Nanotubes and nanowires are nanomaterials that basically represent the quasi-one-dimensional (1D) conductors and semiconductors available for nanotechnology to use. The nanotechnology rush, currently in progress, was generated by nanotubes and nanowires that have evolved into some of the most intensively studied materials (Mao et al., 2013). Carbon nanotubes (CNTs), discovered by Iijima (1991), are exhibiting outstanding mechanical, thermal, and conductive properties. Rolling-up one or more graphene sheets generates CNTs with excellent chemical and thermal stability, extreme electronic properties, large surface area and high mechanical strength while carrying ultralight weight (Ajayan, 1999).

Under well-defined conditions of synthesis, two forms of CNTs can efficiently be prepared: single-wall carbon nanotubes (SWCNTs) or nested multiwall carbon nanotubes (MWCNTs) (Figure 2). Being so close to graphene, CNTs are usually near to atomic-scale perfection making CNTs chemically inert (Enachescu et al., 1999; Bota et al., 2014a,b). Although the CNTs have 1/6th of the weight of steel, similar to graphene under tension, nanotubes are two orders of magnitude stronger than steel. Computer simulations estimation of melting point of nanotubes of about 3700°C is higher than that of any metal, but close to that of graphite. SWCNTs can act as very good conductors of electrons or can show semiconducting behavior, depending on their diameter and the atomic structure of nanotubes. Even the very high thermal conductivity of isotopically pure diamond is expected to be exceeded by that of CNTs (CNTs being excellent conductors of heat) that can be perfectly positioned in the devices to dissipate heat from PC chips. Additionally, CNTs are biocompatible in many environments, similar to the related graphite.

Different technological fields are witnessing great developments in CNTs devices based on their unique properties: probes, conductive composites, nanometer sized semiconductor devices, field emission displays and radiation sources, hydrogen storage media, sensors, energy storage, and energy conversion devices (Sharma and Ahuja, 2008). Functionalization of the CNTs surface was performed during the last decade using many different approaches, some of which focused on increasing CNTs solubility and lowering their toxic effects in order to fit biomedical applications (Bianco et al., 2011), because CNTs were showing poor solubility and apparently high toxicity (Fabbro et al., 2012). CNTs have been proposed either by themselves or as components for biosensors (Wenrong et al., 2010), ion channel blockers (Park et al., 2003), biocatalysts (Feng and Ji, 2011), photo-thermal probes in cancer therapy (Moon et al., 2009), nanovectors (Klumpp et al., 2006) and imaging applications (Kam et al., 2005a; Wu et al., 2005; Klumpp et al., 2006).

A nanotube can be conjugated with multi-functional agents (Wu et al., 2005). The particularity of CNTs to have a higher surface area to volume ratio compared to spheres allows high loads of therapeutic agents (Kam et al., 2005a). CNTs are very good candidates for new delivery vehicles as well increasing the therapeutic effect of drugs. Viral vectors, lipids (positive charged), polymers, liposomes, and NPs represent the previous delivery vehicles. However, reduced penetration into the cell of certain therapeutic agents is one issue of concern despite the versatility of shape, size and materials of non-viral vehicles (Endo et al., 1990). After surface modification via functionalization, CNTs show low cytotoxicity as measured over a few days (Kam et al., 2004; Lu et al., 2004; Pantarotto et al., 2004a,b; Cai et al., 2005; Kam and Dai, 2005; Kam et al., 2005a; Wu et al., 2005) while CNTs are readily internalized by cells.

In addition, single-walled carbon nanotubes (SWCNTs) show confined heating at near infrared (NIR) absorption because SWCNTs absorb strongly in NIR wavelengths range (Endo et al., 1990). In his work, Kam et al. (2005b) showed that SWCNTs released DNA upon exposure to NIR radiation, which permits its translocation into the cell nucleus. Cell death was demonstrated in the same study by using the same technique. SWCNTs were adopted by folate labeled cells, increasing the CNTs functionalization using a folate moiety.

