicochemical analysis of genipin-crosslinked gelatine sponge as a potential resorbable nasal pack

Nasal injury following nasal surgery is an adverse consequence, and prompt treatment should be initiated. Nasal packing, either non-absorbable or absorbable, are commonly used after nasal surgery to prevent bleeding and promote wound healing. In the current study, a novel gelatine sponge crosslinked with genipin was evaluated for suitability to be used as nasal packing and compared to one of the frequently used commercial nasal packing made up of polyurethane. Gelatine at 7% and 10% (w/v) concentration were crosslinked with varying concentrations of genipin, 0.5%, 0.25%, and 0.2% (v/v). The gelatine sponges were further characterised by its water uptake ability, biodegradation, water vapour transmission rate, porosity, contact angle, chemical composition, crosslinking degree, and mechanical properties. The gelatine sponges absorbed five times more water than their dry weight and were degraded within five days. The water vapour transmission rate of the gelatine sponges was 1187.7 ± 430.2 g/(m^-2 day) for 7% gelatine and 779.4 ± 375.5 g/(m^-2 day) for 10% gelatine. Crosslinking of gelatine with genipin resulted in lower porosity and did not affect the wettability of gelatine sponge (contact angle: 95.3 ± 12.1° for 7% gelatine and 88.4 ± 7.2° for 10% gelatine). In terms of biodegradability, the gelatine sponges took 24-48 h to degrade completely. Genipin crosslinking improved the degradation resistance and mechanical strength of gelatine sponge. The physical and chemical properties of the gelatine sponge, i.e. biodegradability and mechanical durability, support its potential as nasal packing.

Tunable bioactivity and mechanics of collagen-based tissue engineering constructs: A comparison of EDC-NHS, genipin and TG2 crosslinkers

Due to its ubiquity and versatility in the human body, collagen is an ideal base material for tissue-engineering constructs. Chemical crosslinking treatments allow precise control of the biochemical and mechanical properties through macromolecular modifications to the structure of collagen. In this work, three key facets regarding the collagen crosslinking process are explored. Firstly, a comparison is drawn between the carbodiimide-succinimide (EDC-NHS) system and two emerging crosslinkers utilising alternate chemistries: genipin and tissue transglutaminase (TG2). By characterising the chemical changes upon treatment, the effect of EDC-NHS, genipin and TG2 crosslinking mechanisms on the chemical structure of collagen, and thus the mechanical properties conferred to the substrate is explored. Secondly, the relative importance of mechanical and biochemical cues on cellular phenomena are investigated, including cell viability, integrin-specific attachment, spreading and proliferation. Here, we observe that for human dermal fibroblasts, long-term, stable proliferation is preconditioned by the availability of suitable binding sites, irrespective of the substrate modulus post-crosslinking. Finally, as seen in the graphical abstract we show that by choosing the appropriate crosslinker chemistries, a materials selection map can be drawn for collagen films, encompassing both a range of tensile modulus and fibroblast proliferation which can be modified independently. Thus, in addition to a range of parameters that can be modified in collagen constructs, we demonstrate a route to obtaining tunable bioactivity and mechanics in collagen constructs is uncovered, that is exclusively driven by the crosslinking process.

Recent Development in the Fabrication of Collagen Scaffolds for Tissue Engineering Applications: A Review

Tissue engineering focuses on developing biological substitutes to restore, maintain or improve tissue functions. The three main components of its application are scaffold, cell and growthstimulating signals. Scaffolds composed of biomaterials mainly function as the structural support for ex vivo cells to attach and proliferate. They also provide physical, mechanical and biochemical cues for the differentiation of cells before transferring to the in vivo site. Collagen has been long used in various clinical applications, including drug delivery. The wide usage of collagen in the clinical field can be attributed to its abundance in nature, biocompatibility, low antigenicity and biodegradability. In addition, the high tensile strength and fibril-forming ability of collagen enable its fabrication into various forms, such as sheet/membrane, sponge, hydrogel, beads, nanofibre and nanoparticle, and as a coating material. The wide option of fabrication technology together with the excellent biological and physicochemical characteristics of collagen has stimulated the use of collagen scaffolds in various tissue engineering applications. This review describes the fabrication methods used to produce various forms of scaffolds used in tissue engineering applications.

