Cell immobilization in gelatin–hydroxyphenylpropionic acid hydrogel fibers

Long-Term Subconjunctival Delivery of Brimonidine Tartrate for Glaucoma Treatment Using a Microspheres/Carrier System

Core-shell polymer microspheres with poly(d,l-lactic-co-glycolic acid) core and poly(l-lactic acid) (PLLA) shell are developed for the long-term subconjunctival release of brimonidine tartrate (BT) in order to reduce intraocular pressure (IOP) in the treatment of glaucoma. The PLLA-rich shell acts as a diffusion barrier, enabling linear release of BT over an extended period of 40 d. The microspheres are encased in a porous non-degradable methacrylate-based carrier for ease of subconjunctival implantation in a glaucoma-induced rabbit model. In vivo release of BT from the microspheres/carrier system has enabled a significant, immediate IOP reduction of 20 mmHg, which is sustained for 55 d. Long-term IOP reduction may be maintained by periodic replacement of the microspheres/carrier system.

Novel control of gel fraction and enhancement of bonding strength for constructing 3D architecture of tissue engineering scaffold with alginate tubular fiber

Alginate tubular fiber has been successfully prepared via coaxial fluid crosslink mode, which is potentially used for the construction of vascularized tissue engineering scaffolds (VTES). However, its elastic and smooth surface is negative for the adhesion of fibers. In this study, the gel fractions were controlled in a novel way of two-step crosslink process in order to meet the needs of each processing link. Based on such consideration, an appropriate formulation was selected to direct write single fiber, which ensured the tubular structure with enough gel portion as well as adhesion between fibers with the reserved sol. Finally, the integrity of the scaffolds had a further development within the 2nd crosslink bath process, which would help to solve the question of poor shear resistance for hydrogel scaffolds.

Curcumin cross-linked collagen aerogels with controlled anti-proteolytic and pro-angiogenic efficacy

This paper elucidates the development of a curcumin cross-linked collagen aerogel system with controlled anti-proteolytic activity and pro-angiogenic efficacy. The results of this study showed that in situ cross-linking of curcumin with collagen leads to the development of aerogels with enhanced physical and mechanical properties. The integrity of collagen after cross-linking with curcumin was studied via FTIR spectroscopy. The results confirmed that the cross-linking with curcumin did not induce any structural changes in the collagen. The curcumin cross-linked collagen aerogels exhibited potent anti-proteolytic and anti-microbial activity. Scanning electron and atomic force microscopic analysis of curcumin cross-linked collagen aerogels showed a 3D microstructure that enhanced the adhesion and proliferation of cells. The highly organized geometry of collagen-curcumin aerogels enhanced the permeability and water-retaining ability required for the diffusion of nutrients that aid cellular growth. The pro-angiogenic properties of collagen-curcumin aerogels were ascribed to the cumulative effect of the nutraceutical and the collagen molecule, which augmented the restoration of damaged tissue. Further, these aerogels exhibited controlled anti-proteolytic activity, which makes them suitable 3D scaffolds for biomedical applications. This study provides scope for the development of biocompatible and bioresorbable collagen aerogel systems that use a nutraceutical as a cross-linker for biomedical applications.

Tumor Growth Suppression Induced by Biomimetic Silk Fibroin Hydrogels

Protein-based hydrogels with distinct conformations which enable encapsulation or differentiation of cells are of great interest in 3D cancer research models. Conformational changes may cause macroscopic shifts in the hydrogels, allowing for its use as biosensors and drug carriers. In depth knowledge on how 3D conformational changes in proteins may affect cell fate and tumor formation is required. Thus, this study reports an enzymatically crosslinked silk fibroin (SF) hydrogel system that can undergo intrinsic conformation changes from random coil to β-sheet conformation. In random coil status, the SF hydrogels are transparent, elastic, and present ionic strength and pH stimuli-responses. The random coil hydrogels become β-sheet conformation after 10 days in vitro incubation and 14 days in vivo subcutaneous implantation in rat. When encapsulated with ATDC-5 cells, the random coil SF hydrogel promotes cell survival up to 7 days, whereas the subsequent β-sheet transition induces cell apoptosis in vitro. HeLa cells are further incorporated in SF hydrogels and the constructs are investigated in vitro and in an in vivo chick chorioallantoic membrane model for tumor formation. In vivo, Angiogenesis and tumor formation are suppressed in SF hydrogels. Therefore, these hydrogels provide new insights for cancer research and uses of biomaterials.

Simple Spinning of Heterogeneous Hollow Microfibers on Chip

A novel and simple chip-based microfluidic strategy is proposed for continuously controlled spinning of desirable hollow microfibers. These fabricated fiber-shaped materials exhibit extraordinary morphological and structural complexity, as well as a heterogeneous composition. The resulting specific hollow microfibers have potential applications in numerous chemical and biomedical fields.

