A Critical Review on Classified Excipient Sodium-Alginate-Based Hydrogels: Modification, Characterization, and Application in Soft Tissue Engineering

Alginates are polysaccharides that are produced naturally and can be isolated from brown sea algae and bacteria. Sodium alginate (SA) is utilized extensively in the field of biological soft tissue repair and regeneration owing to its low cost, high biological compatibility, and quick and moderate crosslinking. In addition to their high printability, SA hydrogels have found growing popularity in tissue engineering, particularly due to the advent of 3D bioprinting. There is a developing curiosity in tissue engineering with SA-based composite hydrogels and their potential for further improvement in terms of material modification, the molding process, and their application. This has resulted in numerous productive outcomes. The use of 3D scaffolds for growing cells and tissues in tissue engineering and 3D cell culture is an innovative technique for developing in vitro culture models that mimic the in vivo environment. Especially compared to in vivo models, in vitro models were more ethical and cost-effective, and they stimulate tissue growth. This article discusses the use of sodium alginate (SA) in tissue engineering, focusing on SA modification techniques and providing a comparative examination of the properties of several SA-based hydrogels. This review also covers hydrogel preparation techniques, and a catalogue of patents covering different hydrogel formulations is also discussed. Finally, SA-based hydrogel applications and future research areas concerning SA-based hydrogels in tissue engineering were examined.

Modification, 3D printing process and application of sodium alginate based hydrogels in soft tissue engineering: A review

Sodium alginate (SA) is an inexpensive and biocompatible biomaterial with fast and gentle crosslinking that has been widely used in biological soft tissue repair/regeneration. Especially with the advent of 3D bioprinting technology, SA hydrogels have been applied more deeply in tissue engineering due to their excellent printability. Currently, the research on material modification, molding process and application of SA-based composite hydrogels has become a hot topic in tissue engineering, and a lot of fruitful results have been achieved. To better help readers have a comprehensive understanding of the development status of SA based hydrogels and their molding process in tissue engineering, in this review, we summarized SA modification methods, and provided a comparative analysis of the characteristics of various SA based hydrogels. Secondly, various molding methods of SA based hydrogels were introduced, the processing characteristics and the applications of different molding methods were analyzed and compared. Finally, the applications of SA based hydrogels in tissue engineering were reviewed, the challenges in their applications were also analyzed, and the future research directions were prospected. We believe this review is of great helpful for the researchers working in biomedical and tissue engineering.

Alginate-Based Biomaterials in Tissue Engineering and Regenerative Medicine

Today, with the salient advancements of modern and smart technologies related to tissue engineering and regenerative medicine (TE-RM), the use of sustainable and biodegradable materials with biocompatibility and cost-effective advantages have been investigated more than before. Alginate as a naturally occurring anionic polymer can be obtained from brown seaweed to develop a wide variety of composites for TE, drug delivery, wound healing, and cancer therapy. This sustainable and renewable biomaterial displays several fascinating properties such as high biocompatibility, low toxicity, cost-effectiveness, and mild gelation by inserting divalent cations (e.g., Ca2+). In this context, challenges still exist in relation to the low solubility and high viscosity of high-molecular weight alginate, high density of intra- and inter-molecular hydrogen bonding, polyelectrolyte nature of the aqueous solution, and a lack of suitable organic solvents. Herein, TE-RM applications of alginate-based materials are deliberated, focusing on current trends, important challenges, and future prospects.

Alginate: Enhancement Strategies for Advanced Applications

Alginate is an excellent biodegradable and renewable material that is already used for a broad range of industrial applications, including advanced fields, such as biomedicine and bioengineering, due to its excellent biodegradable and biocompatible properties. This biopolymer can be produced from brown algae or a microorganism culture. This review presents the principles, chemical structures, gelation properties, chemical interactions, production, sterilization, purification, types, and alginate-based hydrogels developed so far. We present all of the advanced strategies used to remarkably enhance this biopolymer's physicochemical and biological characteristics in various forms, such as injectable gels, fibers, films, hydrogels, and scaffolds. Thus, we present here all of the material engineering enhancement approaches achieved so far in this biopolymer in terms of mechanical reinforcement, thermal and electrical performance, wettability, water sorption and diffusion, antimicrobial activity, in vivo and in vitro biological behavior, including toxicity, cell adhesion, proliferation, and differentiation, immunological response, biodegradation, porosity, and its use as scaffolds for tissue engineering applications. These improvements to overcome the drawbacks of the alginate biopolymer could exponentially increase the significant number of alginate applications that go from the paper industry to the bioprinting of organs.