Force-dependent extracellular matrix remodeling by early-stage cancer cells alters diffusion and induces carcinoma-associated fibroblasts

It is known cancer cells secrete cytokines inducing normal fibroblasts (NFs) to become carcinoma-associated fibroblasts (CAFs). However, it is not clear how the CAF-promoting cytokines can effectively navigate the dense ECM, a diffusion barrier, in the tumor microenvironment to reach NFs during the early stages of cancer development. In this study, we devised a 3D coculture system to investigate the possible mechanism of CAF induction at early stages of breast cancer. We found that in a force-dependent manner, ECM fibrils are radially aligned relative to the tumor spheroid. The fibril alignment enhances the diffusion of exosomes containing CAF-promoting cytokines towards NFs. Suppression of force generation or ECM remodeling abolishes the enhancement of exosome diffusion and the subsequent CAF induction. In summary, our finding suggests that early-stage, pre-metastatic cancer cells can generate high forces to align the ECM fibrils, thereby enhancing the diffusion of CAF-promoting exosomes to reach the stroma and induce CAFs.

Development and Biocompatibility of Collagen-Based Composites Enriched with Nanoparticles of Strontium Containing Mesoporous Glass

In the last years bone tissue engineering has been increasingly indicated as a valid solution to meet the challenging requirements for a healthy bone regeneration in case of bone loss or fracture. In such a context, bioactive glasses have already proved their great potential in promoting the regeneration of new bone tissue due to their high bioactivity. In addition, their composition and structure enable us to incorporate and subsequently release therapeutic ions such as strontium, enhancing the osteogenic properties of the material. The incorporation of these inorganic systems in polymeric matrices enables the formulation of composite systems suitable for the design of bone scaffolds or delivery platforms. Among the natural polymers, type I collagen represents the main organic phase of bone and thus is a good candidate to develop biomimetic bioactive systems for bone tissue regeneration. However, alongside the specific composition and structure, the key factor in the design of new biosystems is creating a suitable interaction with cells and the host tissue. In this scenario, the presented study aimed at combining nano-sized mesoporous bioactive glasses produced by means of a sol–gel route with type I collagen in order to develop a bioactive hybrid formulation suitable for bone tissue engineering applications. The designed system has been fully characterized in terms of physico-chemical and morphological analyses and the ability to release Sr²⁺ ions has been studied observing a more sustained profile in presence of the collagenous matrix. With the aim to improve the mechanical and thermal stability of the resulting hybrid system, a chemical crosslinking approach using 4-star poly (ethylene glycol) ether tetrasuccinimidyl glutarate (4-StarPEG) has been explored. The biocompatibility of both non-crosslinked and 4-StarPEG crosslinked systems was evaluated by in vitro tests with human osteoblast-like MG-63 cells. Collected results confirmed the high biocompatibility of composites, showing a good viability and adhesion of cells when cultured onto the biomaterial samples.

The effect of different cross-linking conditions of EDC/NHS on type II collagen scaffolds: an in vitro evaluation

The purpose of this paper is to analyze the properties of porcine cartilage type II collagen scaffolds crosslinked with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxy-succinamide (EDC/NHS) under different conditions. The porous EDC/NHS-crosslinked scaffolds were obtained through a two-step freeze-drying process. To determine the optimal crosslinking condition, we used different solvents and various crosslinking temperatures to prepare the scaffolds. Three crosslinking solutions were prepared with different solvents, photographs were taken with a flash in the darkroom, and light transmission was observed. Type II collagen was crosslinked on a horizontal shaker at a speed of 60 r/min according to the above grouping conditions, and then the structural change of the scaffold in each group was observed. To investigate the swelling ratio and the in vitro degradation of the collagen scaffold, tests were also carried out by immersion of the scaffolds in a PBS solution and digestion in type II collagenase, respectively. The influence of the scaffolds on the proliferation of chondrocytes was assessed by the methyl thiazolyl tetrazolium colorimetric assay. The morphology of the crosslinked scaffolds cocultured with chondrocytes was characterized by a scanning electron microscope. The results proved that 75% alcohol and a crosslinking temperature of 37 °C are recommended. Collagen fibrils are more densely packed after crosslinking with EDC/NHS and have a more uniform structure than that of noncrosslinked ones. The EDC-crosslinked scaffolds possessed excellent mechanical property and biocompatibility.

