The article inclusion to this review was determined by the following inclusion criteria:

• •

  The manuscript should be written in English.

• •

  The time of Publication should be until 2020.

• •

  The syntaxis of the search should be: [nervous system] AND [scaffolds], [stem cells scaffolds], [bioengineering].

• •

  The article should be indexed in one of the following data bases: Pubmed.

• •

  The article selection included original articles, review articles, case reports and clinical trials.

The article exclusion to this review was determined by the following inclusion criteria:

• •

  Articles in any other language as English.

• •

  Articles without an Indexing in PubMed.

• •

  Letter to editors or any other article type not included in the inclusion criteria.

In order to identify and eliminated duplicated articles, we imported the data to the Endnote X8 reference Manager Software. Then we performed a selection of articles based on the presence of the inclusion criteria in the title or abstracts of the articles. This process was executed by three independent researchers in a parallel search. In the extracted data, we included the name of the biomaterial, the bioactive effect, the experimental model (in the case of an experimental study), translational perspective in the clinic and the advantages and disadvantages of each approach.

This qualitative systematic review shows the extracted data in the tables at the end of the manuscript each table expose the different approaches with the use of bioactive scaffolds and stem cells for diseases in central nervous system and peripheral nervous system. A more detailed overview of the biomaterial constitution, pharmacological kinetics and dynamics is presented in each sub-topic of this manuscript.

Stem cells are unspecialised cells with the capacity for continuous self-renewal and differentiation into a range of cell types. The recent efforts for the regeneration of neural stem cells (NSC) have been focused on biomaterials and bioengineering that emulate the NSC niche in order to establish an adequate microenvironment that promotes neurogenesis and differentiation. The biophysical signals and the release of biochemical factors are the main parameters that regulate NSC survival and differentiation.8

Nevertheless, because the NSC niche in adult mammalians has limited regenerative capabilities, the effective regeneration of the central nervous system (CNS) requires the reconstitution of its embryonic counterpart. During the early developmental phase, the radial glia (RG) is the main NSC that differentiates into neurons and glia. The RG encompass the whole CNS parenchyma and serves as a substrate for neural migration. During the early neurogenesis, blood vessels invade the CNS and interact with the NSCs, giving rise to a neurovascular niche. In the adult brain, the neurogenic RG can be recovered to some extent after a lesion, suggesting an endogenous attempt of reconstitution of the embryonic niche of NSC.9

Glycoproteins have been shown to play a major role in the migration of neural progenitors. In particular, tenascin-C contributes to the stem cell niche in the subventricular zone (SVZ) where it is highly expressed, and some of its main functions are changing the response to growth factors, enhancing sensitivity to FGF2 and decreasing sensitivity to BMP4 which promotes EGF receptor acquisition. A new role for endothelial cells and oligodendrocytes is key for such growth factor production. Many primary neurons such as cerebellar granule neurons, spinal motor neurons and dorsal root ganglion neurons have shown to exhibit increased average neurite lengths when they are cultured in-vitro on polyamide fibre coatings with tenascin-C.4

Platelet-derived growth factor-AA (PDGF-AA) is another potentially useful factor for in-vitro and in-vivo differentiation of neural stem/progenitor cells (NSPCs), and affects the expansion of NSPCs-derived oligodendrocyte population by acting as a survival, proliferation and differentiation factor. Thus, it is promising for use in conjunction with stem cell transplantation.10

Recent research advances have also shown that stem cell lineages other than NSCs can be used for treatment of CNS injuries, in particular the bone marrow mesenchymal stem cells (BMSCs). Some studies described the neural differentiation from BMSCs generated using butylated hydroxyanisole, noggin and retinoic acid. This approach opens new possibilities for the substitution of damaged neurons through so-called transdifferentiation.11