Physically entrapped gelatin in polyethylene glycol scaffolds for three-dimensional chondrocyte culture

Developing tissue-engineered constructs for clinical use must satisfy the fundamental biologic parameters of biocompatibility, cell adhesiveness, and biodegradability. Physical entrapment of bioactive agents into synthetic polymers, as three-dimensional scaffolds, holds great promise for cell culture applications. Here, in an attempt to elucidate the effects of physical interlocking of natural and synthetic gel networks on cell responses within three-dimensional microenvironments, gelatin (of different concentrations) was physically incorporated into macroporous polyethylene glycol (PEG) hydrogels to fabricate PEG-GEL1 (10:1, PEG:gelatin) and PEG-GEL5 (10:5, PEG:gelatin). The effect of the physically entrapped gelatin on primary chondrocytes was investigated in relation to cell distribution, morphology and viability, proliferation, gene expression, and extracellular matrix accumulation in vitro. Our findings have shown successful incorporation of two different concentrations of gelatin into polyethylene glycol macroporous hydrogels through physical mixing. These physical blends not only enhanced chondrocyte adhesion and proliferation but also boosted gene expression of collagen II and aggrecan after 14 days in culture. Although results demonstrated that gelatin levels dropped sharply in PEG-GEL1 and PEG-GEL5 in the first 7 days, however evidently, after days 14 and 21 gelatin levels in both groups remained substantially unchanged and in turn enhanced glycosaminoglycan formation in vitro. Thus, the modification of polyethylene-glycol-based scaffolds with physically entrapped gelatin may be sufficient for dictating three-dimensional microenvironments for chondrocyte cultures.

Can microfluidics address biomanufacturing challenges in drug/gene/cell therapies?

Translation of any inventions into products requires manufacturing. Development of drug/gene/cell delivery systems will eventually face manufacturing challenges, which require the establishment of standardized processes to produce biologically-relevant products of high quality without incurring prohibitive cost. Microfluidicu technologies present many advantages to improve the quality of drug/gene/cell delivery systems. They also offer the benefits of automation. What remains unclear is whether they can meet the scale-up requirement. In this perspective, we discuss the advantages of microfluidic-assisted synthesis of nanoscale drug/gene delivery systems, formation of microscale drug/cell-encapsulated particles, generation of genetically engineered cells and fabrication of macroscale drug/cell-loaded micro-/nano-fibers. We also highlight the scale-up challenges one would face in adopting microfluidic technologies for the manufacturing of these therapeutic delivery systems.

A reactor-like spinneret used in 3D printing alginate hollow fiber: a numerical study of morphological evolution

In this paper, a reactor-like spinneret is proposed to generate a continuous hollow hydrogel fiber. In order to reliably control the deforming dynamics, the components of the spinneret are standardized in order to ease the online observation of morphological evolution. We found that not only did a co-flow occur in the tubular space, but a relatively large shrinkage of the shell layer at the outlet also occurred. Whereupon a weak coupling of the velocity field and diffusion-reacting co-flow was developed to describe the monitored co-flow morphology and to simulate the intermediate state of the concentration field, as well as to calculate the shrinkage profile with an integral formula. And a critical isogram [G]cri was determined to correspond to the morphological segmental feature, to trigger gelation and shrinkage as a threshold of solubility and the integral upper limit of the shrinkage region. Experimental evidence indicates that: the simulation is able to effectively predict the inner diameter of the hollow fiber; the transient inner diameter of the fiber at the outlet is expanded by approximately 70 μm (co-flow distance = 15 mm) as compared to the initial fluid dynamics value, and that the relative mean error of the simulated inner diameter was less than 8%. The proposed study provides deeper insight into the printing of hollow fibers and other gelling processes which utilize a reactor-like spinneret.

Well-defined and biocompatible hydrogels with toughening and reversible photoresponsive properties

In the present study, novel hydrogels with extremely high strength, reversible photoresponsive and excellent biocompatible properties were prepared. The functional hydrogels were synthesized from a well-defined poly (ethylene glycol) polymer with spiropyran groups at a given position (PEG-SP) via a Cu(i)-catalyst Azide-Alkyne Cycloaddition (CuAAC) reaction. The molecular structures of the sequential intermediates for PEG-SP hydrogel preparation were verified by (1)HNMR and FT-IR. The mechanical property, swelling ratio, compression strength, surface hydrophilicity, and biocompatibility of the resulting hydrogel were characterized. Since spiropyran is pivotal to the switch in hydrophilicity on the hydrogel surface, the swelling ratio of PEG-SP hydrogel under Vis irradiation has a major decrease (155%). Before and after UV light irradiation, the contact angle of the hydrogel has a change of 13.8°. The photoresponsive property of this hydrogel was thus demonstrated, and such a property was also shown to be reversible. The well-defined PEG-SP hydrogel can also sustain a compressive stress of 49.8 MPa without any macro- or micro-damage, indicating its outstanding mechanical performance. Furthermore, it possessed excellent biocompatibility as demonstrated by its performance in an in vivo porcine subcutaneous implantation environment. No inflammation was observed and it got along well with the adjacent tissue. The above features indicate that PEG-SP hydrogels are promising as an implantable matrix for potential applications in biomaterial.

Injectable hydrogel systems crosslinked by horseradish peroxidase

Hydrogels are widely used as reservoirs in drug delivery and scaffolds for tissue engineering. In particular, injectable hydrogel systems, which are formed by physical, chemical, or enzyme-mediated crosslinking reactions in situ, offer the advantages of minimal invasiveness, ease of application, and void-filling property. Examples of these hydrogels are provided in the first part of this paper. In the second part, hydrogels that are formed by the enzymatic activity of horseradish peroxidase (HRP) are highlighted. HRP catalyzes the crosslinking reaction of polymer-phenol conjugates in the presence of hydrogen peroxide (H2O2), resulting in hydrogels with tunable gelation rate and crosslinking density. The catalytic mechanism of the HRP-mediated crosslinking reaction is discussed in detail, and the recent biomedical applications of the HRP-crosslinked hydrogels are described. Lastly, the concerns associated with HRP-mediated crosslinking and the future outlook of HRP-crosslinked hydrogels are addressed.