The transport of nanotubes into cells is of fundamental importance for the biomedical applications mentioned above and below. As yet, the way in which CNTs enter cells is still under hot debate, generating both controversy and confusion about the mechanism of entering cells. Bianco et al. (Pantarotto et al., 2004b) suggested that ammonium-functionalized SWCNTs and MWCNTs are formed via a passive, endocytosis-independent mechanism; however, Dai et al. (Kam et al., 2004) came to the conclusion that the mechanism of the acid-functionalized SWCNTs entering the cell involves an endocytosis pathway. Another mechanism proposed for MWCNTs, which cannot use the endocytosis pathway due to their size takes into consideration the flipping of lipid molecules of the membrane to allow CNTs to enter the cell (Kam et al., 2004; Lopez et al., 2004; Lu et al., 2004; Pantarotto et al., 2004a,b; Cai et al., 2005; Kam and Dai, 2005). The communication between cell and nanotube is constrained by the type of coating on the nanotube surface. By transferring CNTs into cells, proteins are absorbed to the nanotube surface, coating the nanotube with serum-containing proteins, such as albumin and fibronectin. It has been suggested that the CNT transfer into cells has a natural switching mechanism of lipids in the membrane (Lopez et al., 2004; Pantarotto et al., 2004a) and to not exceed an endocytotic pathway for the MWCNTs that are 200 nm in length with 10 nm radius.

An open subject remains regarding what happens to CNTs once they have entered the cell and also about when or how they would be exocytosed by the cells (Sakhtianchi et al., 2013). The possibility for the nanotubes to be subsequently expelled from the cell would be advantageous for most biological applications; however, as yet, this has not been reported in the literature. There is still much work necessary to understand the CNTs cellular transport in order to be able to control the CNTs placement inside cells.

In biology the range of length scale varies by orders of magnitudes—from nanometer sized nucleic or amino acids to several centimeters for organs and neuronal circuits. There is a need for interfaces with nanoscale spatial resolution in order to investigate processes at the subcellular level. Besides carbon nanotubes, these interfaces can be achieved through the use of other nanostructures, such as semiconductor nanowires (NWs). With dimensions that are as small as a protein molecule, CNTs and NWs present the building blocks for nanoscale electronics (McEuen et al., 2002; Lieber, 2003). The critical feature sizes (atomic scale) of these building blocks can be well-controlled during synthesis, in contrast with nanostructures fabricated by “top-down” process. Even for isolated CNTs transistors that have shown exceptional properties (Javey et al., 2003), large scale integration challenges remain due to difficulties in preparing pure semiconductor nanotubes. The issues faced by CNTs could be overcome by nanowires because of the reproducible control over size and electronic properties that current growth methods enable (Cui and Lieber, 2001; Cui et al., 2001a, 2003; Wu et al., 2004). A wide class of NWs have been developed, ranging from NWs based on classic semiconductors, such as silicon NWs (Chen et al., 2006a; Goncher et al., 2006; Yajie et al., 2008), GaP (Dujavova-Laurencikova et al., 2013), GaN (Lee et al., 2007), CdS and ZnS (Barrelet et al., 2003), heterostructures as Ge-Si (Xiang et al., 2006a,b), InAs-InP (Jiang et al., 2007), oxide nanowires MgO (Yin et al., 2002), Cu₂O (Jiang et al., 2002), SiO₂ (Yu et al., 1998; Liu et al., 2001; Zheng et al., 2002), Ga₂O₃ (Wu et al., 2000; Sharma and Sunkara, 2002), Al₂O₃ (Valcarcel et al., 1998; Xiao et al., 2002), In₂O₃ (Li et al., 2003), SnO₂ (Dai et al., 2001), MnO₂ (Wang and Li, 2002), Sb₂O₃ (Guo et al., 2000), TiO₂ (Seraji et al., 2000; Miao et al., 2002), ZnO (Tian et al., 2003; Vayssieres, 2003), and LiNiO₂ (Zhou et al., 2002).

The field-effect transistors (FETs) configuration of semiconductor NWs is one of the most appropriate detection schemes (Cui and Lieber, 2001; Cui et al., 2001b, 2003; Zheng et al., 2004; Xiang et al., 2006a). Binding to the dielectric gate of a polar/charged species appears analogous to applying a voltage to a gate electrode. For example, accumulation of positive carriers (holes) together with an increase/variation in device conductance can be generated by binding a protein with negative charge to the surface of a p-type FET. Silicon based NWs (or composed of other types of semiconductors) also may function as FET devices (Cui and Lieber, 2001; Cui et al., 2003; Lieber, 2003; Zheng et al., 2004; Li et al., 2006; Xiang et al., 2006a). One-dimensional morphology of NWs is the main feature that determines overcoming sensitivity limitations for planar FET devices. Thus, a more substantial change in device conductance for the NW vs. a planar FET will take place if any analyte binding event will happen (this event leads to accumulation or depletion of carriers).