Engineering Polysaccharides for Tissue Repair and Regeneration

The success of repair or regeneration depends greatly on the architecture of 3D scaffolds that finely mimic natural extracellular matrix to support cell growth and assembly. Polysaccharides have excellent biocompatibility with intrinsic biological cues and they have been extensively investigated as scaffolds for tissue engineering and regenerative medicine (TERM). The physical and biochemical structures of natural polysaccharides, however, can barely meet all the requirements of tissue-engineered scaffolds. To take advantage of their inherent properties, many innovative approaches including chemical, physical, or joint modifications have been employed to improve their properties. Recent advancement in molecular and material building technology facilitates the fabrication of advanced 3D structures with desirable properties. This review focuses on the latest progress of polysaccharide-based scaffolds for TERM, especially those that construct advanced architectures for tissue regeneration.

Hydrogel scaffolds with elasticity-mimicking embryonic substrates promote cardiac cellular network formation

Hydrogels are a class of biomaterials used for a wide range of biomedical applications, including as a three-dimensional (3D) scaffold for cell culture that mimics the extracellular matrix (ECM) of native tissues. To understand the role of the ECM in the modulation of cardiac cell function, alginate was used to fabricate crosslinked gels with stiffness values that resembled embryonic (2.66 ± 0.84 kPa), physiologic (8.98 ± 1.29 kPa) and fibrotic (18.27 ± 3.17 kPa) cardiac tissues. The average pore diameter and hydrogel swelling were seen to decrease with increasing substrate stiffness. Cardiomyocytes cultured within soft embryonic gels demonstrated enhanced cell spreading, elongation, and network formation, while a progressive increase in gel stiffness diminished these behaviors. Cell viability decreased with increasing hydrogel stiffness. Furthermore, cells in fibrotic gels showed enhanced protein expression of the characteristic cardiac stress biomarker, Troponin-I, while reduced protein expression of the cardiac gap junction protein, Connexin-43, in comparison to cells within embryonic gels. The results from this study demonstrate the role that 3D substrate stiffness has on cardiac tissue formation and its implications in the development of complex matrix remodeling-based conditions, such as myocardial fibrosis.

Improving alginate printability for biofabrication: establishment of a universal and homogeneous pre-crosslinking technique

Many different biofabrication approaches as well as a variety of bioinks have been developed by researchers working in the field of tissue engineering. A main challenge for bioinks often remains the difficulty to achieve shape fidelity after printing. In order to overcome this issue, a homogeneous pre-crosslinking technique, which is universally applicable to all alginate-based materials, was developed. In this study, the Young's Modulus after post-crosslinking of selected hydrogels, as well as the chemical characterization of alginate in terms of M/G ratio and molecular weight, were determined. With our technique it was possible to markedly enhance the printability of a 2% (w/v) alginate solution, without using a higher polymer content, fillers or support structures. 3D porous scaffolds with a height of around 5 mm were printed. Furthermore, the rheological behavior of different pre-crosslinking degrees was studied. Shear forces on cells as well as the flow profile of the bioink inside the printing nozzle during the process were estimated. A high cell viability of printed NIH/3T3 cells embedded in the novel bioink of more than 85% over a time period of two weeks could be observed.

Aerosol-Jet-Printed Preferentially Aligned Carbon Nanotube Twin-Lines for Printed Electronics

The alignment of carbon nanotubes (CNTs) is of great importance for the fabrication of high-speed electronic devices such as a transistor as the electron mobilities can be greatly enhanced with aligned CNT architectures. Here, we report, for the first time, a methodology to obtain preferentially aligned CNT traces on a flexible polyimide substrate utilizing the high-resolution aerosol jet printing technique and evaporation-driven self-assembly process. A self-assembled twin-line of CNT ("coffee-ring" effect) is observed in the deposit patterns, and the field-emission scanning electron microscopy (FESEM) images reveal highly self-ordered CNT in the resulting CNT twin-line. Various aerosol jet parameters have been investigated to obtain printed tracks in the range of 30-80 μm and conductive tracks (single CNT twin-line width) in the range of 600-1500 nm. The smallest CNT twin-line obtained in this experiment is found to be approximately 16 μm using a suitable sheath-to-atomizer flow ratio. Image analysis of FESEM images confirms the formation of aligned CNT traces at the ink periphery. The effect of the line width on the degree of alignment of the CNT is studied and evaluated. The electrical resistance of the CNT trace is adjustable by controlling the number of print passes and print speed.

