Preparation of Polyurethane-Graphene Nanocomposite and Evaluation of Neurovascular Regeneration

Development and physiochemical assessment of graphene-bioactive glass-P(3HB-co-4HB) composite scaffold as prospect biomaterial for wound healing

The clinical management of wounds presents a considerable challenge because dressing selection must prioritise the provision of appropriate barrier and the healing properties, consider patient's compliance factors such as comfort, functionality and practicality. This study primarily aimed to develop a composite scaffold patch for potential application in wound healing. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] is a biopolymer that originated from bacteria. It is well-recognised owing to its distinctive mechanical and physical characteristics suitable for biomedical applications. Graphene (G) and bioactive glass (BG) are biocompatible towards humans, and enhanced properties are achievable by adding biopolymer. In this study, composite scaffolds were developed by combining P(3HB-co-4HB) at a distinct proportion of 4HB monomer reinforced with G (3.0 wt.%) and BG (2.5 wt.%) by using solvent casting, resulting in two types of composite scaffolds: P(3HB-co-25%4HB)/G/BG and P(3HB-co-37%4HB)/G/BG. A successful composite scaffold as a unified structure was achieved based on chemical assessments of organic and inorganic elements within the composites. The pure polymer displayed a smooth surface, and the BG and G addition into the composite scaffolds increased surface roughness, forming irregular pores and protuberances. The wettability and hydrophilicity of the composites significantly improved up to 40% in terms of water uptake. An increment in crystallisation temperature diminished the flexibility of the composite's scaffolds. Evaluation of Presto Blue biocompatibility demonstrated nontoxic behaviour with a dosage of less than 25.00 mg ml-1of composite scaffold-conditioned media. The L929 fibroblast cells displayed excellent adhesion to both types of composite scaffolds, as evidenced by the increased percentage of cell viability observed throughout 14 d of exposure. These findings demonstrate the importance of optimising each component within the composite scaffolds and their interrelation, paving the way for excellent material properties and enhancing the potential for wound healing applications.

Injectable Hydrogels Based on Hyaluronic Acid and Gelatin Combined with Salvianolic Acid B and Vascular Endothelial Growth Factor for Treatment of Traumatic Brain Injury in Mice

Traumatic brain injury (TBI) leads to structural damage in the brain, and is one of the major causes of disability and death in the world. Herein, we developed a composite injectable hydrogel (HA/Gel) composed of hyaluronic acid (HA) and gelatin (Gel), loaded with vascular endothelial growth factor (VEGF) and salvianolic acid B (SAB) for treatment of TBI. The HA/Gel hydrogels were formed by the coupling of phenol-rich tyramine-modified HA (HA-TA) and tyramine-modified Gel (Gel-TA) catalyzed by horseradish peroxidase (HRP) in the presence of hydrogen peroxide (H2O2). SEM results showed that HA/Gel hydrogel had a porous structure. Rheological test results showed that the hydrogel possessed appropriate rheological properties, and UV spectrophotometry results showed that the hydrogel exhibited excellent SAB release performance. The results of LIVE/DEAD staining, CCK-8 and Phalloidin/DAPI fluorescence staining showed that the HA/Gel hydrogel possessed good cell biocompatibility. Moreover, the hydrogels loaded with SAB and VEGF (HA/Gel/SAB/VEGF) could effectively promote the proliferation of bone marrow mesenchymal stem cells (BMSCs). In addition, the results of H&E staining, CD31 and α-SMA immunofluorescence staining showed that the HA/Gel/SAB/VEGF hydrogel possessed good in vivo biocompatibility and pro-angiogenic ability. Furthermore, immunohistochemical results showed that the injection of HA/Gel/SAB/VEGF hydrogel to the injury site could effectively reduce the volume of defective tissues in traumatic brain injured mice. Our results suggest that the injection of HA/Gel hydrogel loaded with SAB and VEGF might provide a new approach for therapeutic brain tissue repair after traumatic brain injury.

