Hydrogel based injectable scaffolds for cardiac tissue regeneration

Tissue engineering promises to be an effective strategy that can overcome the lacuna existing in the current pharmacological and interventional therapies and heart transplantation. Heart failure continues to be a major contributor to the morbidity and mortality across the globe. This may be attributed to the limited regeneration capacity after the adult cardiomyocytes are terminally differentiated or injured. Various strategies involving acellular scaffolds, stem cells, and combinations of stem cells, scaffolds and growth factors have been investigated for effective cardiac tissue regeneration. Recently, injectable hydrogels have emerged as a potential candidate among various categories of biomaterials for cardiac tissue regeneration due to improved patient compliance and facile administration via minimal invasive mode that treats complex infarction. This review discusses in detail on the advances made in the field of injectable materials for cardiac tissue engineering highlighting their merits over their preformed counterparts.     

Structure-property evaluation of thermally and chemically gelling injectable hydrogels for tissue engineering

The impact of synthesis and solution formulation parameters on the swelling and mechanical properties of a novel class of thermally and chemically gelling hydrogels combining poly(N-isopropylacrylamide)-based thermogelling macromers containing pendant epoxy rings with polyamidoamine-based hydrophilic and degradable diamine cross-linking macromers was evaluated. Through variation of network hydrophilicity and capacity for chain rearrangement, the often problematic tendency of thermogelling hydrogels to undergo significant syneresis was addressed. The demonstrated ability to tune postformation dimensional stability easily at both the synthesis and formulation stages represents a significant novel contribution toward efforts to utilize poly(N-isopropylacrylamide)-based polymers as injectable biomaterials. Furthermore, the cytocompatibility of the hydrogel system under relevant conditions was established while demonstrating time- and dose-dependent cytotoxicity at high solution osmolality. Such injectable in situ forming degradable hydrogels with tunable water content are promising candidates for many tissue-engineering applications, particularly for cell delivery to promote rapid tissue regeneration in non-load-bearing defects.     

Synthesis and characterization of thermally and chemically gelling injectable hydrogels for tissue engineering

Novel, injectable hydrogels were developed that solidify through a physical and chemical dual-gelation mechanism upon preparation and elevation of temperature to 37 °C. A thermogelling, poly(N-isopropylacrylamide)-based macromer with pendant epoxy rings and a hydrolytically degradable polyamidoamine-based diamine cross-linker were synthesized, characterized, and combined to produce in situ forming hydrogel constructs. Network formation through the epoxy-amine reaction was shown to be rapid and facile, and the progressive incorporation of the hydrophilic polyamidoamine cross-linker into the hydrogel was shown to mitigate the often problematic tendency of thermogelling materials to undergo significant postformation gel syneresis. The results suggest that this novel class of injectable hydrogels may be attractive substrates for tissue engineering applications due to the synthetic versatility of the component materials and beneficial hydrogel gelation kinetics and stability.     

Photocrosslinkable biodegradable responsive hydrogels as drug delivery systems

Recently, controlled release from biocompatible materials has received much attention for biomedical applications. Due to their biocompatibility and biodegradability, glucopyranosides such as dextran appear as promising polymeric materials if one is able to regulate their rheological properties and the encapsulation/release efficiency. In this work graft polymer hydrogels from dextran and N-isopropylacrylamide (NIPAAm) were prepared and characterized.Dextran molecules were modified with 2-isocyanatoethylmethacrylate (IEMA) in order to obtain a polymer with carbon double bonds. Urethane linkages resulted from the reaction between hydroxyl groups (OH) of the dextran and isocyanate groups (NCO) of the IEMA. The obtained polymer was then crosslinked by UV irradiation in the presence of the photoinitiating agent Irgacure 2959 by CIBA^®. The drug Ondansetron^® was entrapped in the final system and its release profile was determined at 25 and 37 °C.The characterization of the materials was accomplished by: ATR-FTIR (Attenuated Total Reflectance-Fourier Transform Infrared) spectroscopy, elemental analysis, lower critical solution temperature (LCST) determination, swelling behaviour evaluation, determination of surface energy by contact angle measurement and drug delivery profile studies.     