Overview of Carbon Nanotubes for Biomedical Applications

The unique combination of mechanical, optical and electrical properties offered by carbon nanotubes has fostered research for their use in many kinds of applications, including the biomedical field. However, due to persisting outstanding questions regarding their potential toxicity when considered as free particles, the research is now focusing on their immobilization on substrates for interface tuning or as biosensors, as load in nanocomposite materials where they improve both mechanical and electrical properties or even for direct use as scaffolds for tissue engineering. After a brief introduction to carbon nanotubes in general and their proposed applications in the biomedical field, this review will focus on nanocomposite materials with hydrogel-based matrices and especially their potential future use for diagnostics, tissue engineering or targeted drug delivery. The toxicity issue will also be briefly described in order to justify the safe(r)-by-design approach offered by carbon nanotubes-based hydrogels.

Carbon Nanotubes and Graphene as Nanoreinforcements in Metallic Biomaterials: a Review

Current challenges in existing metallic biomaterials encourage undertaking research in the development of novel materials for biomedical applications. This paper critically reviews the potential of carbon nanotubes (CNT) and graphene as nanoreinforcements in metallic biomaterials for bone tissue engineering. Unique and remarkable mechanical, electrical, and biological properties of these carbon nanomaterials allow their use as secondary-phase reinforcements in monolithic biomaterials. The nanoscale dimensions and extraordinarily large surface areas of CNT and graphene make them suitable materials for purposeful reaction with living organisms. However, the cytocompatibility of CNT and graphene is still a controversial issue that impedes advances in utilizing these promising materials in clinical orthopedic applications. The interaction of CNT and graphene with biological systems including proteins, nucleic acids, and human cells is critically reviewed to assess their cytocompatibity in vitro and in vivo. It is revealed that composites reinforced with CNT and graphene show enhanced adhesion of osteoblast cells, which subsequently promotes bone tissue formation in vivo. This potential is expected to pave the way for developing ground-breaking technologies in regenerative medicine and bone tissue engineering. In addition, current progress and future research directions are highlighted for the development of CNT and graphene reinforced implants for bone tissue engineering.

Polysaccharide Based Scaffolds for Soft Tissue Engineering Applications

Soft tissue reconstructs require materials that form three-dimensional (3-D) structures supportive to cell proliferation and regenerative processes. Polysaccharides, due to their hydrophilicity, biocompatibility, biodegradability, abundance, and presence of derivatizable functional groups, are distinctive scaffold materials. Superior mechanical properties, physiological signaling, and tunable tissue response have been achieved through chemical modification of polysaccharides. Moreover, an appropriate formulation strategy enables spatial placement of the scaffold to a targeted site. With the advent of newer technologies, these preparations can be tailor-made for responding to alterations in temperature, pH, or other physiological stimuli. In this review, we discuss the developmental and biological aspects of scaffolds prepared from four polysaccharides, viz. alginic acid (ALG), chitosan (CHI), hyaluronic acid (HA), and dextran (DEX). Clinical studies on these scaffolds are also discussed.

An Experimental Study on the Mechanical and Biological Properties of Bio-Printed Alginate/Halloysite Nanotube/Methylcellulose/Russian Olive-Based Scaffolds

Purpose: Cartilage shows neither repairs nor regenerative properties after trauma or gradual wear and causes severe pain due to bones rubbing. Bioprinting of tissue-engineered artificial cartilage is one of the most fast-growing sciences in this area that can help millions of people against this disease. Methods: Bioprinting of proper bioscaffolds for cartilage repair was the main goal of this study. The bioprinting process was achieved by a novel composition consisting of alginate (AL), Halloysite nanotube (HNT), and methylcellulose (MC) prepared in bio-ink. Also, the effect of Russian olive (RO) in chondrocytes growth on bioscaffolds was also investigated in this work. Compressive, hardness and viscosity tests, Energy-Dispersive X-Ray Spectroscopy (EDX), Fourier-Transform Infrared Spectroscopy (FT-IR), Differential Scanning Calorimetry (DSC), water-soluble Tetrazolium (WST) assay, and also transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were carried out. Results: The results show that in constant concentrations of AL, MC, and RO (20 mg/ml AL, 20 mg/ml MC, and 10 mg/ml RO) when concentration of HNT increased from 10 mg/ml (T-7) to 20 mg/ml (T-8) compressive stiffness increased from 241±45 kPa to 500.66±19.50 kPa. Also, 20 mg/ml of AL in composition saved proper water content for chondrocyte growth and produced good viscosity properties for a higher printing resolution. Conclusion: RO increased chondrocytes living cell efficiency by 11% on bioprinted scaffolds in comparison with the control group without RO. Results obtained through in-vivo studies were similar to those of in-vitro studies. According to the results, T-7 bio-ink has good potential in bioprinting of scaffolds in cartilage repairs.