Influence of Graphene-Based Nanocomposites in Neurogenesis and Neuritogenesis: A Brief Summary

Graphene is a prospective candidate for various biomedical applications, including drug transporters, bioimaging agents, and scaffolds for tissue engineering, thanks to its superior electrical conductivity and biocompatibility. The clinical issue of nerve regeneration and rehabilitation still has a major influence on people's lives. Nanomaterials based on graphene have been exploited extensively to promote nerve cell differentiation and proliferation. Their high electrical conductivity and mechanical robustness make them appropriate for nerve tissue engineering. Combining graphene with other substances, such as biopolymers, may transmit biochemical signals that support brain cell division, proliferation, and regeneration. The utilization of nanocomposites based on graphene in neurogenesis and neuritogenesis is the primary emphasis of this review. Here are some examples of the many synthetic strategies used. For neuritogenesis and neurogenesis, it has also been explored to combine electrical stimulation with graphene-based materials.

Progress and mechanism of graphene oxide-composited materials in application of peripheral nerve repair

Peripheral nerve injuries (PNI) are one of the most common nerve injuries, and graphene oxide (GO) has demonstrated significant potential in the treatment of PNI. GO could enhance the proliferation, adhesion, migration, and differentiation of neuronal cells by upregulating the expression of relevant proteins, and regulate the angiogenesis process and immune response. Therefore, GO is a suitable additional component for fabricating artificial nerve scaffolds (ANS), in which the slight addition of GO could improve the physicochemical performance of the matrix materials, through hydrogen bonds and electrostatic attraction. GO-composited ANS can increase the expression of nerve regeneration-associated genes and factors, promoting angiogenesis by activating the RAS/MAPK and AKT-eNOS-VEGF signaling pathway, respectively. Moreover, GO could be metabolized and excreted from the body through the pathway of peroxidase degradation in vivo. Consequently, the application of GO in PNI regeneration exhibits significant potential for transitioning from laboratory research to clinical use.

State of the Art and Current Challenges on Electroactive Biomaterials and Strategies for Neural Tissue Regeneration

The loss or failure of an organ/tissue stands as one of the healthcare system's most prevalent, devastating, and costly challenges. Strategies for neural tissue repair and regeneration have received significant attention due to their particularly strong impact on patients' well-being. Many research efforts are dedicated not only to control the disease symptoms but also to find solutions to repair the damaged tissues. Neural tissue engineering (TE) plays a key role in addressing this problem and significant efforts are being carried out to develop strategies for neural repair treatment. In the last years, active materials allowing to tune cell-materials interaction are being increasingly used, representing a recent paradigm in TE applications. Among the most important stimuli influencing cell behavior are the electrical and mechanical ones. In this way, materials with the ability to provide this kind of stimuli to the neural cells seem to be appropriate to support neural TE. In this scope, this review summarizes the different biomaterials types used for neural TE, highlighting the relevance of using active biomaterials and electrical stimulation. Furthermore, this review provides not only a compilation of the most relevant studies and results but also strategies for novel and more biomimetic approaches for neural TE.

Polydopamine-Decorated PLCL Conduit to Induce Synergetic Effect of Electrical Stimulation and Topological Morphology for Peripheral Nerve Regeneration

Due to the limited self-repairing capacity after peripheral nerve injuries (PNI), artificial nerve conduits are widely applied to facilitate neural regeneration. Exogenous electrical stimulation (ES) that is carried out by the conductive conduit regulates the biological behavior of Schwann cells (SCs). Meanwhile, a longitudinal surface structure counts to guide axonal growth to accelerate the end-to-end connection. Currently, there are no conduits equipped with both electrical conduction and axon-guiding surface structure. Herein, a biodegradable, conductive poly(l-lactide-co-caprolactone)/graphene (PLCL/GN) composite conduit is designed. The conduit with 20.96 ± 1.26 MPa tensile strength has a micropatterned surface of 20 µm groove fabricated by microimprint technology and self-assembled polydopamine (PDA). In vitro evaluation shows that the conduits with ES effectively stimulate the directional cell migration, adhesion, and elongation, and enhance neuronal expression of SCs. The rat sciatic nerve crush model demonstrates that the conductive micropatterned conduit with ES promotes the growth of myelin sheath, faster nerve regeneration, and 20-fold functional recovery in vivo. These discoveries prove that the PLCL(G)/PDA/GN composite conduit is a promising tool for PNI treatment by providing the functional integration of physical guidance, biomimetic biological regulation, and bioelectrical stimulation, which inspires a novel therapeutic approach for nerve regeneration in the future.