The peripheral nervous system (PNS) can partially recover from injuries caused by acute trauma and diseases such as diabetes mellitus. The nerve growth factor (NGF) is one of the main neurotropic factors for the survival, maturation and differentiation of sensory neurons. It is upregulated by dedifferentiated Schwann cells and has been used in biomimetic materials for neurite outgrowth.12

Bioactive compounds such as growth factors are soluble-secreted signalling polypeptides that can induce variable cellular responses in a biological environment. This type of response can result in cell actions such as control of migration, differentiation or proliferation of a specific subset of cells and cell survival.13

These signalling polypeptides bind to specific transmembrane receptors on the target cells that ultimately elicit an integrated biological response. Growth factors differ from other oligo−/polypeptide molecules in that they exhibit short-range diffusion through the extracellular matrix and act locally due to their short half-lives and slow diffusion.2

Multiple factors such as the diffusion through ECMs, number of target cells, type of receptor and subsequent intracellular signal transduction, and external factors such as the degradation and binding in the ECM, control the effects of these polypeptides. These variables should be taken into account for the design of biomimetic scaffolds and give rise to some of the pharmacological challenges for the use of such scaffolds.3

Two major strategies have been developed for the delivery of signalling polypeptides in bioactive scaffolds2 (Fig. 2):

• •

  Chemical immobilisation of the growth factor into or onto the matrix. This involves the affinity or chemical interactions between the growth-factor-containing polymer substrate and the surrounding tissue.

• •

  Physical encapsulation of the growth factors in the delivery system. This strategy will achieve an in-demand release of the signalling molecules to the surrounding tissue.

Fig. 2.

Schematic representation of the two main tissue engineering bioactive scaffold strategies used to present bioactive compounds/signalling molecules (Growth factors, mitogens, morphogens) to host tissue in order to promote tissue regeneration. (A) Physical encapsulation of the growth factors in the delivery system. This strategy will achieve an in-demand release of the signalling molecules to the surrounding tissue. (B) Chemical immobilisation of the growth factor into or onto the matrix. This involves the affinity or chemical interactions between the growth-factor-containing polymer substrate and the surrounding tissue.

(0.19MB).

One of the most important signalling molecules used as bioactive compounds in regenerative CNS and PNS medicine are neurotropic factors (NF) that are promising compounds for the regeneration of nervous tissue and can be potentially used for the treatment of several diseases related to neuroinflammatory or neurodegenerative processes.14 However, generally, these compounds show either unfavourable pharmacokinetics, or a short half-life, or are unable to cross the blood–brain barrier (BBB). Local injections have shown limited results, and the continuous administration has short duration and requires the use of invasive techniques such as osmotic pumps and direct interstitial therapy as the delivery vehicle, or the use of implants, hindering its application in clinical practice.

The supply of a dynamic dose of substances is one of the main challenges for the use of bioactive scaffolds because the vehicle of the compound should be suitable to the local physiology, and adapt to pH, temperature and proteolysis. Therefore, nano-scaffolds are an excellent option for the delivery of bioactive compounds. Nano-scaffolds are synthetised as reticulated polymers, can be designed with a wide spectrum of morphological and mechanical characteristics, and can be bound to pre-designed molecules for their controlled release. In addition to the emulation of the ECM, the scaffolds can be designed for recruitment, differentiation, proliferation and release of cells. However, the creation of a scaffold system with a controlled and precise release of NF is likely difficult and impractical, and mostly involves the use of polymers that require complex synthesis, in-situ assembly with toxic co-solvents, or the use of external releasants.15

The use of precise chronometrical studies to measure the release of the bioactive compound and the cellular response is not practical and in some cases, these studies cannot be used for the systematic analysis of the performance of these materials. In such a case, ultimately it is necessary for the scaffolds to bind a bioactive compound to obtain biocompatibility with a reproducible morphology and a load of NF.