Advances in Carbon Nanotubes-Hydrogel Hybrids in Nanomedicine for Therapeutics

In spite of significant advancement in hydrogel technology, low mechanical strength and lack of electrical conductivity have limited their next-level biomedical applications for skeletal muscles, cardiac and neural cells. Host-guest chemistry based hybrid nanocomposites systems have gained attention as they completely overcome these pitfalls and generate bioscaffolds with tunable electrical and mechanical characteristics. In recent years, carbon nanotube (CNT)-based hybrid hydrogels have emerged as innovative candidates with diverse applications in regenerative medicines, tissue engineering, drug delivery devices, implantable devices, biosensing, and biorobotics. This article is an attempt to recapitulate the advancement in synthesis and characterization of hybrid hydrogels and provide deep insights toward their functioning and success as biomedical devices. The improved comparative performance and biocompatibility of CNT-hydrogels hybrids systems developed for targeted biomedical applications are addressed here. Recent updates toward diverse applications and limitations of CNT hybrid hydrogels is the strength of the review. This will provide a holistic approach toward understanding of CNT-based hydrogels and their applications in nanotheranostics.

Microstructure and Cytocompatibility of Electrospun Nanocomposites Based on Poly(ɛ-Caprolactone) and Carbon Nanostructures

Carbon nanostructures (CNSs) are attractive and promising nanomaterials for the next generation of tissue engineering scaffolds, especially in neural prosthesis. Optimizing scaffold vascularization may be an important strategy to promote the repair of damaged brain tissue. In this context, the idea was to evaluate the cell response of electrospun nanohybrid scaffolds loaded with CNSs. Fibrous composites based on poly(ɛ-caprolactone) (PCL) and CNSs were fabricated by means of electrospinning technique. High-purity carbon nanofibers (CNFs) and single-wall carbon nanotubes (SWNTs) were studied. A detailed microstructural characterization was performed to evaluate the most favorable experimental conditions for the realization of fibrous PCL/CNS fabrics. Electrospun mats comprised of rather uniform and homogeneous submicrometric fibers were obtained starting from 1:1 v/v mixture of tetrahydrofuran (THF) and N,N dimethylformamide (DMF). In vitro cytocompatibility tests were performed using rat cerebro-microvascular endothelial cells (CECs). Acquired results showed an increased cell viability for PCL/CNS nanocomposites, suggesting these materials as a suitable environment for endothelial cells. These results are indicative of the promising potential of CNS-based nanocomposites in biomedical devices for tissue engineering applications where endothelial functional properties are required.

CNT-decellularized cartilage hybrids for tissue engineering applications

An optimized scaffold with adaptable architectural and biochemical properties is a goal for articular cartilage (AC) repair. A mechanically enhanced decellularized AC can be an optimistic ECM-derived scaffold. In this study, reinforced decellularized bovine AC was evaluated as a potential scaffold for cartilage repair applications. Individually dispersed single-wall carbon nanotubes (CNTs) were incorporated into chemically decellularized bovine AC samples. The mechanical and thermodynamic properties as well as the biocompatibility of the samples were evaluated by a compressive test, SEM, AFM, FTIR, TGA, DSC and a resazurin test. The Young's modulus of the CNT-incorporated samples (0.67 ± 0.09 MPa) was significantly higher compared to the decellularized ones (0.43 ± 0.06 MPa) (P = 0.001). A higher cell proliferation in the resazurin reduction test after 7 days in culture with human-adipose-derived stem cells (hADSCs) (P < 0.001) was reconfirmed with SEM. FTIR, TGA and DSC confirmed the higher stability when CNT was incorporated into the decellularized AC samples. Our findings indicate that the incorporation of CNTs can substantially enhance the mechanical properties of decellularized AC while retaining its biocompatibility, hence suggesting CNT-incorporated decellularized AC as potential scaffolds for cartilage tissue engineering applications.