Graphene/Polymer Nanocomposites: Preparation, Mechanical Properties, and Application

Although polymers are very important and vastly used materials, their physical properties are limited. Therefore, they are reinforced with fillers to relieve diverse restrictions and expand their application areas. The exceptional properties of graphene make it an interesting material with huge potential for application in various industries and devices. The interfacial interaction between graphene and the polymer matrix improved the uniform graphene dispersion in the polymer matrix, enhancing the general nanocomposite performance. Therefore, graphene functionalization is essential to enhance the interfacial interaction, maintain excellent properties, and obstruct graphene agglomeration. Many studies have reported that graphene/polymer nanocomposites have exceptional properties that enable diverse applications. The use of graphene/polymer nanocomposites is expected to increase sustainably and to transform from a basic to an advanced material to offer optimum solutions to industry and consumers.

Influence of Starch on the Structure–Properties Relationship in Polyethylene Glycol/Polycaprolactone Diol Polyurethanes

Improvements in the antithrombogenicity activity of biomaterials for cardiovascular applications are necessary to meet the demand for vascular grafts in the world. Zwitterionic compounds tend to be used due to their anti-fouling properties, which reduce platelet adhesions and protein absorptions. Therefore, in this research, potato starch (AL-N) and zwitterionic starch (AL-Z) (obtained by Williamson etherification) were included as fillers in polyurethane (PU) matrices from polycaprolactone diol (PCL), polyethylene glycol (PEG), pentaerythritol (PE) and isophorone diisocyanate (IPDI) in order to study their effect in terms of their physicochemical, mechanical and thermal properties. We conducted our evaluation using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), contact angle analysis, swelling behavior, thermogravimetric analysis (TGA), tensile/strain analysis, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS), dynamic mechanic analysis (DMA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results showed that AL-N and AL-Z modified these properties, where AL-N improved tensile strength, and AL-Z increased the hydrophilicity of polyurethanes matrices; additionally, AL-N had interactions with the soft segments, and AL-Z had interactions with the hard segments. Finally, both fillers reduced the degree of crystallinity and did not affect the thermal stability of polyurethanes.

Biodegradable polyurethane scaffolds in regenerative medicine: Clinical translation review

Early explorations of tissue engineering and regenerative medicine concepts commonly utilized simple polyesters such as polyglycolide, polylactide, and their copolymers as scaffolds. These biomaterials were deemed clinically acceptable, readily accessible, and provided processability and a generally known biological response. With experience and refinement of approaches, greater control of material properties and integrated bioactivity has received emphasis and a broadened palette of synthetic biomaterials has been employed. Biodegradable polyurethanes (PUs) have emerged as an attractive option for synthetic scaffolds in a variety of tissue applications because of their flexibility in molecular design and ability to fulfill mechanical property objectives, particularly in soft tissue applications. Biodegradable PUs are highly customizable based on their composition and processability to impart tailored mechanical and degradation behavior. Additionally, bioactive agents can be readily incorporated into these scaffolds to drive a desired biological response. Enthusiasm for biodegradable PU scaffolds has soared in recent years, leading to rapid growth in the literature documenting novel PU chemistries, scaffold designs, mechanical properties, and aspects of biocompatibility. Despite the enthusiasm in the field, there are still few examples of biodegradable PU scaffolds that have achieved regulatory approval and routine clinical use. However, there is a growing literature where biodegradable PU scaffolds are being specifically developed for a wide range of pathologies and where relevant pre-clinical models are being employed. The purpose of this review is first to highlight examples of clinically used biodegradable PU scaffolds, and then to summarize the growing body of reports on pre-clinical applications of biodegradable PU scaffolds.