Self-assembly of collagen bundles and enhanced piezoelectricity induced by chemical crosslinking

The piezoelectricity of collagen is purported to be linked to many biological processes including bone formation and wound healing. Although the piezoelectricity of tissue-derived collagen has been documented across the length scales, little work has been undertaken to characterise the local electromechanical properties of processed collagen, which is used as a base for tissue-engineering implants. In this work, three chemically distinct treatments used to form structurally and mechanically stable scaffolds-EDC-NHS, genipin and tissue transglutaminase-are investigated for their effect on collagen piezolectricity. Crosslinking with EDC-NHS is noted to produce a distinct self-assembly of the fibres into bundles roughly 300 nm in width regardless of the collagen origin. These fibre bundles also show a localised piezoelectric response, with enhanced vertical piezoelectricity of collagen. Such topographical features are not observed with the other two chemical treatments, although the shear piezoelectric response is significantly enhanced upon crosslinking. These observations are reconciled by a proposed effect of the crosslinking mechanisms on the molecular and nanostructure of collagen. These results highlight the ability to modify the electromechanical properties of collagen using chemical crosslinking methods.

Modifying collagen with alendronate sodium for bone regeneration applications

Phosphorylated materials are attractive candidates for bone regeneration because they may facilitate the construction of a phosphorylated bone extracellular matrix (ECM) to build a beneficial environment for bone formation. Here, we designed and synthesized a new phosphorylated material, collagen type I phosphorylated with alendronate sodium (Col-Aln), based on the biodegradable osteoconductive collagen backbone. Col-Aln can distinctly accelerate in vitro mineralization in simulated body fluid. Col-Aln showed good biocompatibility with bone marrow mesenchymal stem cells (BMSCs) and promoted their adhesion as well as the osteogenic differentiation of BMSCs more effectively than did pure collagen. Furthermore, collagen and Col-Aln scaffolds implanted into a critical-sized rat cranial defect for 4 and 8 weeks were shown to degrade in vivo and helped to facilitate bone growth in the defect, while the phosphate-containing Col-Aln scaffold significantly promoted new bone formation. Col-Aln provides a new strategy to integrate bioactive phosphate molecules via covalent grafting onto biopolymers and has promise for bone regeneration applications.

Efficient covalent bonding with phosphate-containing alendronate prompts the fast mineralization and osteoinduction of the collagen scaffold.

Preparation and evaluation of decellularized porcine carotid arteries cross-linked by genipin: the preliminary results

Decellularized arteries have been considered as promising scaffolds for small-diameter vascular substitutes. However, weakened mechanical properties, immunological rejection and rapid degradation after transplantation still exist after decellularization. Previous studies indicated that genipin cross-linking can solve these problems. Therefore, genipin was selected as the cross-linking agent for the pre-treatment of decellularized arteries in our study. Histological analysis, scanning electron microscopy, mechanical properties analysis and subcutaneous embedding experiment were adopted to investigate the efficiency of decellularization and the effect of genipin cross-linking on improving mechanical, structural and biological properties of decellularized arteries. Decellularization protocols based on three trypsin concentrations were used to prepare decellularized arteries, after decellularization, arteries were cross-linked with genipin. Results showed that 0.5% trypsin was the most efficient concentration to remove cellular components and preserve ECM. However, mechanical properties of 0.5% trypsin decellularized arteries weakened significantly, while genipin cross-linking improved mechanical properties of decellularized arteries to the same level as fresh arteries. After 4 weeks subcutaneous embedding, cross-linked arteries caused the mildest inflammatory response. In conclusion, genipin could be employed as an ideal cross-linking agent to strengthen mechanical properties, enhance the resistance to degradation and reduce the antigenicity of decellularized arteries for small-diameter blood vessel tissue engineering applications.