Another type of materials called self-assembling peptides (SAP) can spontaneously form scaffolds through intermolecular interactions such as the van der Waals and electrostatic interactions and hydrogen bonds. In particular, Arginine–Alanine–Aspartic Acid–Alanine (RADA) has a long history of use in cell cultures and tissue bioengineering and can emulate the local ECM structure. Specifically, in neural tissues, it has been used in the NSCs of rats, mice and humans. It has been demonstrated to be fully biocompatible with NSCs and with hippocampal cells.

The use of controlled release of pharmacological compounds by enzymes had been studied using different approaches such as nano-scaffolds, nanoparticles and hydrogels. Many pharmacological therapies have focused on neuroinflammatory diseases in which different types of proteases are secreted. For example, matrix-metallo-proteinases (MMP) can be designed for specific peptide sequences and adjusted for on-demand release. In particular, MMP-2 has many substrates and a variable activity and is secreted by the active glia during inflammation. Two excision peptides are potentially good for the controlled release of bioactive compounds: Glycine–Proline–Glutamine glycine + Isoleucine–Alanine–Serine–Glutamine (GPQG+IASQ, CP1) that has a high MMP-2 activity and Glycine Proline–Glutamine–Glycine + Proline–Alanine–Glycine–Glutamine (GPQG + PAGQ, CP2) with low MMP-2 activity. The incorporation of these peptides in RADA 4 hydrogels is relatively simple and reproducible because these molecules can be formed during the initial synthesis of peptides with predictable purity.

NF are fragile molecules that depend on their three-dimensional structures for bioactivity, and usually are 10–20 times larger than the SAP molecules. This problematic property can severely affect the trade-off between the function of the NF and the self-assembling capability of the RADA 4 scaffold. The use of peptide analogues (PA) can potentially resolve this challenge with the smaller size of PAs enabling the self-assembly of RADA 4.16

Two promising NF PAs have been investigated. The first is the brain-derived neurotrophic factor (BDNF) associated with the MVG (DP1) sequence, and known as Neuro-Pep-1 that stimulated the release of BDNF and Trkb in H19-7 cells in hippocampal tissue of rats, and has been demonstrated to improve the spatial learning and memory of adult rats. The second is the ciliary neurotropic factor (CNTF) associated with the DGGL (DP2) sequence and modified for its diffusion through the BBB that has been shown to promote neurogeny in the dentate circumvolution and synaptic plasticity in the rat brain.

To date, tissue bioengineering approaches based on the use of electrospun nanofibre mats have been shown to induce the axonal outgrowth and prompt tissue regeneration after a CNS injury. The porous structures of these electrospun nanofibre mats are biologically similar to the ECM and their correspondingly similar microenvironment is favourable for adhesion and cell proliferation. The physico-chemical characteristics of the nanofibres are useful for the regulation of critical physiological and biochemical activities of cells, enabling the nanofibre mats to function as scaffolds for adhesion and support of the cells cultivated ex-vivo as well as their use in the in-vivo setting.17,18

Blue-green microalgae spirulina has been considered to be a highly promising possible food source in the future and contains many bioactive compounds with therapeutic functions. It is expected that the bioactive molecules of spirulina that resemble the ECM can promote the microenvironment conditions for cultivated cells in nanofibre spheres, forming a complex matrix similar to the host tissue.17

ECMs are essential structures that contain a wide variety of bioactive molecules that induce the growth and regeneration of tissues. The development of synthetic ECMs for tissue bioengineering is an active field of study. A wide variety of scaffolding materials based on synthetic and natural materials have been studied for application in neural tissue. However, achieving the desired biochemical and morphological characteristics through modification is difficult for many of these polymers. Generally, these scaffolds require an additional design modification, toxic co-solvents or a chemical trigger in order to maintain or form a suitable structure. One of the major problems in the application of these scaffolds is the lack of methods for their effective biodegradation. Because of this, these materials can potentially lead to chronic inflammation at the site of administration. It is also possible that the degradation products of these biomaterials will be toxic. It has been shown that the peptide derived from laminin isoleucine–lysine–valine–alanine–valine (IKVAV) inhibits the formation of glial scars and induces axonal outgrowth after spinal cord lesions. IKVAV was attached to RADA 4 in order to attenuate the potential glial response regulated by the host and therefore have a beneficial interaction.16

For materials such as polyglycolic acid (PGA) and polylactic acid, another potential risk that should be taken into account is the decrease of local pH induced by the degradation by-products such as lactic acid and glycolic acid. This effect has been shown to be detrimental to the surrounding cells and tissue.