One of the most powerful and versatile platforms based on NWs devices has emerged to build functional interfaces for biological (including neurons) systems. NWs are non-invasive (highly local) probes of neuronal projections; individual NWs devices are becoming optimal for interfacing with neurons due to the fact that the contact length along the axon (or the dendrite projection crossing a NW) is just about 20 nm. Compared to other electrophysiological methods, with micro-fabricated electrodes and planar FETs, the active junction area for NWs devices is orders of magnitude smaller and is quite similar to natural synapses. This small size creates advantages, such as: (a) spatially resolved signal detection without complicated averaging of extracellular potentials that change over a large portion of a given neuron, and (b) integration of axon's elements together with the dendrite projections from a single cell. The stimulation of neuronal activity through NW/axon junctions is also achievable using NWs devices. Somatic action potential spikes detected with intracellular electrodes, are generated by applying excitatory sequences of biphasic pulses to the NWs of NW/axon junctions (Patolsky et al., 2007).

Also, NW-based FET device can be designed into a device array; neuron growth over dense NWs device arrays is usually achievable nowadays (Patolsky et al., 2007). Thus, interfacing ensembles of NWs inputs and outputs to different neural networks and neurons enables the implementation of stimulation, inhibition, or reversibly blocking signal propagation through specific pathways (while the signal flow is simultaneously mapped throughout the network). Besides single NW-based FET devices or arrays of NW-based FET devices used for investigating neuronal activity, the NWs are also used to design and build NWs-based electrodes for neural recordings in the brain.

The potential to revolutionize neuroscience research and clinical therapy (Benabid, 2007; Kipke et al., 2008; Vaadia and Birbaumer, 2009; Suyatin et al., 2013) is represented even by implantable neural interfaces (Rutten, 2002; Fromherz, 2003; Cogan, 2008). However, the recorded neurons and tissue reactions that encapsulate and insulate the implant are still presenting instability results (Schouenborg, 2011). The nanostructured electrodes are considered as a promising alternative to conventional neuronal interfaces because the recording properties depend, primarily on electrode surface properties and tissue reactions to the surface (Kotov et al., 2009; Timko et al., 2010; Dvir et al., 2011; Voge and Stegemann, 2011; Suyatin et al., 2013). Nanostructured electrodes provide additional advantages such as improved electrical properties (Keefer et al., 2008; Cellot et al., 2009; Martin et al., 2010; Ansaldo et al., 2011; Duan et al., 2012), shorter cell-to-electrode distance (Tian et al., 2010; Duan et al., 2012; Xie et al., 2012), as well as a better spatial resolution. They also have a potential for less tissue damage (Almquist and Melosh, 2010; Martin et al., 2010; Tian et al., 2010; Duan et al., 2012), better biocompatibility (Hallstrom et al., 2007; Kim et al., 2007; Martin et al., 2010; Berthing et al., 2011) and new functionalities, such as selective guidance of neuronal fibers (Hallstrom et al., 2009). Importantly, recent cell signal recordings with different nanowire-based electrodes have been achieved in vitro (Tian et al., 2010; Timko et al., 2010; Brueggemann et al., 2011; Dvir et al., 2011; Duan et al., 2012; Robinson et al., 2012; Xie et al., 2012), demonstrating the epitaxially grown wires of small diameter may provide minimally invasive tissue penetration (Kawano et al., 2010; Takei et al., 2010; Tian et al., 2010; Duan et al., 2012; Xie et al., 2012). Up to now, using mainly carbon nanotubes without structural feature control and in combination with rather big surfaces has been performed with in vivo studies using nanostructured neuronal electrodes (Keefer et al., 2008; Ansaldo et al., 2011; Suyatin et al., 2013). However, recently, it has been shown that with neuronal interfaces for improved cell survival (Hallstrom et al., 2007) and improved cell adhesion with focal adhesions forming specifically on the nanowires, the epitaxially grown gallium phosphide (GaP) NWs have beneficial properties (Prinz et al., 2008). Compared to other material NWs, GaP NWs can be synthesized with very little tapering and exceptional control over their position and geometry, and with a high aspect ratio (over 50) (Suyatin et al., 2009). Also recently, the design and fabrication of a first generation of GaP NW-based electrode with a controllable nanomorphology was reported (Suyatin et al., 2013). The first functional testing in vivo of a NWs-based device was performed during acute recordings in the rat cerebral cortex, where the NWs were used as a backbone for a metal nanostructured electrode with a three-dimensional (3D) structure. This electrode design opened the development of a new model system, with the prospect of enabling more reliable tissue anchoring as well as a more intimate contact between the electrode and the neurons, Xie et al., (2010) furthering research on the functionality of nanostructure-based neuronal interfaces in vivo, given the better electrode-cell electrical coupling (Hai et al., 2010; Robinson et al., 2012; Xie et al., 2012).