Biological properties of dialdehyde carboxymethyl cellulose crosslinked gelatin-PEG composite hydrogel fibers for wound dressings

Gelatin-based composite hydrogel fibers were prepared by gel-spinning with PEG6000 as the modifier. Dialdehyde carboxymethyl cellulose (DCMC), as an ideal crosslinking reagent for protein, was used to fix the composite hydrogel fibers. Then the biological properties of the hydrogel fibers for wound dressings were evaluated. The results indicate that the hydrogen bond interactions and CN linkages between gelatin and DCMC can be formed. The addition of DCMC can efficiently improve the mechanical properties, enzymatic stability and blood compatibility of the hydrogel fibers. Crosslinking with DCMC can reduce the degree of swelling of the hydrogel fibers, which is beneficial for hydrogel fibers to avoid undesired reduction in mechanical properties. Moreover, the composite hydrogel fibers present three-dimensional structure, porous networks and low cytotoxicity. The study suggests that DCMC is an effective crosslinking reagent for biomaterials fixation. The developed composite hydrogel fibers can be well-suited for biomedical applications such as wound dressings.

A Biological 3D Printer for the Preparation of Tissue Engineering Micro-Channel Scaffold

The clinical applications of tissue engineering are still limited by the lack of a functional vascular supply in tissue-engineered constructs. In order to improve the pre-vascularization of tissue-engineered scaffold during in vitro culture, in this study, based on three-dimensional (3D) printing technology, using the crosslinking effect of coaxial fluids (sodium alginate and CaCl2) to prepare vessel-like hollow gel fibers, then layer by layer overlapping into 3D scaffold. The biological 3D printing platform was successfully developed and a coaxial nozzle module was introduced to generate a CaCl2-in-Alginate coaxial microfluidic. The inner core diameters of the prepared hollow gel fibers were 220~380 micrometers. In addition, the influence of materials concentration and dispensing rates on hollow fiber dimension were investigated, the cell-encapsulated in the printed hollow fibers was realized and the viability of endothelial cells (ECs) was studied with Laser scanning confocal microscopy (LSCM) and Live-Dead cell staining. The 3D scaffold built by hollow fibers could improve the phenomenon of diffusion constrain and enhance the survival rate of those ECs growing at a greater depth in the construct. This study provides a new theoretical basis for the vascularization of bone scaffold.

Cell-selective encapsulation in hydrogel sheaths via biospecific identification and biochemical cross-linking

Selective encapsulation of a particular cell population from heterogeneous cell populations has potential applications such as studies in cell-to-cell communication, regenerative medicine, and cell therapies. However, there are no versatile methods for realizing this. Here we report a method based on biospecific identification of the target cells through antigen-antibody reaction and subsequent enzymatic hydrogel sheath formation on the cell surfaces by horseradish peroxidase (HRP). Human hepatoma cell line HepG2 cells were selectively encapsulated in alginate-based hydrogel sheath from the mixture with mouse embryo fibroblast-like cell line 10T1/2 fibroblasts using anti-human CD326 antibody conjugated with HRP. The viability of the encapsulated cells was 93%. The cells released at 6 days of the encapsulation by degrading the sheath using alginate lyase grew almost the same as those free from encapsulation. The versatility of the method was confirmed using another antibody, cells, and hydrogel sheath material: Only human vein endothelial cells were encapsulated in gelatin-based hydrogel sheath from the mixture with 10T1/2 fibroblasts using anti-human CD31 antibody conjugated with HRP. The cell-selective encapsulation was also achieved by a system using a primary antibody with a secondary antibody conjugated with HRP.

Multi stimuli-responsive hydrogel microfibers containing magnetite nanoparticles prepared using microcapillary devices

Extensive research efforts have been devoted to the development of hydrogel microfibers for tissue engineering, because the vascular structure is related to the transport of nutrients and oxygen as well as the control of metabolic and mechanical functions in the human body. Even though stimuli-responsive properties would enhance the potential applicability of hydrogel microfibers for artificial tissue architectures, previous studies of their fabrication have not considered changes in the microfibers in response to external stimuli. In this work, we prepared temperature-responsive poly(N-isopropylacrylamide) (PNIPAm) microfibers with controlled shapes and sizes by the in situ photo-polymerization of aqueous monomers loaded in calcium alginate templates generated from microcapillary devices. We found that the shape and size of the hydrogel microfibers could be controlled by adjusting the injection positions of the solutions and varying the diameters of the inner capillary, respectively. We further fabricated light-responsive materials by incorporating photothermal magnetite nanoparticles (MNPs) within the temperature-responsive PNIPAm hydrogel microfibers. Because the MNPs incorporated into the PNIPAm microfibers generated heat upon the absorption of visible light, we could demonstrate volume changes in the microfibers triggered by both visible light irradiation and temperature.