An Update on Graphene-Based Nanomaterials for Neural Growth and Central Nervous System Regeneration

Thanks to their reduced size, great surface area, and capacity to interact with cells and tissues, nanomaterials present some attractive biological and chemical characteristics with potential uses in the field of biomedical applications. In this context, graphene and its chemical derivatives have been extensively used in many biomedical research areas from drug delivery to bioelectronics and tissue engineering. Graphene-based nanomaterials show excellent optical, mechanical, and biological properties. They can be used as a substrate in the field of tissue engineering due to their conductivity, allowing to study, and educate neural connections, and guide neural growth and differentiation; thus, graphene-based nanomaterials represent an emerging aspect in regenerative medicine. Moreover, there is now an urgent need to develop multifunctional and functionalized nanomaterials able to arrive at neuronal cells through the blood-brain barrier, to manage a specific drug delivery system. In this review, we will focus on the recent applications of graphene-based nanomaterials in vitro and in vivo, also combining graphene with other smart materials to achieve the best benefits in the fields of nervous tissue engineering and neural regenerative medicine. We will then highlight the potential use of these graphene-based materials to construct graphene 3D scaffolds able to stimulate neural growth and regeneration in vivo for clinical applications.

Two-Dimensional Nanomaterials for Peripheral Nerve Engineering: Recent Advances and Potential Mechanisms

Peripheral nerve tissues possess the ability to regenerate within artificial nerve scaffolds, however, despite the advance of biomaterials that support nerve regeneration, the functional nerve recovery remains unsatisfactory. Importantly, the incorporation of two-dimensional nanomaterials has shown to significantly improve the therapeutic effect of conventional nerve scaffolds. In this review, we examine whether two-dimensional nanomaterials facilitate angiogenesis and thereby promote peripheral nerve regeneration. First, we summarize the major events occurring after peripheral nerve injury. Second, we discuss that the application of two-dimensional nanomaterials for peripheral nerve regeneration strategies by facilitating the formation of new vessels. Then, we analyze the mechanism that the newly-formed capillaries directionally and metabolically support neuronal regeneration. Finally, we prospect that the two-dimensional nanomaterials should be a potential solution to long range peripheral nerve defect. To further enhance the therapeutic effects of two-dimensional nanomaterial, strategies which help remedy the energy deficiency after peripheral nerve injury could be a viable solution.

Influence of the Halloysite Nanotube (HNT) Addition on Selected Mechanical and Biological Properties of Thermoplastic Polyurethane

Halloysite nanotube (HNT) additions to the thermoplastic polyurethane (TPU) system were thoroughly evaluated in this study. The resultant composites have been designed for future personalized intervertebral disc implant applications, which requires additional technology to obtain the appropriate geometry unique to each patient. These requirements can be fulfilled using 3D printing. In this work, a technology was developed to produce filaments for fused deposition modeling (FDM). Nanocomposites were prepared using variable HNT content (1, 2, and 3 wt.%). The nanostructure of the resultant composites was confirmed using scanning transmission electron microscopy (STEM). Mechanical tests were used to measure the tensile modulus, stress, and elongation the composites and TPU matrix. Nanocomposites with 2% HNT content were able to withstand 26% increased stress and 50% increased elongation compared to pure TPU before fracturing in addition to a 13% reduction in the friction coefficient. A MTT cytotoxicity assay confirmed the cytotoxicity of all tested materials against human epidermal keratinocyte cells (HaCaT).

Graphene and water-based elastomer nanocomposites - a review

Water-based elastomers (WBEs) are polymeric elastomers in aqueous systems. WBEs have recently continued to gain wide acceptability by both academia and industry due to their remarkable environmental and occupational safety friendly nature, as a non-toxic elastomeric dispersion with low-to-zero volatile organic compound (VOC) emission. However, their inherent poor mechanical and thermal properties remain a drawback to these sets of elastomers. Hence, nano-fillers such as graphene oxide (GO), reduced graphene oxide (rGO) and graphene nanoplatelets (GNPs) are being employed for the reinforcement and enhancement of this set of elastomers. This work is geared towards a critical review and summation of the state-of-the-art developments of graphene enhanced water-based elastomer composites (G-WBEC), including graphene and composite production processes, properties, characterisation techniques and potential commercial applications. The dominant production techniques, such as emulsion mixing and in situ polymerisation processes, which include Pickering emulsion, mini-emulsion and micro-emulsion, as well as ball-milling approach, are systematically evaluated. Details of the account of mechanical properties, electrical conductivity, thermal stability and thermal conductivity enhancements, as well as multifunctional properties of G-WBEC are discussed, with further elaboration on the structure-property relationship effects (such as dispersion and filler-matrix interface) through effective and non-destructive characterisation tools like Raman and XRD, among others. The paper also evaluates details of the current application attempts and potential commercial opportunities for G-WBEC utilisation in aerospace, automotive, oil and gas, biomedicals, textiles, sensors, electronics, solar energy, and thermal management.