Development of functionalized multi-walled carbon-nanotube-based alginate hydrogels for enabling biomimetic technologies

Alginate is a hydrogel commonly used for cell culture by ionically crosslinking in the presence of divalent Ca(2+) ions. However these alginate gels are mechanically unstable, not permitting their use as scaffolds to engineer robust biological bone, breast, cardiac or tumor tissues. This issue can be addressed via encapsulation of multi-walled carbon nanotubes (MWCNT) serving as a reinforcing phase while being dispersed in a continuous phase of alginate. We hypothesized that adding functionalized MWCNT to alginate, would yield composite gels with distinctively different mechanical, physical and biological characteristics in comparison to alginate alone. Resultant MWCNT-alginate gels were porous, and showed significantly less degradation after 14 days compared to alginate alone. In vitro cell-studies showed enhanced HeLa cell adhesion and proliferation on the MWCNT-alginate compared to alginate. The extent of cell proliferation was greater when cultured atop 1 and 3 mg/ml MWCNT-alginate; although all MWCNT-alginates lead to enhanced cell cluster formation compared to alginate alone. Among all the MWCNT-alginates, the 1 mg/ml gels showed significantly greater stiffness compared to all other cases. These results provide an important basis for the development of the MWCNT-alginates as novel substrates for cell culture applications, cell therapy and tissue engineering.

Repairing Peripheral Nerves: Is there a Role for Carbon Nanotubes?

Peripheral nerve injury continues to be a major global health problem that can result in debilitating neurological deficits and neuropathic pain. Current state-of-the-art treatment involves reforming the damaged nerve pathway using a nerve autograft. Engineered nerve repair conduits can provide an alternative to the nerve autograft avoiding the inevitable tissue damage caused at the graft donor site. Commercially available nerve repair conduits are currently only considered suitable for repairing small nerve lesions; the design and performance of engineered conduits requires significant improvements to enable their use for repairing larger nerve defects. Carbon nanotubes (CNTs) are an emerging novel material for biomedical applications currently being developed for a range of therapeutic technologies including scaffolds for engineering and interfacing with neurological tissues. CNTs possess a unique set of physicochemical properties that could be useful within nerve repair conduits. This progress report aims to evaluate and consolidate the current literature pertinent to CNTs as a biomaterial for supporting peripheral nerve regeneration. The report is presented in the context of the state-of-the-art in nerve repair conduit design; outlining how CNTs may enhance the performance of next generation peripheral nerve repair conduits.

Interpenetration of Natural Polymer Aerogels by Supercritical Drying

Natural polymers, such as alginate and gelatin, can be used to produce scaffolds for tissue engineering applications; but, their mechanical and biochemical performance should be improved. A possible solution to obtain this result, is the generation of multi-component scaffolds, by blending two or more polymers. One way to realize it, is the formation of an interpenetrating polymer network (IPN). In this work, the interpenetration of alginate and gelatin hydrogels has been successfully obtained and preserved by supercritical CO₂ (SC-CO₂) drying performed at 200 bar and 35 °C, using different blend compositions: from alginate/gelatin = 20:80 v/v to alginate/gelatin = 80:20 v/v. The process allowed modulation of morphology and mechanical properties of these blends. The overall result was made possible by the supercritical drying process that, working at zero surface tension, allows preserving the hydrogels nanostructure in the corresponding aerogels.

Combined additive manufacturing approaches in tissue engineering

Advances introduced by additive manufacturing (AM) have significantly improved the control over the microarchitecture of scaffolds for tissue engineering. This has led to the flourishing of research works addressing the optimization of AM scaffolds microarchitecture to optimally trade-off between conflicting requirements (e.g. mechanical stiffness and porosity level). A fascinating trend concerns the integration of AM with other scaffold fabrication methods (i.e. "combined" AM), leading to hybrid architectures with complementary structural features. Although this innovative approach is still at its beginning, significant results have been achieved in terms of improved biological response to the scaffold, especially targeting the regeneration of complex tissues. This review paper reports the state of the art in the field of combined AM, posing the accent on recent trends, challenges, and future perspectives.