Microfluidic strategies for design and assembly of microfibers and nanofibers with tissue engineering and regenerative medicine applications

Fiber-based materials provide critical capabilities for biomedical applications. Microfluidic fiber fabrication has recently emerged as a very promising route to the synthesis of polymeric fibers at the micro and nanoscale, providing fine control over fiber shape, size, chemical anisotropy, and biological activity. This Progress Report summarizes advanced microfluidic methods for the fabrication of both microscale and nanoscale fibers and illustrates how different methods are enabling new biomedical applications. Microfluidic fabrication methods and resultant materials are explained from the perspective of their microfluidic device principles, including co-flow, cross-flow, and flow-shaping designs. It is then detailed how the microchannel design and flow parameters influence the variety of synthesis chemistries that can be utilized. Finally, the integration of biomaterials and microfluidic strategies is discussed to manufacture unique fiber-based systems, including cell scaffolds, cell encapsulation, and woven tissue matrices.

Liposomal delivery of horseradish peroxidase for thermally triggered injectable hyaluronic acid–tyramine hydrogel scaffolds

A thermally triggered injectable scaffold was developed by utilizing thermoresponsive liposomes to segregate the crosslinking agent from a polymer.

In situ generation of tunable porosity gradients in hydrogel-based scaffolds for microfluidic cell culture

Compared with preformed anisotropic matrices, an anisotropic matrix that allows users to alter its properties and structure in situ after synthesis offers the important advantage of being able to mimic dynamic in vivo microenvironments, such as in tissues undergoing morphogenesis or in wounds undergoing tissue repair. In this study, porous gradients are generated in situ in a hydrogel comprising enzymatically crosslinked gelatin hydroxyphenylpropionic acid (GTN-HPA) conjugate and carboxylmethyl cellulose tyramine (CMC-TYR) conjugate. The GTN-HPA component acts as the backbone of the hydrogel, while CMC-TYR acts as a biocompatible sacrificial polymer. The hydrogel is then used to immobilize HT1080 human fibrosarcoma cells in a microfluidic chamber. After diffusion of a biocompatible cellulase enzyme through the hydrogel in a spatially controlled manner, selective digestion of the CMC component of the hydrogel by the cellulase gives rise to a porosity gradient in situ instead of requiring its formation during hydrogel synthesis as with other methods. The influence of this in situ tunable porosity gradient on the chemotactic response of cancer cells is subsequently studied both in the absence and presence of chemoattractant. This platform illustrates the potential of hydrogel-based microfluidics to mimic the 3D in vivo microenvironment for tissue engineering and diagnostic applications.

Microfluidic spinning of micro- and nano-scale fibers for tissue engineering

Microfluidic technologies have recently been shown to hold significant potential as novel tools for producing micro- and nano-scale structures for a variety of applications in tissue engineering and cell biology. Over the last decade, microfluidic spinning has emerged as an advanced method for fabricating fibers with diverse shapes and sizes without the use of complicated devices or facilities. In this critical review, we describe the current development of microfluidic-based spinning techniques for producing micro- and nano-scale fibers based on different solidification methods, platforms, geometries, or biomaterials. We also highlight the emerging applications of fibers as bottom-up scaffolds such as cell encapsulation or guidance for use in tissue engineering research and clinical practice.

Injectable 3D hydrogel scaffold with tailorable porosity post-implantation

Since rates of tissue growth vary significantly between tissue types, and also between individuals due to differences in age, dietary intake, and lifestyle-related factors, engineering a scaffold system that is appropriate for personalized tissue engineering remains a significant challenge. In this study, a gelatin-hydroxyphenylpropionic acid/carboxylmethylcellulose-tyramine (Gtn-HPA/CMC-Tyr) porous hydrogel system that allows the pore structure of scaffolds to be altered in vivo after implantation is developed. Cross-linking of Gtn-HPA/CMC-Tyr hydrogels via horseradish peroxidase oxidative coupling is examined both in vitro and in vivo. Post-implantation, further alteration of the hydrogel structure is achieved by injecting cellulase enzyme to digest the CMC component of the scaffold; this treatment yields a structure with larger pores and higher porosity than hydrogels without cellulase injection. Using this approach, the pore sizes of scaffolds are altered in vivo from 32-87 μm to 74-181 μm in a user-controled manner. The hydrogel is biocompatible to COS-7 cells and has mechanical properties similar to those of soft tissues. The new hydrogel system developed in this work provides clinicians with the ability to tailor the structure of scaffolds post-implantation depending on the growth rate of a tissue or an individual's recovery rate, and could thus be ideal for personalized tissue engineering.

Use of myocardial matrix in a chitosan-based full-thickness heart patch

A novel cardiac scaffold comprised of decellularized porcine heart matrix was investigated for use as a biodegradable patch with a potential for surgical reconstruction of the right ventricular outflow tract. Powdered heart matrix solution was blended with chitosan and lyophilized to form three-dimensional scaffolds. For this investigation, we examined the influence of different blending ratios of heart matrix to chitosan on porosity and mechanical properties, then gene expression and electrophysiological function of invading neonatal rat ventricular myocytes (NRVM) compared to type-A gelatin/chitosan composite scaffolds. Heart matrix/chitosan-blended hydrogels (1.6 mg/mL heart matrix) had similar porosity (109±34 μm), and elastic modulus (13.2±4.0 kPa) as previously published gelatin/chitosan scaffolds. Heart matrix/chitosan hydrogels maintained>80% viability and had higher NRVM retention (∼1000 cells/mm(2)) than gelatin/chitosan scaffolds. There was a significant increase in α-myosin heavy chain and connexin-43 expression in NRVM cultured on heart matrix/chitosan scaffolds after 14 days compared with gelatin/chitosan scaffolds. Further, heart matrix/chitosan scaffolds had significantly higher conduction velocity (12.6±4.9 cm/s) and contractile stress (0.79±0.13 mN/mm(2)) than gelatin/chitosan scaffolds. In summary, NRVM cultured on heart matrix scaffold showed improvements in contractile and electrophysiological function.