Evaluation of genipin for stabilization of decellularized porcine cartilage

We speculate that an acellular osteochondral xenograft may be a good alternative to allografts for repair of focal articular cartilage lesions. In order to make a xenograft resistant to enzymatic degradation and to prevent a chronic immune response it may be beneficial to stabilize it through crosslinking. The concept is analogous to treatment of porcine bioprosthetic heart valves with glutaraldehyde. The purpose of this study was to evaluate genipin, a natural crosslinking agent with low cytotoxicity, for stabilization of decellularized cartilage. Porcine articular cartilage discs were decellularized in SDS and nucleases and then crosslinked in genipin. The utility of genipin was determined from its effects on degree of crosslinking, mechanical properties, dimensional stability, enzymatic resistance, and in vitro biocompatibility. Degree of crosslinking, compressive moduli, and collagenase resistance varied over a wide range depending on genipin concentration. The equilibrium compressive modulus could be increased from approximately 50% to more than 120% that of native cartilage, and the time to complete degradation by collagenase could be extended from less than 12 h to more than 15 days. Radial shrinkage of approximately 4% was observed at a genipin concentration of 0.1% wt/vol, and cartilage coefficient of friction against glass increased in a concentration-dependent manner. Autologous chondrocytes displayed little difference in viability or their ability to attach and spread over the surface of genipin-fixed cartilage compared to non-crosslinked cartilage during 6 weeks of culture. These results indicate that genipin may be efficacious for stabilization of a decellularized porcine osteochondral xenograft. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1949-1957, 2017.

Construction and biocompatibility of a thin type I/II collagen composite scaffold

Articular cartilage injury is a common type of damage observed in clinical practice. A matrix-induced autologous chondrocyte implant was developed to repair articular cartilage as an advancement on the autologous chondrocyte implant procedure. Here, we establish a thin double layer of collagen as a novel and effective bioscaffold for the regeneration of cartilaginous lesions. We created a collagen membrane with double layers using a cover slip, a cover slip, and the collagen was then freeze-dried under vacuum. Carbodiimide as a crosslinking agent was used to obtain a relatively stable collagen construction. The thickness of the knee joint cartilage from grown rabbits was measured from a frozen section. Both type I and type II collagens were characterized using Sodium dodecylsulfate/polyacrylamide gel electrophoresis (SDS-PAGE) and ultraviolet absorption peaks. The aperture size of the scaffold was observed using a scanning electron microscope (SEM). The degradation of the scaffolds in vitro was tested through digestion using collagenase solution. The mechanical capacity of the scaffolds was assessed under dynamic compression. The influence of the scaffold on chondrocyte proliferation was assessed using the methyl thiazolyl tetrazolium (MTT) colourimetric assay and scanning electron microscopy. The frozen sections of the rabbit femoral condyle showed that the thickness of the weight-bearing area of the articular cartilage was less than 1 mm. The results of the SDS-PAGE and ultraviolet absorption peaks of the collagens were in agreement with the standard photographs in the references. SEM showed that the aperture size of the cross-linked scaffold was 82.14 ± 15.70 μm. The in vitro degradation studies indicated that Carbodiimide cross-linking can effectively enhance the biostability of the scaffolds. The Carbodiimide cross-linking protocol resulted in a mean value for the samples that ranged from 8.72 to 15.95 MPa for the compressive strength. The results of the MTT demonstrated that the scaffold had promoted chondrocyte proliferation and SEM observations showed that the scaffold was a good adhesive and growth material for chondrocytes. Thin type I/II collagen composite scaffold can meet the demands of cartilage tissue engineering and have good biocompatibility.

Bioinspired Hydrogels to Engineer Cancer Microenvironments

Recent research has demonstrated that tumor microenvironments play pivotal roles in tumor development and metastasis through various physical, chemical, and biological factors, including extracellular matrix (ECM) composition, matrix remodeling, oxygen tension, pH, cytokines, and matrix stiffness. An emerging trend in cancer research involves the creation of engineered three-dimensional tumor models using bioinspired hydrogels that accurately recapitulate the native tumor microenvironment. With recent advances in materials engineering, many researchers are developing engineered tumor models, which are promising platforms for the study of cancer biology and for screening of therapeutic agents for better clinical outcomes. In this review, we discuss the development and use of polymeric hydrogel materials to engineer native tumor ECMs for cancer research, focusing on emerging technologies in cancer engineering that aim to accelerate clinical outcomes.