Injectable in situ forming biodegradable chitosan-hyaluronic acid based hydrogels for adipose tissue regeneration

An injectable, biodegradable and glucose-responsive hydrogel derived from natural polysaccharide derivatives was synthesized to deliver adipogenic factor of insulin in vitro for adipose tissue engineering. The biodegradable hydrogel based N-succinyl-chitosan (SCS) and aldehyde hyaluronic acid (AHA) with covalently conjugated glucose oxidase and catalase. The gelation is attributed to the Schiff-base reaction between amino and aldehyde groups of SCS and AHA, respectively. The morphologies and compressive modulus of the freeze-dried hydrogels demonstrated that the incorporated insulin and enzymes results in the formation of a tighter network structure in composite hydrogels. The immobilized enzymes triggered conversion of glucose reduces the pH value of the microenvironment, and results in hydrolysis and increasing swelling of the network basing on Schiff-base cross-linking. The pH inside the hydrogel, kept in PBS solution at pH 7.4 and 37°C, linearly dropped from 7.40 to 7.17 during 4 h of initial period, then slowly increased to 7.36 after 24 h. Correspondingly, the swelling ratio increased from 20.8 to 28.6 at 37°C in PBS with 500 mg/dL glucose. In PBS buffer with 500 mg/dL glucose, about 10.8% of insulin was released from the hydrogel after 8 h of incubation while upon observation. The results demonstrated that the adipogenic factor of insulin would be released from this biodegradable hydrogel device into the local microenvironment in a controlled fashion by the swelling of hydrogel network. These preliminary studies indicate that the biodegradable and glucose-responsive hydrogel may have potential uses in adipose tissue engineering applications.     

Thermoreversible hydrogels from RAFT-synthesized BAB triblock copolymers: steps toward biomimetic matrices for tissue regeneration

Narrowly dispersed, temperature-responsive BAB block copolymers capable of forming physical gels under physiological conditions were synthesized via aqueous reversible addition fragmentation chain transfer (RAFT) polymerization. The use of a difunctional trithiocarbonate facilitates the two-step synthesis of BAB copolymers with symmetrical outer blocks. The outer B blocks of the triblock copolymers consist of poly(N-isopropylacrylamide) (PNIPAM) and the inner A block consists of poly(N,N-dimethylacrylamide). The copolymers form reversible physical gels above the phase transition temperature of PNIPAM at concentrations as low as 7.5 wt % copolymer. Mechanical properties similar to collagen, a naturally occurring polypeptide used as a three-dimensional in vitro cell growth scaffold, have been achieved. Herein, we report the mechanical properties of the gels as a function of solvent, polymer concentration, and inner block length. Structural information about the gels was obtained through pulsed field gradient NMR experiments and confocal microscopy.     

Biomimetic approaches to protein and gene delivery for tissue regeneration

Novel therapeutic strategies that promote wound healing seek to mimic the response of the body to wounding, to regenerate rather than repair injured tissues. Many synthetic or natural biomaterials have been developed for this purpose and are used to deliver wound therapeutics in a controlled manner that prevents unwanted and potentially harmful side-effects. Here, we review the natural and synthetic biomaterials that have been developed for protein and gene delivery to enhance tissue regeneration. Particular emphasis is placed on novel biomimetic materials that respond to environmental stimuli or release their cargo according to cellular demand. Engineering biomaterials to release therapeutic agents in response to physiologic signals mimics the natural healing process and can promote faster tissue regeneration and reduce scarring in severe acute or chronic wounds.     

Combined systemic and local delivery of stem cell inducing/recruiting factors for in situ tissue regeneration

Whereas the conventional tissue engineering strategy involves the use of scaffolds combined with appropriate cell types to restore normal functions, the concept of in situ tissue regeneration uses host responses to a target-specific scaffold to mobilize host cells to a site of injury without the need for cell seeding. For this purpose, local delivery of bioactive molecules from scaffolds has been generally used. However, this approach has limited stem cell recruitment into the implants. Thus, we developed a combination of systemic delivery of substance P (SP) and local release of stromal-derived factor-1α (SDF-1α) from an implant. In this study, we examined whether this combined system would significantly enhance recruitment of host stem cells into the implants. Flow cytometry and immunohistochemistry for CD29/CD45, CD146/α-smooth muscle actin, and c-kit demonstrated that this system significantly increased the number of stem cell-like cells within the implants when compared with other systems. In vitro culture of the cells that had infiltrated into the scaffolds from the combined system confirmed that host stem cells were recruited into these implants and indicated that they were capable of differentiation into multiple lineages. These results indicate that this combined system may lead to more efficient tissue regeneration.     

Current strategies for cell delivery in cartilage and bone regeneration

Several cell-based tissue-engineering therapies are emerging to regenerate damaged tissues. These strategies use autologous cells in combination with bioresorbable delivery materials. Major functions of a delivery scaffold are to provide initial mechanical stability, homogenous three-dimensional cell distribution, improved tissue differentiation, suitable handling and properties for delivery and fixation into patients. Delivery of cells can be achieved using injectable matrices, soft scaffolds, membranes, solid load-bearing scaffolds or immunoprotective macroencapsulation. Thus, to expand the clinical potential, next generation therapies will depend on smart delivery concepts that make use of the regenerative potential of stem cells, morphogenetic growth factors and biomimetic materials.     