Cell encapsulation via microtechnologies

The encapsulation of living cells in a variety of soft polymers or hydrogels is important, particularly, for the rehabilitation of functional tissues capable of repairing or replacing damaged organs. Cellular encapsulation segregates cells from the surrounding tissue to protect the implanted cell from the recipient's immune system after transplantation. Diverse hydrogel membranes have been popularly used as encapsulating materials and permit the diffusion of gas, nutrients, wastes and therapeutic products smoothly. This review describes a variety of methods that have been developed to achieve cellular encapsulation using microscale platform. Microtechnologies have been adopted to precisely control the encapsulated cell number, size and shape of a cell-laden polymer structure. We provide a brief overview of recent microtechnology-based cell encapsulation methods, with a detailed description of the relevant processes. Finally, we discuss the current challenges and future directions likely to be taken by cell microencapsulation approaches toward tissue engineering and cell therapy applications.

Fibrous hydrogel scaffolds with cells embedded in the fibers as a potential tissue scaffold for skin repair

A novel approach was undertaken to create a potential skin wound dressing. L929 fibroblast cells and alginate solution were simultaneously dispensed into a calcium chloride solution using a three-dimensional plotting system to manufacture a fibrous alginate scaffold with interconnected pores. These cells were then embedded in the alginate hydrogel fibers of the scaffold. A conventional scaffold with cells directly seeded on the fiber surface was used as a control. The encapsulated fibroblasts made using the co-dispensing method distributed homogeneously within the scaffold and showed the delayed formation of large cell aggregates compared to the control. The cells embedded in the hydrogel fibers also deposited more type I collagen in the extracellular matrix and expressed higher levels of fgf11 and fn1 than the control, indicating increased cellular proliferation and attachment. The results indicate that the novel co-dispensing alginate scaffold may promote skin regeneration better than the conventional directly-seeded scaffold.

Interpenetrating networks based on gelatin methacrylamide and PEG formed using concurrent thiol click chemistries for hydrogel tissue engineering scaffolds

The integration of biological extracellular matrix (ECM) components and synthetic materials is a promising pathway to fabricate the next generation of hydrogel-based tissue scaffolds that more accurately emulate the microscale heterogeneity of natural ECM. We report the development of a bio/synthetic interpenetrating network (BioSINx), containing gelatin methacrylamide (GelMA) polymerized within a poly(ethylene glycol) (PEG) framework to form a mechanically robust network capable of supporting both internal cell encapsulation and surface cell adherence. The covalently crosslinked PEG network was formed by thiol-yne coupling, while the bioactive GelMA was integrated using a concurrent thiol-ene coupling reaction. The physical properties (i.e. swelling, modulus) of BioSINx were compared to both PEG networks with physically-incorporated gelatin (BioSINP) and homogenous hydrogels. BioSINx displayed superior physical properties and significantly lower gelatin dissolution. These benefits led to enhanced cytocompatibility for both cell adhesion and encapsulation; furthermore, the increased physical strength provided for the generation of a micro-engineered tissue scaffold. Endothelial cells showed extensive cytoplasmic spreading and the formation of cellular adhesion sites when cultured onto BioSINx; moreover, both encapsulated and adherent cells showed sustained viability and proliferation.

A Fibrin-Based Tissue-Engineered Renal Proximal Tubule for Bioartificial Kidney Devices: Development, Characterization and In Vitro Transport Study

A bioartificial renal proximal tubule is successfully engineered as a first step towards a bioartificial kidney for improved renal substitution therapy. To engineer the tubule, a tunable hollow fiber membrane with an exterior skin layer that provides immunoprotection for the cells from extracapillary blood flow and a coarse inner surface that facilitates a hydrogel coating for cell attachment was embedded in a “lab-on-a-chip” model for the small-scale exploratory testing under flow conditions. Fibrin was coated onto the inner surface of the hollow fiber, and human renal proximal tubule epithelial cells were then seeded. Using this model, we successfully cultured a confluent monolayer, as ascertained by immunofluorescence staining for ZO-1 tight junctions and other proximal tubule markers, scanning electron microscopy, and FITC-inulin recovery studies. Furthermore, the inulin studies, combined with the creatinine and glucose transport profiles, suggested that the confluent monolayer exhibits functional transport capabilities. The novel approaches here may eventually improve current renal substitution technology for renal failure patients.

Manipulation of viscous all-aqueous jets by electrical charging

We demonstrate the manipulation of viscous all-aqueous jets by electrical charging. At sufficiently high voltages, the folding of an uncharged viscous jet is suppressed, and the jet diameter can be adjusted by varying the applied voltage or the fluid flow rates. This inspires new ways to fabricate biocompatible fibers.