Light-Triggered In Situ Gelation of Hydrogels using 2D Molybdenum Disulfide (MoS2 ) Nanoassemblies as Crosslink Epicenter

Light-responsive biomaterials are an emerging class of materials used for developing noninvasive, noncontact, precise, and controllable biomedical devices. Long-wavelength near-infrared (NIR) radiation is an attractive light source for in situ gelation due to its higher penetration depth and minimum side effects. The conventional approach to obtain crosslinked biomaterials relies heavily on the use of a photoinitiator by generating reactive species when exposed to short-wavelength radiation, which is detrimental to surrounding cells and tissue. Here, a new class of NIR-triggered in situ gelation system based on defect-rich 2D molybdenum disulfide (MoS2 ) nanoassemblies and thiol-functionalized thermoresponsive polymer in the absence of a photoinitiator is introduced. Exposure to NIR radiation activates the dynamic polymer-nanomaterials interactions by leveraging the photothermal characteristics of MoS2 and intrinsic phase transition ability of the thermoresponsive polymer. Specifically, upon NIR exposure, MoS2 acts as a crosslink epicenter by connecting with multiple polymeric chains via defect-driven click chemistry. As a proof-of-concept, the utility of NIR-triggered in situ gelation is demonstrated in vitro and in vivo. Additionally, the crosslinked gel exhibits the potential for NIR light-responsive release of encapsulated therapeutics. These light-responsive biomaterials have strong potential for a range of biomedical applications, including artificial muscle, smart actuators, 3D/4D printing, regenerative medicine, and therapeutic delivery.

Graphene-based hybrid materials as promising scaffolds for peripheral nerve regeneration

Peripheral nerve injury (PNI) is a serious clinical health problem caused by the damage of peripheral nerves which results in neurological deficits and permanent disability. There are several factors that may cause PNI such as localized damage (car accident, trauma, electrical injury) and outbreak of the systemic diseases (autoimmune or diabetes). While various diagnostic procedures including X-ray, magnetic resonance imaging (MRI), as well as other type of examinations such as electromyography or nerve conduction studies have been efficiently developed, a full recovery in patients with PNI is in many cases deficient or incomplete. This is the reason why additional therapeutic strategies should be explored to favor a complete rehabilitation in order to get appropriate nerve injury regeneration. The use of biomaterials acting as scaffolds opens an interesting approach in regenerative medicine and tissue engineering applications due to their ability to guide the growth of new tissues, adhesion and proliferation of cells including the expression of bioactive signals. This review discusses the preparation and therapeutic strategies describing in vitro and in vivo experiments using graphene-based materials in the context of PNI and their ability to promote nerve tissue regeneration.

Eco-Friendly Preparation of Epoxy-Rich Graphene Oxide for Wound Healing

Despite the ever-growing endangerment caused by the multidrug resistance (MDR) of bacteria, the development of effective antibacterial materials still remains a global challenge. Current antibiotic therapies cannot simultaneously inactivate bacteria and accelerate wound healing. This study aimed to originally separate the intercalation of MnO₃⁺ and the oxidation processes to synthesize epoxy-rich graphene oxide (erGO) nanofilms via an eco-friendly synthetic route, which possessed low density and large lamellar distribution and was rich in epoxide. Importantly, the MnO₃⁺ could be separated from the product and recycled for preparing the next generation of erGO nanofilms, which was quite economical and eco-friendly. The erGO nanofilm was capable of successfully inhibiting Gram-negative bacteria and even had excellent growth-inhibitory effects on Gram-positive bacteria including multidrug resistance (MDR) bacteria, as evidenced by antibacterial phenomena. Additionally, the erGO nanofilm with high ^•C density formed from epoxide exerted excellent antibacterial effects through tight membrane wrapping and induction of lipid peroxidation. The wound-healing property of the erGO nanofilm was evaluated via treatments of wounds infected by Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), which not only killed bacteria but also accelerated wound healing in mice with a skin infection. The novel erGO nanofilm with dual antimicrobial mechanisms might serve as a promising multifunctional antimicrobial agent for medical wound dressing with high biocompatibility.