* Collagen Cross-Linking: Biophysical, Biochemical, and Biological Response Analysis

Extracted forms of collagen are subjected to chemical cross-linking to enhance their stability. However, traditional cross-linking approaches are associated with toxicity and inflammation. This work investigates the stabilization capacity, cytotoxicity and inflammatory response of collagen scaffolds cross-linked with glutaraldehyde (GTA), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, 4-arm polyethylene glycol (PEG) succinimidyl glutarate (4SP), genipin (GEN), and oleuropein. Although all cross-linking methods reduced free amine groups, variable data were obtained with respect to denaturation temperature, resistance to collagenase digestion, and mechanical properties. With respect to biological analysis, fibroblast cultures showed no significant difference between the treatments. Although direct cultures with human-derived leukemic monocyte cells (THP-1) clearly demonstrated the cytotoxic effect of GTA, THP-1 cultures supplemented with conditioned medium from the various groups showed no significant difference between the treatments. With respect to cytokine profile, no significant difference in secretion of proinflammatory (e.g., interleukin [IL]-1β, IL-8, tumor necrosis factor-α) and anti-inflammatory (e.g., vascular endothelial growth factor) cytokines was observed between the noncross-linked and the 4SP and GEN cross-linked groups, suggesting the suitability of these agents as collagen cross-linkers.

Comparison of natural crosslinking agents for the stabilization of xenogenic articular cartilage

Osteochondral xenografts are potentially inexpensive, widely available alternatives to fresh allografts. However, antigen removal from xenogenic cartilage may damage the extracellular matrix and reduce compressive stiffness. Non-crosslinked xenogenic cartilage may also undergo rapid enzymatic degradation in vivo. We hypothesized that natural crosslinking agents could be used in place of glutaraldehyde to improve the mechanical properties and enzymatic resistance of decellularized cartilage. This study compared the effects of genipin (GNP), proanthocyanidin (PA), and epigallocatechin gallate (EGCG), on the physical and mechanical properties of decellularized porcine cartilage. Glutaraldehyde (GA) served as a positive control. Porcine articular cartilage discs were decellularized in 2% sodium dodecyl sulfate and DNase I followed by fixation in 0.25% GNP, 0.25% PA, 0.25% EGCG, or 2.5% GA. Decellularization decreased DNA by 15% and GAG by 35%. For natural crosslinkers, the average degree of crosslinking ranged from approximately 50% (EGCG) to 78% (GNP), as compared to 83% for the GA control. Among the natural crosslinkers, only GNP significantly affected the disc diameter, and shrinkage was under 2%. GA fixation had no significant effect on disc diameter. Decellularization decreased aggregate modulus; GA and GNP, but not EGCG and PA, were able to restore it to its original level. GNP, PA, and GA conferred a similar, almost complete resistance to collagenase degradation. EGCG also conferred substantial resistance but to a lesser degree. Overall, the data support our hypothesis and suggest that natural crosslinkers may be suitable alternatives to glutaraldehyde for stabilization of decellularized cartilage. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1037-1046, 2016.

The effects of different cross-linking conditions on collagen-based nanocomposite scaffolds-an in vitro evaluation using mesenchymal stem cells

Nanocomposite scaffolds which aimed to imitate a bone extracellular matrix were prepared for bone surgery applications. The scaffolds consisted of polylactide electrospun nano/sub-micron fibres, a natural collagen matrix supplemented with sodium hyaluronate and natural calcium phosphate nano-particles (bioapatite). The mechanical properties of the scaffolds were improved by means of three different cross-linking agents: N-(3-dimethylamino propyl)-N'-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide in an ethanol solution (EDC/NHS/EtOH), EDC/NHS in a phosphate buffer saline solution (EDC/NHS/PBS) and genipin. The effect of the various cross-linking conditions on the pore size, structure and mechanical properties of the scaffolds were subsequently studied. In addition, the mass loss, the swelling ratio and the pH of the scaffolds were determined following their immersion in a cell culture medium. Furthermore, the metabolic activity of human mesenchymal stem cells (hMSCs) cultivated in scaffold infusions for 2 and 7 days was assessed. Finally, studies were conducted of cell adhesion, proliferation and penetration into the scaffolds. With regard to the structural stability of the tested scaffolds, it was determined that EDC/NHS/PBS and genipin formed the most effectively cross-linked materials. Moreover, it was discovered that the genipin cross-linked scaffold also provided the best conditions for hMSC cultivation. In addition, the infusions from all the scaffolds were found to be non-cytotoxic. Thus, the genipin and EDC/NHS/PBS cross-linked scaffolds can be considered to be promising biomaterials for further in vivo testing and bone surgery applications.

Wound Healing: Biologics, Skin Substitutes, Biomembranes and Scaffolds

This review will explore the latest advancements spanning several facets of wound healing, including biologics, skin substitutes, biomembranes and scaffolds.