Collagen nerve guidance channels have also been fabricated with good results. However, collagen is difficult to handle during suturing and is expensive; on the other hand, collagen denaturalisation produces gelatine, which is easier to manage and does not express antigenicity in physiological conditions.12

The use of hydrogels is an excellent strategy for minimising risks because they are usually inert to biological processes, are degradation-resistant, can be easily processed, withstand the heat of sterilisation without damage and can be synthetised in many shapes and forms.19

When the scaffolds are combined with treatment strategies such as stem cell therapy, an additional spectrum of risks must be considered and studied. Although stem cell therapy is a highly promising approach for the treatment of many CNS and PNS diseases, the true utility of these approaches in the clinical setting is still unclear because many challenges must still be overcome before large-scale clinical trials can be performed. The potential risks for cell therapy products (CTPs) are related to the inherent complexity and heterogeneous nature of CTPs. The most commonly used products such as pluripotent stem cells (hPSCs), human embryonic stem cells (hESCs) and human-induced pluripotent stem cells (hiPSCs) have inherent risks for potential tumorigenicity which is a key safety concern.20

Tumorigenicity risk has been primarily associated with hPSCs because these cell lines can form teratomas and other types of tumours in immunodeficient animals. This effect is related to several residual, undifferentiated hPSCs that persist in the final CTP, and can initiate tumour development in transplanted patients. It has been reported that other factors such as cell transformation and mutations in proto-oncogenes or tumour-suppressor genes, accumulation of genomic aberrations during the establishment of an hPSC line, or genomic instability following prolonged cell passage also may play an important role in tumour development. Due to these unexpected effects, safety protocols should be developed prior to the use of this therapy in clinical trials.8,20

In stem cell-based therapies for brain injury, the use of bone marrow mesenchymal stem cells (BMSCs) offers several advantages because BMSCs show a strong safety record in clinical trials, absence of tumorigenicity and ethical controversies, and relatively easy isolation and growth. Their potential use in brain injury treatment will be carried out through the induction of neural transdifferentiation.11

Due to the above-described possible side effects, awareness of the bioethical implications of these procedures is crucial, and in making therapeutic decisions, balance in the risk/benefit ratio to the patient must be sought at all times and harm to the patient must be avoided to ensure the safety and benefit for the patient.21,22

Regeneration of CNS in adult mammals is extremely limited after an injury, and moreover, several pathological process cause rapid damage of the surrounding nervous tissue accompanied by release of inflammatory mediators, loss of local blood vessels and a cellular immune response that ultimately leads to further axonal injury and cell death.11

Furthermore, the CNS presents other challenging characteristics that must be considered for the creations of biomaterials. The most important is the CNS segregation from the circulating blood by the BBB and the blood spinal cord barrier (BSCB). In normal physiological conditions, these barriers allow the passage of only a few molecules through the tight junctions between the extracellular membranes of endothelial cells and astrocytes. This condition complicates the intravenous delivery of drugs and other therapeutic molecules. For this reason, polymers that can be intrathecally placed show great potential for the delivery of therapeutics into the brain and the spinal cord.

Biomaterial scaffolds are a promising therapeutic approach because they can be used as cell and bioactive compound vehicles that promote the host tissue response and protection, emulating the ECM of the brain with bioactive signals. The concomitant use of stem-cell based therapies can also promote regeneration and neuroprotection. Here, we present a list of the main therapeutic approaches for CNS obtained from the survey of the literature (Table 1).