Personalizing Biomaterials for Precision Nanomedicine Considering the Local Tissue Microenvironment

New advances in (nano)biomaterial design coupled with the detailed study of tissue-biomaterial interactions can open a new chapter in personalized medicine, where biomaterials are chosen and designed to match specific tissue types and disease states. The notion of a "one size fits all" biomaterial no longer exists, as growing evidence points to the value of customizing material design to enhance (pre)clinical performance. The complex microenvironment in vivo at different tissue sites exhibits diverse cell types, tissue chemistry, tissue morphology, and mechanical stresses that are further altered by local pathology. This complex and dynamic environment may alter the implanted material's properties and in turn affect its in vivo performance. It is crucial, therefore, to carefully study tissue context and optimize biomaterials considering the implantation conditions. This practice would enable attaining predictable material performance and enhance clinical outcomes.

In vitro evaluation of alginate/halloysite nanotube composite scaffolds for tissue engineering

In this study, a series of alginate/halloysite nanotube (HNTs) composite scaffolds were prepared by solution-mixing and freeze-drying method. HNTs are incorporated into alginate to improve both the mechanical and cell-attachment properties of the scaffolds. The interfacial interactions between alginate and HNTs were confirmed by the atomic force microscope (AFM), transmission electron microscope (TEM) and FTIR spectroscopy. The mechanical, morphological, and physico-chemical properties of the composite scaffolds were investigated. The composite scaffolds exhibit significant enhancement in compressive strength and compressive modulus compared with pure alginate scaffold both in dry and wet states. A well-interconnected porous structure with size in the range of 100-200μm and over 96% porosity is found in the composite scaffolds. X-ray diffraction (XRD) result shows that HNTs are uniformly dispersed and partly oriented in the composite scaffolds. The incorporation of HNTs leads to increase in the scaffold density and decrease in the water swelling ratio of alginate. HNTs improve the stability of alginate scaffolds against enzymatic degradation in PBS solution. Thermogravimetrica analysis (TGA) shows that HNTs can improve the thermal stability of the alginate. The mouse fibroblast cells display better attachment to the alginate/HNT composite than those to the pure alginate, suggesting the good cytocompatibility of the composite scaffolds. Alginate/HNT composite scaffolds exhibit great potential for applications in tissue engineering.

In vitro evaluation of carbon-nanotube-reinforced bioprintable vascular conduits

Vascularization of thick engineered tissue and organ constructs like the heart, liver, pancreas or kidney remains a major challenge in tissue engineering. Vascularization is needed to supply oxygen and nutrients and remove waste in living tissues and organs through a network that should possess high perfusion ability and significant mechanical strength and elasticity. In this paper, we introduce a fabrication process to print vascular conduits directly, where conduits were reinforced with carbon nanotubes (CNTs) to enhance their mechanical properties and bioprintability. In vitro evaluation of printed conduits encapsulated in human coronary artery smooth muscle cells was performed to characterize the effects of CNT reinforcement on the mechanical, perfusion and biological performance of the conduits. Perfusion and permeability, cell viability, extracellular matrix formation and tissue histology were assessed and discussed, and it was concluded that CNT-reinforced vascular conduits provided a foundation for mechanically appealing constructs where CNTs could be replaced with natural protein nanofibers for further integration of these conduits in large-scale tissue fabrication.

Controlling mechanical properties of cell-laden hydrogels by covalent incorporation of graphene oxide

Graphene-based materials are useful reinforcing agents to modify the mechanical properties of hydrogels. Here, an approach is presented to covalently incorporate graphene oxide (GO) into hydrogels via radical copolymerization to enhance the dispersion and conjugation of GO sheets within the hydrogels. GO is chemically modified to present surface-grafted methacrylate groups (MeGO). In comparison to GO, higher concentrations of MeGO can be stably dispersed in a pre-gel solution containing methacrylated gelatin (GelMA) without aggregation or significant increase in viscosity. In addition, the resulting MeGO-GelMA hydrogels demonstrate a significant increase in fracture strength with increasing MeGO concentration. Interestingly, the rigidity of the hydrogels is not significantly affected by the covalently incorporated GO. Therefore, this approach can be used to enhance the structural integrity and resistance to fracture of the hydrogels without inadvertently affecting their rigidity, which is known to affect the behavior of encapsulated cells. The biocompatibility of MeGO-GelMA hydrogels is confirmed by measuring the viability and proliferation of the encapsulated fibroblasts. Overall, this study highlights the advantage of covalently incorporating GO into a hydrogel system, and improves the quality of cell-laden hydrogels.