A multilayered scaffold of a chitosan and gelatin hydrogel supported by a PCL core for cardiac tissue engineering

A three-dimensional scaffold composed of self-assembled polycaprolactone (PCL) sandwiched in a gelatin-chitosan hydrogel was developed for use as a biodegradable patch with a potential for surgical reconstruction of congenital heart defects. The PCL core provides surgical handling, suturability and high initial tensile strength, while the gelatin-chitosan scaffold allows for cell attachment, with pore size and mechanical properties conducive to cardiomyocyte migration and function. The ultimate tensile stress of the PCL core, made from blends of 10, 46 and 80kDa (Mn) PCL, was controllable in the range of 2-4MPa, with lower average molecular weight PCL blends correlating with lower tensile stress. Blends with lower molecular weight PCL also had faster degradation (controllable from 0% to 7% weight loss in saline over 30 days) and larger pores. PCL scaffolds supporting a gelatin-chitosan emulsion gel showed no significant alteration in tensile stress, strain or tensile modulus. However, the compressive modulus of the composite tissue was similar to that of native tissue (∼15kPa for 50% gelatin and 50% chitosan). Electron microscopy revealed that the gelatin-chitosan gel had a three-dimensional porous structure, with a mean pore diameter of ∼80μm, showed migration of neonatal rat ventricular myocytes (NRVM), maintained NRVM viability for over 7 days, and resulted in spontaneously beating scaffolds. This multi-layered scaffold has sufficient tensile strength and surgical handling for use as a cardiac patch, while allowing migration or pre-loading of cardiac cells in a biomimetic environment to allow for eventual degradation of the patch and incorporation into native tissue.

Novel methodology based on biomimetic superhydrophobic substrates to immobilize cells and proteins in hydrogel spheres for applications in bone regeneration

Cell-based therapies for regenerative medicine have been characterized by the low retention and integration of injected cells into host structures. Cell immobilization in hydrogels for target cell delivery has been developed to circumvent this issue. In this work mesenchymal stem cells isolated from Wistar rats bone marrow (rMSCs) were immobilized in alginate beads fabricated using an innovative approach involving the gellification of the liquid precursor droplets onto biomimetic superhydrophobic surfaces without the need of any precipitation bath. The process occurred in mild conditions preventing the loss of cell viability. Furthermore, fibronectin (FN) was also immobilized inside alginate beads with high efficiency in order to mimic the composition of the extracellular matrix. This process occurred in a very fast way (around 5 min), at room temperature, without aggressive mechanical strengths or particle aggregation. The methodology employed allowed the production of alginate beads exhibiting a homogenous rMSCs and FN distribution. Encapsulated rMSCs remained viable and were released from the alginate for more than 20 days. In vivo assays were also performed, by implanting these particles in a calvarial bone defect to evaluate their potential for bone tissue regeneration. Microcomputed tomography and histological analysis results showed that this hybrid system accelerated bone regeneration process. The methodology employed had a dual role by preventing cell and FN loss and avoiding any contamination of the beads or exchange of molecules with the surrounding environment. In principle, the method used for cell encapsulation could be extended to other systems aimed to be used in tissue regeneration strategies.

Enzymatically cross-linked injectable gelatin gel as osteoblast delivery vehicle

Injectable and degradable hydrogels are potential candidates as cell delivery vehicles for the regeneration of osseous defects. We evaluated the potential of injectable enzymatically cross-linked gelatin gel as an osteoblast delivery vehicle using murine preosteoblast MC3T3-E1 cells. Injectable hydrogels were prepared by enzymatic cross-linking of the phenol derivatives of gelatin (tyramine-modified) in the presence of hydrogen peroxide (H2O2) and horseradish peroxidase. The effect of gelatin concentration on gel morphology and in supporting the adhesion and spreading of encapsulated MC3T3-E1 cells, activation of intercellular signaling in MC3T3-E1 cells by extracellular signal-regulated kinase phosphorylation, β-catenin and Runx2 was evaluated. Both tyramine-modified and unmodified gelatins as well as gelatin gels did not activate intercellular signaling pathways in MC3T3-E1 cells. The encapsulated cells in gelatin gel showed extracellular signal-regulated kinase phosphorylation and active β-catenin expression in the presence of inductive molecules such as insulin and LiCl. The gelatin gels formed from 10 to 25 mg/mL tyramine-modified gelatin supported the adhesion, spreading, and three-dimensional growth of MC3T3-E1 cells. However, the lack of activation of intercellular signaling in the gelatin gel indicates the need to add exogenous bioactive molecules to modulate the osteogenic functions of the encapsulated cells.

Enzymatically cross-linked gelatin-phenol hydrogels with a broader stiffness range for osteogenic differentiation of human mesenchymal stem cells