Tissue engineering: TGF-beta superfamily members and delivery systems in bone regeneration

The induction of bone formation requires three parameters that interact in a highly regulated process: soluble osteoinductive signals, capable responding cells, and a supporting matrix substratum or insoluble signal. The use of recombinant and naturally derived bone morphogenetic proteins and transforming growth factor beta(s) (TGF-beta(s)) has increased our understanding of the functions of these morphogens during the induction of endochondral bone formation. In addition, growing understanding of the cellular interactions of living tissues with synthetic biomaterials has led to the in vivo induction of bone formation using porous biomimetic matrices as an alternative to the use of autografts for bone regeneration. This review outlines the basis of bone tissue engineering by members of the TGF-beta superfamily, focusing on their delivery systems and the intrinsic induction of bone formation by specific biomimetic matrices with a defined geometry.     

Drug delivery systems for fluoroquinolones: New prospects in tuberculosis treatment

The review focuses on new delivery systems of fluoroquinolones, the highly active antibiotics, the therapeutic application of which is still limited due to low bioavailability and low solubility in biological media. The development of suitable delivery systems seems to be a promising solution to these problems. Here, we consider the delivery systems based on synthetic polymers (polylactic and polyglycolic acids and their copolymers, polycaprolactones, etc.) and natural polymers, in particular, polysaccharides. Oligosaccharide delivery systems, conjugates of fluoroquinolones with natural polymers, as well as lipid delivery systems, including liposomes, solid lipid particles, and hybrid particles, are also discussed. The characteristic features of oral, intravenous, and aerosol delivery methods for fluoroquinolones are revealed, which is especially important in the development of new drugs for the treatment of tuberculosis.     

Development of an injectable composite for bone regeneration

With the development of minimally invasive surgical techniques, there is a growing interest in the research and development of injectable biomaterials especially for orthopedic applications. In a view to enhance the overall surgery benefits for the patient, the BIOSINJECT project aims at preparing a new generation of mineral-organic composites for bone regeneration exhibiting bioactivity, therapeutic activity and easiness of use to broaden the application domains of the actual bone mineral cements and propose an alternative strategy with regard to their poor resorbability, injectability difficulties and risk of infection. First, a physical-chemical study demonstrated the feasibility of self-setting injectable composites associating calcium carbonate-calcium phosphate cement and polysaccharides (tailor-made or commercial polymer) in the presence or not of an antibacterial agent within the composite formulation. Then, bone cell response and antimicrobial activity of the composite have been evaluated in vitro. Finally, in order to evaluate resorption rate and bone tissue response an animal study has been performed and the histological analysis is still in progress. These multidisciplinary and complementary studies led to promising results in a view of the industrial development of such composite for dental and orthopaedic applications.     

Injectable in situ forming biodegradable chitosan-hyaluronic acid based hydrogels for adipose tissue regeneration

An injectable, biodegradable and glucose-responsive hydrogel derived from natural polysaccharide derivatives was synthesized to deliver adipogenic factor of insulin in vitro for adipose tissue engineering. The biodegradable hydrogel based N-succinyl-chitosan (SCS) and aldehyde hyaluronic acid (AHA) with covalently conjugated glucose oxidase and catalase. The gelation is attributed to the Schiff-base reaction between amino and aldehyde groups of SCS and AHA, respectively. The morphologies and compressive modulus of the freeze-dried hydrogels demonstrated that the incorporated insulin and enzymes results in the formation of a tighter network structure in composite hydrogels. The immobilized enzymes triggered conversion of glucose reduces the pH value of the microenvironment, and results in hydrolysis and increasing swelling of the network basing on Schiff-base cross-linking. The pH inside the hydrogel, kept in PBS solution at pH 7.4 and 37oC, linearly dropped from 7.40 to 7.17 during 4 h of initial period, then slowly increased to 7.36 after 24 h. Correspondingly, the swelling ratio increased from 20.8 to 28.6 at 37oC in PBS with 500 mg/dL glucose. In PBS buffer with 500mg/dL glucose, about 10.8 % of insulin was seen to be released from the hydrogel after 8 h of incubation. The results demonstrated that the adipogenic factor of insulin would be released from this biodegradable hydrogel device into the local microenvironment in a controlled fashion by the swelling of hydrogel network. These preliminary studies indicate that the biodegradable and glucose-responsive hydrogel may have potential uses in adipose tissue engineering applications.     