In the CNS, laminin is a critical protein in ECM and is a major component of the basal lamina surrounding the brain blood vessels. It has functions related to neural stem cell expansion, migration and differentiation, and neural development, and enhances the growth of neurites. Supramolecular nanostructures of laminin mimetic peptide amphiphiles displaying the α1-chain laminin-derived epitope (IKVAV) are self-assembling nanofibres with properties for differentiation of neural progenitors into neurons and transdifferentiation of BMSCs into neuronal lineage.

In another approach, the controlled release of bioactive PDGF-AA was carried out with an injectable nanoparticle/hydrogel drug delivery system in poly(lactide-co-glycolide) nanoparticles. This compound can be potentially used in the treatment of spinal cord injuries due to its activity for NSPC oligodendrocyte differentiation.10

Currently, the field of electronic-brain interface shows great potential, with electronic devices interfacing with the brain used for applications such as monitoring or stimulation of brain areas. Some examples are the use of such devices as interfaces for robotic controlled movements, deep brain stimulators or cochlear implants. To make these devices biocompatible and to aid nerve regrowth, biomimetic materials have been used for the delivery of neurotrophins/anti-inflammatory drugs. The poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel-coated substrates used as a neurotrophin delivery matrix are very useful materials because they can be designed to possess mechanical properties similar to those of brain tissue. It was shown that the use of these substrates enhanced survival and sprouting of dorsal root ganglion cells in vitro.

One of the main challenges in the peripheral nervous system regeneration is the promotion of neural outgrowth and structural guidance of the traumatic nerve gap. An appropriate recovery process can be stimulated with a favourable microenvironment at the site of the injury. Thus, the use of a scaffold that emulates the biochemical signals and structural support of healthy tissue can be an appropriate strategy in this context. Currently used therapeutic strategies such as end-to-end suturing, nerve autografting and local delivery of NGF are only useful in a few select cases. In Table 2, we provide a list of the main therapeutic approaches for PNS assembled from the survey of the literature.

NGF is an important compound for the survival, maturation and differentiation of peripheral nerves, and after injury, the production of this growth factor is upregulated by Schwan cells. Therefore, it has a central role in nerve reconnection following axonal damage. NGF binds to its high affinity receptor TrkA that is expressed by the nerve-cells-activating pathways such MAPK, Akt and PKC.

The binding of this compound in a scaffold can only be achieved through high-affinity epitopes for the stability, increased local concentrations and prolonged release at the site of action. PA nanofibres showed high affinity binding of NGF and good cellular differentiation in a PC 12 cells in-vitro model. This binding was achieved due to its epitope sequence that is crucial for the growth factor affinity. The use of collagen binding epitopes for NGF has also been reported.5

Following nerve regeneration, non-degradable materials used as scaffolds may have deleterious effects due to infection or mechanical impingent. Therefore, biodegradable materials such as collagen may obtain more favourable results, particularly for the use of collagen gelatines. The use of crosslinking agents such as glutaraldehyde allows the modulation of the mechanico-chemical characteristics of gelatine films. NGF, glial cell line derived neurotrophic factor (GDNF) and neurotrophin (NT-3) have been shown to enhance regeneration across nerve gaps. In this case, the gelatine contains primary carboxyl groups on its chain backbones that endow the collagen gelatine with a capability to bind covalently to the amine groups of these neurotrophic molecules with an appropriate cross-linking agent.12

The use of the tenascin-C biomimetic materials has also been explored for the regeneration of spinal cord injuries, and a high expression of this glycoprotein was observed after spinal cord injuries and regions of axon ingrowth. Tenascin-C also plays an important role during brain development by providing guidance to neural progenitor cells. It has been used in self-assembling nanofibre gels formed by PAs, where it encapsulates and provides physical support to the cells. The self-assembly process is triggered upon exposure to the physiological divalent cations such as Ca2+ 4.