An injectable hydrogel system, composed of gelatin-hydroxyphenylpropionic acid (Gtn-HPA) conjugates chemically cross-linked by an enzyme-mediated oxidation reaction, has been designed as a biodegradable scaffold for tissue engineering. In light of the role of substrate stiffness on cell differentiation, we herein report a newly improved Gtn hydrogel system with a broader range of stiffness control that uses Gtn-HPA-tyramine (Gtn-HPA-Tyr) conjugates to stimulate the osteogenic differentiation of human mesenchymal stem cells (hMSCs). The Gtn-HPA-Tyr conjugate was successfully synthesized through a further conjugation of Tyr to Gtn-HPA conjugate by means of a general carbodiimide/active ester-mediated coupling reaction. Proton nuclear magnetic resonance and UV-visible measurements showed a higher total phenol content in the Gtn-HPA-Tyr conjugate than that content in the Gtn-HPA conjugate. The Gtn-HPA-Tyr hydrogels were formed by the oxidative coupling of phenol moieties catalyzed by hydrogen peroxide (H(2)O(2)) and horseradish peroxidase (HRP). Rheological studies revealed that a broader range of storage modulus (G') of Gtn-HPA-Tyr hydrogel (600-26,800 Pa) was achieved using different concentrations of H(2)O(2), while the G' of the predecessor Gtn-HPA hydrogels was limited to the range of 1000 to 13,500 Pa. The hMSCs on Gtn-HPA-Tyr hydrogel with G' greater than 20,000 showed significantly up-regulated expressions of osteocalcin and runt-related transcription factor 2 (RUNX2) on both the gene and protein level, with the presence of alkaline phosphatase, and the evidence of calcium accumulation. These studies with the Gtn-HPA-Tyr hydrogel with G' greater than 20,000 collectively suggest the stimulation of the hMSCs into osteogenic differentiation, while these same observations were not found with the Gtn-HPA hydrogel with a G' of 13,500.

Injectable and biodegradable hydrogels: gelation, biodegradation and biomedical applications

Injectable hydrogels with biodegradability have in situ formability which in vitro/in vivo allows an effective and homogeneous encapsulation of drugs/cells, and convenient in vivo surgical operation in a minimally invasive way, causing smaller scar size and less pain for patients. Therefore, they have found a variety of biomedical applications, such as drug delivery, cell encapsulation, and tissue engineering. This critical review systematically summarizes the recent progresses on biodegradable and injectable hydrogels fabricated from natural polymers (chitosan, hyaluronic acid, alginates, gelatin, heparin, chondroitin sulfate, etc.) and biodegradable synthetic polymers (polypeptides, polyesters, polyphosphazenes, etc.). The review includes the novel naturally based hydrogels with high potential for biomedical applications developed in the past five years which integrate the excellent biocompatibility of natural polymers/synthetic polypeptides with structural controllability via chemical modification. The gelation and biodegradation which are two key factors to affect the cell fate or drug delivery are highlighted. A brief outlook on the future of injectable and biodegradable hydrogels is also presented (326 references).

The effect of injectable gelatin-hydroxyphenylpropionic acid hydrogel matrices on the proliferation, migration, differentiation and oxidative stress resistance of adult neural stem cells

Transplanted or endogenous neural stem cells often lack appropriate matrix in cavitary lesions in the central nervous system. In this study, gelatin-hydroxyphenylpropionic acid (Gtn-HPA), which could be enzymatically crosslinked with independent tuning of crosslinking degree and gelation rate, was explored as an injectable hydrogel for adult neural stem cells (aNSCs). The storage modulus of Gtn-HPA could be tuned (449-1717 Pa) to approximate adult brain tissue. Gtn-HPA was cytocompatible with aNSCs (yielding high viability >93%) and promoted aNSC adhesion. Gtn-HPA demonstrated a crosslinking-based approach for preconditioning aNSCs and increased the resistance of aNSCs to oxidative stress, improving their viability from 8-15% to 84% when challenged with 500 μM H(2)O(2). In addition, Gtn-HPA was able to modulate proliferation and migration of aNSCs in relation to the crosslinking degree. Finally, Gtn-HPA exhibited bias for neuronal cells. In mixed differentiation conditions, Gtn-HPA increased the proportion of aNSCs expressing neuronal marker β-tubulin III to a greater extent than that for astrocytic marker glial fibrillary acidic protein, indicating an enhancement in differentiation towards neuronal lineage. Between neuronal and astrocytic differentiation conditions, Gtn-HPA also selected for higher survival in the former. Overall, Gtn-HPA hydrogels are promising injectable matrices for supporting and influencing aNSCs in ways that may be beneficial for brain tissue regeneration after injuries.

A synthetic cartilage extracellular matrix model: hyaluronan and collagen hydrogel relaxivity, impact of macromolecular concentration on dGEMRIC

OBJECTIVE

To develop and characterize the MR properties of a synthetic model for cartilage extra-cellular matrix using hydrogels and to determine the concentration dependence of spin-lattice (T1) and spin-spin (T2) relaxation times of hydrogels and their glycosaminoglycan and collagen components in the presence and absence of gadopentetate dimeglumine (Gd-DTPA) for use in dGEMRIC.

MATERIALS AND METHODS

T1 and T2 measurements were made at 3 Tesla on a range of gelatin (i.e., collagen) and hyaluronan (i.e., glycosaminoglycan) solutions (6.25-100 g/l), alone, together in a composite, and as dityramine-bridged hydrogels. Relaxivity was calculated as a function of macromolecular concentration.

RESULTS

Even at the highest concentrations, gelatin and hyaluronan solutions had T1 and T2 values significantly larger than those reported for cartilage. Only composite hydrogels with gelatin and hyaluronan concentrations naturally found in cartilage resulted in T1 values, but not T2 values, representative of cartilage. Relaxivities were slightly dependent on both hyaluronan concentration (R1 = 0.0027 l g(-1) s(-1); R2 = 0.025 l g(-1) s(-1)) and gelatin concentration (R1 = 0.0032 l g(-1) s(-1); R2 = 0.020 l g(-1) s(-1)) alone and as a composite (R1 = 0.0068 l g(-1) s(-1); R2 = 0.101 l g(-1) s(-1)). Gd-DTPA relaxivities were dependent upon macromolecular concentration and varied by 14-32% (R1 = 4.24 to 5.55 mM(-1) s(-1); R2 = 4.60 to 6.27 mM(-1) s(-1)) over the range of cartilage biochemistry.