Thermoresponsive and Conductive Chitosan-Polyurethane Biocompatible Thin Films with Potential Coating Application

Conductive thin films have great potential for application in the biomedical field. Herein, we designed thermoresponsive and conductive thin films with hydrophilicity, strain sensing, and biocompatibility. The crosslinked dense thin films were synthesized and prepared through a Schiff base reaction and ionic interaction from dialdehyde polyurethane, N-carboxyethyl chitosan, and double-bonded chitosan grafted polypyrrole. The thin films were air-dried under room temperature. These thin films showed hydrophilicity and conductivity (above 2.50 mS/cm) as well as responsiveness to the deformation. The tensile break strength (9.72 MPa to 15.07 MPa) and tensile elongation (5.76% to 12.77%) of conductive thin films were enhanced by heating them from 25 °C to 50 °C. In addition, neural stem cells cultured on the conductive thin films showed cell clustering, proliferation, and differentiation. The application of the materials as a conductive surface coating was verified by different coating strategies. The conductive thin films are potential candidates for surface modification and biocompatible polymer coating.

Development of MOF Reinforcement for Structural Stability and Toughness Enhancement of Biodegradable Bioinks

Three-dimensional (3D) bioprinting is a technology that can precisely fabricate customized tissues and organs. Hydrogel materials that can embed living cells for use in 3D printing are called bioinks. However, there are only limited options of bioinks currently because they require the following features at once, such as printability, repetitive layer-by-layer stacking (stackability), structure stabilization, and biological properties. A polyurethane-gelatin double network hydrogel bioink was previously reported to own tunable modulus through changing the solid content, but cell viability at the high solid content is inevitably reduced. In the present study, the reinforcement effects of a metal-organic framework (MOF), zeolitic imidazolate framework-8 (ZIF-8), in the PUG bioink were evaluated. The printability, stackability, thermoresponsiveness, and shear-thinning behavior of the PUG-ZIF-8 composite hydrogels were examined. It was found that the PUG composite hydrogel containing 1250 μg/mL ZIF-8 crystals showed significant structural stability and modulus enhancement (∼2.5-fold). However, the PUG bioink containing 1250 μg/mL ZIF-8 crystals may lead to cell senescence or death. The cytocompatible concentration of ZIF-8 crystals in the bioink was about 875 μg/mL, and this concentration was much higher than the reported tolerable amount (∼50 μg/mL) of ZIF-8 for biomedical applications. The strong reinforcement effect of ZIF-8 and the drug-loading/sensing possibilities of MOFs may open new opportunities for using MOFs in 3D bioprinting applications.

Tracing Carbon Nanotubes (CNTs) in Rat Peripheral Nerve Regenerated with Conductive Conduits Composed of Poly(lactide-co-glycolide) and Fluorescent CNTs

Nerve regeneration can be promoted using nerve guide conduits (NGCs). Carbon nanotubes (CNTs) are often used to prepare conductive NGCs, however, the major concern for their applications is the final location of the implanted CNTs in vivo. Herein, photoluminescent multiwalled CNTs (MWCNTs) were prepared and electrospun with poly(lactide-co-glycolide) (PLGA), followed by shaping into multichannel NGCs for repairing of injured rat sciatic nerve, thereby the distribution of CNTs in vivo could be detected via bioimaging. Photoluminescent MWCNTs (MWCNT-FITC) were prepared by functionalization with poly(glycidyl methacrylate) (PGMA) and fluorescein-isothiocyanate-isomer I (FITC) subsequently. The conductivity of the PLGA/MWCNT-FITC fibers was approx. 10^-4 S/cm at 3 wt % MWCNTs. Compared with PLGA fibers, Schwann cells on PLGA/MWCNT-FITC fibers matured at a faster rate, accordingly, nerve regeneration was promoted by the PLGA/MWCNT-FITC NGC. With a confocal laser scanning microscope and small-animal imaging system, the location of MWCNTs was detected. Alongside the degradation of PLGA, MWCNTs intended to aggregate and were entrapped in the regenerated nerve tissue without migrating into surrounding tissues and other organs (liver, kidneys, and spleen). This study provides a useful characterization method for MWCNTs and the guidance for in vivo applications of MWCNTs in tissue engineering.