Combined multi-nozzle deposition and freeze casting process to superimpose two porous networks for hierarchical three-dimensional microenvironment

An engineered three-dimensional scaffold with hierarchical porosity and multiple niche microenvironments is produced using a combined multi-nozzle deposition-freeze casting technique. In this paper we present a process to fabricate a scaffold with improved interconnectivity and hierarchical porosity. The scaffold is produced using a two-stage manufacturing process which superimposes a printed porous alginate (Alg) network and a directionally frozen ceramic-polymer matrix. The combination of two processes, multi-nozzle deposition and freeze casting, provides engineering control of the microenvironment of the scaffolds over several length scales; including the addition of lateral porosity and the ratio of polymer to ceramic microstructures. The printed polymer scaffold is submerged in a ceramic-polymer slurry and subsequently, both structures are directionally frozen (freeze cast), superimposing and patterning both microenvironments into a single hierarchical architecture. An optional additional sintering step removes the organic material and densifies the ceramic phase to produce a well-defined network of open pores and a homogenous cell wall material composition. The techniques presented in this contribution address processing challenges, such as structure definition, reproducibility and fine adjustments of unique length scales, which one typically encounters when fabricating topological channels between longitudinal and transverse porous networks.

Carbon nanotubes: properties, synthesis, purification, and medical applications

Current discoveries of different forms of carbon nanostructures have motivated research on their applications in various fields. They hold promise for applications in medicine, gene, and drug delivery areas. Many different production methods for carbon nanotubes (CNTs) have been introduced; functionalization, filling, doping, and chemical modification have been achieved, and characterization, separation, and manipulation of individual CNTs are now possible. Parameters such as structure, surface area, surface charge, size distribution, surface chemistry, and agglomeration state as well as purity of the samples have considerable impact on the reactivity of carbon nanotubes. Otherwise, the strength and flexibility of carbon nanotubes make them of potential use in controlling other nanoscale structures, which suggests they will have a significant role in nanotechnology engineering.

Carbon nanotubes reinforced chitosan films: mechanical properties and cell response of a novel biomaterial for cardiovascular tissue engineering

Carbon nanotubes have been proposed as fillers to reinforce polymeric biomaterials for the strengthening of their structural integrity to achieve better biomechanical properties. In this study, a new polymeric composite material was introduced by incorporating various low concentrations of multiwalled carbon nanotubes (MWCNTs) into chitosan (CS), aiming at achieving a novel composite biomaterial with superior mechanical and biological properties compared to neat CS, in order to be used in cardiovascular tissue engineering applications. Both mechanical and biological characteristics in contact with the two relevant cell types (endothelial cells and vascular myofibroblasts) were studied. Regarding the mechanical behavior of MWCNT reinforced CS (MWCNT/CS), 5 and 10 % concentrations of MWCNTs enhanced the mechanical behavior of CS, with that of 5 % exhibiting a superior mechanical strength compared to 10 % concentration and neat CS. Regarding biological properties, MWCNT/CS best supported proliferation of endothelial and myofibroblast cells, MWCNTs and MWCNT/CS caused no apoptosis and were not toxic of the examined cell types. Conclusively, the new material could be suitable for tissue engineering (TE) and particularly for cardiovascular TE applications.

Biocomposites reinforced by fibers or tubes as scaffolds for tissue engineering or regenerative medicine

As a dynamic and hierarchically organized composite, native extracellular matrix (ECM) not only supplies mechanical support, which the embedded cells need, but also regulates various cellular activities through interaction with them. On the basis of the ECM-mimetic principle, good biocompatibility and appropriate mechanical properties are the two basic requirements that the ideal scaffolds for the tissue engineering or regenerative medicine need. Some fibers and tubes have been shown effective to reinforce scaffolds for tissue engineering or regenerative medicine. In this review, three parts, namely properties affected by the addition of fibers or tubes, scaffolds reinforced by fibers or tubes for soft tissue repair, and scaffolds reinforced by fibers or tubes for hard tissue repair are stated, which shows that tissue repair or regeneration efficacy was enhanced significantly by fiber or tube reinforcement. In addition, it indicates that these reinforcing agents can improve the biocompatibility and biodegradation of the scaffolds in most cases. However, there are still some concerns, such as the homogeneousness in structure or composition throughout the reinforced scaffolds, the adhesive strength between the matrix and the fibers or tubes, cytotoxicity of nanoscaled reinforcing agents, etc., which were also discussed in the conclusion and perspectives part.