CONCLUSIONS

Without the contrast agent, hyaluronan and gelatin, alone or in a composite, have a very small impact on the relaxivities of the model system. The impact on R1 was approximately tenfold less than on R2. In contrast, macromolecular concentrations above 50 g/l significantly impacted Gd-DTPA relaxivity and should be accounted for when measuring the glycosaminoglycan content of cartilage in vivo using dGEMRIC.

Microfluidic fabrication of microengineered hydrogels and their application in tissue engineering

Microfluidic technologies are emerging as an enabling tool for various applications in tissue engineering and cell biology. One emerging use of microfluidic systems is the generation of shape-controlled hydrogels (i.e., microfibers, microparticles, and hydrogel building blocks) for various biological applications. Furthermore, the microfluidic fabrication of cell-laden hydrogels is of great benefit for creating artificial scaffolds. In this paper, we review the current development of microfluidic-based fabrication techniques for the creation of fibers, particles, and cell-laden hydrogels. We also highlight their emerging applications in tissue engineering and regenerative medicine.

3D cell entrapment in crosslinked thiolated gelatin-poly(ethylene glycol) diacrylate hydrogels

The combined use of natural ECM components and synthetic materials offers an attractive alternative to fabricate hydrogel-based tissue engineering scaffolds to study cell-matrix interactions in three-dimensions (3D). A facile method was developed to modify gelatin with cysteine via a bifunctional PEG linker, thus introducing free thiol groups to gelatin chains. A covalently crosslinked gelatin hydrogel was fabricated using thiolated gelatin and poly(ethylene glycol) diacrylate (PEGdA) via thiol-ene reaction. Unmodified gelatin was physically incorporated in a PEGdA-only matrix for comparison. We sought to understand the effect of crosslinking modality on hydrogel physicochemical properties and the impact on 3D cell entrapment. Compared to physically incorporated gelatin hydrogels, covalently crosslinked gelatin hydrogels displayed higher maximum weight swelling ratio (Q(max)), higher water content, significantly lower cumulative gelatin dissolution up to 7 days, and lower gel stiffness. Furthermore, fibroblasts encapsulated within covalently crosslinked gelatin hydrogels showed extensive cytoplasmic spreading and the formation of cellular networks over 28 days. In contrast, fibroblasts encapsulated in the physically incorporated gelatin hydrogels remained spheroidal. Hence, crosslinking ECM protein with synthetic matrix creates a stable scaffold with tunable mechanical properties and with long-term cell anchorage points, thus supporting cell attachment and growth in the 3D environment.

Defining and designing polymers and hydrogels for neural tissue engineering

The use of biomaterials, such as hydrogels, as neural cell delivery devices is becoming more common in areas of research such as stroke, traumatic brain injury, and spinal cord injury. When reviewing the available research there is some ambiguity in the type of materials used and results are often at odds. This review aims to provide the neuroscience community who may not be familiar with fundamental concepts of hydrogel construction, with basic information that would pertain to neural tissue applications, and to describe the use of hydrogels as cell and drug delivery devices. We will illustrate some of the many tunable properties of hydrogels and the importance of these properties in obtaining reliable and consistent results. It is our hope that this review promotes creative ideas for ways that hydrogels could be adapted and employed for the treatment of a broad range of neurological disorders.

Microencapsulating and Banking Living Cells for Cell-Based Medicine

A major challenge to the eventual success of the emerging cell-based medicine such as tissue engineering, regenerative medicine, and cell transplantation is the limited availability of the desired cell sources. This challenge can be addressed by cell microencapsulation to overcome the undesired immune response (i.e., to achieve immunoisolation) so that non-autologous cells can be used to treat human diseases, and by cell/tissue preservation to bank living cells for wide distribution to end users so that they are readily available when needed in the future. This review summarizes the status quo of research in both cell microencapsulation and banking the microencapsulated cells. It is concluded with a brief outlook of future research directions in this important field.

The performance of primary human renal cells in hollow fiber bioreactors for bioartificial kidneys

Bioartificial kidneys (BAKs) containing human primary renal proximal tubule cells (HPTCs) have been applied in clinical trials. The results were encouraging, but also showed that more research is required. Animal cells or cell lines are not suitable for clinical applications, but have been mainly used in studies on BAK development as large numbers of such cells could be easily obtained. It is difficult to predict HPTC performance based on data obtained with other cell types. To enable more extensive studies on HPTCs, we have developed a bioreactor containing single hollow fiber membranes that requires relatively small amounts of cells. Special hollow fiber membranes with the skin layer on the outer surface and consisting of polyethersulfone/polyvinylpyrrolidone were developed. The results suggested that such hollow fiber membranes were more suitable for the bioreactor unit of BAKs than membranes with an inner skin layer. An HPTC-compatible double coating was applied to the insides of the hollow fiber membranes, which sustained the formation of functional epithelia under bioreactor conditions. Nevertheless, the state of differentiation of the primary human cells remained a critical issue and should be further addressed. The bioreactor system described here will facilitate further studies on the relevant human cell type.