Bifunctional monolithic affinity hydrogels for dual-protein delivery

Multiple-protein delivery has been proven to be a critical consideration for promoting tissue regeneration. Many polymeric composite biomaterials have been designed and used for modulating dual-protein delivery to enhance tissue regeneration in vitro or in vivo. However, the fabrication conditions and low water contents within the portions of these composite matrices that determine protein release rates are not optimal for maintaining the stability of encapsulated macromolecular therapeutics. In this proof-of-concept work, we aim to resolve this deficiency by single-step fabrication of affinity hydrogels capable of independently delivering two or more proteins. Selective protein-binding sites were incorporated into poly(ethylene glycol) hydrogels via copolymerization with glycidyl methacrylate-iminodiacetic acid (GMIDA) ligands to modulate release of two model proteins, lysozyme and hexahistidine tagged green fluorescent protein (hisGFP), via two distinct matrix-binding mechanisms, namely electrostatic interaction and metal-ion chelation. Differing from composite matrices for dual-protein delivery, the results reported herein indicate that injectable monolithic affinity hydrogels are capable of rapidly encapsulating multiple therapeutic agents under mild physiological conditions and independently controlling their localized delivery. Most importantly, these affinity hydrogels retain high water permeabilities throughout the entire device, characteristics that are necessary for maintaining the stability and viability of encapsulated proteins and cells.     

Stem cell systems and regeneration in planaria

Planarians are members of the Platyhelminthes (flatworms). These animals have evolved a remarkable stem cell system. A single pluripotent adult stem cell type (“neoblast”) gives rise to the entire range of cell types and organs in the planarian body plan, including a brain, digestive-, excretory-, sensory- and reproductive systems. Neoblasts are abundantly present throughout the mesenchyme and divide continuously. The resulting stream of progenitors and turnover of differentiated cells drive the rapid self-renewal of the entire animal within a matter of weeks. Planarians grow and literally de-grow (“shrink”) by the food supply-dependent adjustment of organismal turnover rates, scaling body plan proportions over as much as a 50-fold size range. Their dynamic body architecture further allows astonishing regenerative abilities, including the regeneration of complete and perfectly proportioned animals even from tiny tissue remnants. Planarians as an experimental system, therefore, provide unique opportunities for addressing a spectrum of current problems in stem cell research, including the evolutionary conservation of pluripotency, the dynamic organization of differentiation lineages and the mechanisms underlying organismal stem cell homeostasis. The first part of this review focuses on the molecular biology of neoblasts as pluripotent stem cells. The second part examines the fascinating mechanistic and conceptual challenges posed by a stem cell system that epitomizes a universal design principle of biological systems: the dynamic steady state.     

Stem cell and tissue engineering therapies for ocular regeneration

The eye is a relatively small but very complex organ. It is responsible for vision. Most of its cells are terminally differentiated, and several pathologies affecting those cells lead to vision loss and eventual blindness. Several years ago, a group of cells, located in the limbus, was identified as having the capacity of self-renewal and later on found to feed the renewal of the corneal epithelial layer. Since then, this niche of stem cells has been studied in order to provide clues that can be valuable for the regeneration of ocular structures. The worldwide shortage of donors, increased risk of transmissible diseases and immune rejection and the increased life expectancy, all contributed for the development of strategies to regenerate or repair ocular tissues. In this review we focus on two approaches for ocular regeneration: one based on stem cells and the other one based on tissue engineering strategies, and present examples where these two strategies overlap. We review the sources of cells and tissue engineering strategies for the regeneration of the cornea and of the retina, summarizing the most relevant and recent findings.     

New perspectives on spinal motor systems

The production and control of complex motor functions are usually attributed to central brain structures such as cortex, basal ganglia and cerebellum. In traditional schemes the spinal cord is assigned a subservient function during the production of movement, playing a predominantly passive role by relaying the commands dictated to it by supraspinal systems. This review challenges this idea by presenting evidence that the spinal motor system is an active participant in several aspects of the production of movement, contributing to functions normally ascribed to 'higher' brain regions.     

Thermo-sensitive alginate-based injectable hydrogel for tissue engineering

A thermo-sensitive comb-like copolymer was synthesized by grafting PNIPAAm-COOH with a single carboxy end group onto aminated alginate (AAlg) through amide bond linkages. In the copolymer, alginate was the backbone and poly(N-isopropylacrylamide) (PNIPAAm) was the pendant group. The structures of AAlg and three AAlg-g-PNIPAAm copolymers with different PNIPAAm grafting ratios were determined by FTIR and ¹H NMR. The rheological properties of AAlg-g-PNIPAAm copolymer hydrogels were measured by monitoring the viscosity, storage modulus and loss modulus as a function of temperature. The lower critical solution temperature of AAlg-g-PNIPAAm copolymers was measured as 35 °C through rheological analysis. An in vitro degradation study was carried out by monitoring weight loss. It was confirmed that degradation can be controlled by PNIPAAm modification. Encapsulation of human bone mesenchymal stem cells (hBMSCs) within hydrogels showed that the AAlg-g-PNIPAAm copolymer was not cytotoxic and preserved the viability of the entrapped cells well. The thermo-sensitive AAlg-g-PNIPAAm copolymer has attractive properties that make it suitable as cell or pharmaceutical delivery vehicles for a variety of tissue engineering applications.