ADDITION OF CONDUCTIVE ELEMENTS TO POLYMERIC SCAFFOLDS FOR MUSCLE TISSUE ENGINEERING

Cardiac and skeletal muscles are two tissues that would benefit from an electrically conductive scaffold to regenerate lost or lower functioning areas. By augmenting polymeric scaffolds with conductive elements, the contractile process for both muscles could increase. In this review, the components reviewed include polyaniline (PANi), gold (Au) nanoparticles, and carbon nanotubes (CNT). PANi has been combined with several polymers and increased the conductivity of the scaffolds. It is biocompatible, but increases mechanical properties and decreases scaffold elongation. Tissue engineering using nanoparticles is an emerging area and considerable research focuses on determining possible toxicity due to nanoparticle concentration. Contradicting data exists for both Au nanoparticles and CNT. Smaller Au nanoparticles damage cardiac tissue in vivo while larger ones do not. By comparison, in vitro data shows no harmful results for skeletal muscle cells. Data for CNT is just as diverse as the amount, orientation and further purification or functionalization could all play a role in determining biocompatibility. Future research should focus on establishing the conductivity level needed for each muscle tissue to ascertain the amount of conductive element needed so the most suitable one can be utilized.

Finely dispersed single-walled carbon nanotubes for polysaccharide hydrogels

Here we demonstrate a polysaccharide hydrogel reinforced with finely dispersed single-walled carbon nanotubes (SWNTs) using biocompatible dispersants O-carboxymethylchitosan (OC) and chondroitin sulfate A (CS-A) as a structural support. Both of the dispersants can disperse SWNTs in aqueous solutions and hydrogel matrix as individual tubes or small bundles. Additionally, we have found that compressive modulus and strain of the hydrogels reinforced with SWNTs were enhanced as much as two times by the addition of a few weight percent of SWNTs. Moreover, the SWNT-incorporated hydrogels exhibited lower impedance and higher charge capacity than the alginate/dispersant hydrogel without SWNTs. The OC and the CS-A demonstrated much higher reinforcing enhancement than a commercially available dispersant, sodium dodecyl sulfate. Combined with the experimental data on the mechanical and electrical properties, the biocompatibility of OC and CS-A can provide the possibility of biomedical application of the SWNT-reinforced hydrogels.

Biopolymer-based hydrogels as scaffolds for tissue engineering applications: a review

Hydrogels are physically or chemically cross-linked polymer networks that are able to absorb large amounts of water. They can be classified into different categories depending on various parameters including the preparation method, the charge, and the mechanical and structural characteristics. The present review aims to give an overview of hydrogels based on natural polymers and their various applications in the field of tissue engineering. In a first part, relevant parameters describing different hydrogel properties and the strategies applied to finetune these characteristics will be described. In a second part, an important class of biopolymers that possess thermosensitive properties (UCST or LCST behavior) will be discussed. Another part of the review will be devoted to the application of cryogels. Finally, the most relevant biopolymer-based hydrogel systems, the different methods of preparation, as well as an in depth overview of the applications in the field of tissue engineering will be given.

Carbon Nanotube Composite Scaffolds and Coatings for Tissue Engineering Applications

Carbon nanotubes (CNTs) are composed of two-dimensional hexagonal graphite sheets rolled up to form into a seamless hollow tube or cylinder of diameters ranging from 0.7 to 100 nm and length of several micrometres up to several millimetres [1, 2]. CNTs can be synthesised in two configurations, as single-walled nanotubes (SWCNTs) and multi-walled nanotubes (MWCNTs). Whereas SWCNTs are made of one tubular structure, MWCNTs consist of concentrically arranged carbon tubes with a typical spacing of ≈ 0.34 nm between the different layers. Owing to their remarkable structural characteristics (light weight, high aspect ratio, high specific surface area), as well as attractive mechanical (high stiffness and strength), electrical (high conductivity) and chemical (versatile surface chemistry, easily to functionalise) properties [2], there is increasing interest in biomedical applications of CNTs.