In recent years, a broad platform for electronic interfaces with cells and tissue using CNTs and NWs devices has been implemented. Compared to standard techniques used to measure, record and observe extracellular signals from individual tissues and cells, CNTs and NWs devices have orders of magnitude smaller recording area. The mV range signals of CNT/NW platforms device are significantly larger than those measured using planar devices or multiple electrode arrays, likely due to enhanced coupling between the cell membrane and nanoscale device. CNTs and NWs represent also the natural building blocks for biological-nanomaterial interfaces; the creation of hybrid nanoelectronic-neuronal devices is permitting novel directions in neuronal research and applications. The possibility of tuning their material composition and corresponding properties at the time of synthesis is opening up the design of ultra-high sensitivity devices at nanoscale for future opportunities.

Carbon nanotubes have an arsenal of properties (electrical, mechanical, and chemical) that make them very promising materials for applications in neuroscience. The ease with which carbon nanotubes can be patterned makes them very useful for studying the organization of neural networks while the electrical conductivity of nanotubes can provide a vital mechanism to monitor or stimulate neurons. Carbon nanotubes themselves can interact with and affect neuronal function, most likely at the level of ion channels (Malarkey and Parpura, 2007, 2010). Both SWCNTs and MWCNTs have been increasingly used as “scaffolds for neuronal growth.” Lately, CNTs were used in the research of neural stem cell growth and differentiation. Additionally, CNTs were applied as interface materials with neurons, where they deliver electrical stimulation to these cells and detect electrical activity. Here are just few applications of the CNTs:

Interfacing cultured neurons with carbon nanotubes. To demonstrate that the electrical simulation produced by single-wall carbon nanotubes (SWCNTs) can indeed induce neuronal signaling, Mazzatenta et al. (2007) developed an integrated SWCNTs neuronal system and demonstrated that hippocampal cells can be grown on pure SWCNTs substrates. Their experimental results point to the fact that SWCNTs can be directly used to stimulate brain circuit activity. These results may have a remarkable impact on the future developments and architectural design of microsystems for neural prosthetics (Mazzatenta et al., 2007).

Intracellular neural recording with pure carbon nanotubes probes. A novel millimeter-long electrode, remarkably simple, can be used to produce extracellular and intracellular recordings from vertebrate neurons in vitro and in vivo experiments, when it is terminated with a tip fabricated from self-entangled pure CNTs with sub-micron dimensions (Yoon et al., 2013). Assembling intracellular electrodes from CNTs using the self-entangled CNTs fabrication technology is opening a new opportunity to harness nanotechnology for neuroscience applications.

Carbon nanotubes in neuro-regeneration and repair. CNTs based technologies are emerging as particularly innovative tools for tissue repair and functional recovery after brain damage, due to their ability to interface neuronal circuits, synapses and membranes (Sakhtianchi et al., 2013). Carbon nanotube technology can now be applied to develop new devices that are capable to drive repair of nerve tissue, neuronal differentiation, growth and network reconstruction.

Analog neuromorphic modules based on carbon nanotube synapses. Shen et al. (2013) recently reported an analog neuromorphic module consisting of an integrate-and-fire circuit and p-type carbon nanotubes (CNTs) synapses. The CNTs synapse resembles a FET structure using as its gate an aluminum oxide dielectric layer implanted with indium ions and as its channel a random CNTs network. Electrons are attracted into the defect sites of the gate aluminum oxide layer by a positive voltage pulse applied to the gate, followed by a gradual release of the trapped electrons after the pulse is removed. Thus, the electrons induce a dynamic postsynaptic current in the CNTs channel by modifying the holes' concentration. The excitatory or inhibitory postsynaptic currents generated by the multiple input pulses via excitatory or inhibitory CNTs synapses flow toward an integrate-and-fire circuit which triggers output pulses. Further, the analysis of the dynamic transfer functions between the input and output pulses of the neuromorphic module are performed. An emulation of biological neural networks and their functions could be implemented by scaling up such a neuromorphic module.

Nanotechnology and nanocomputing. The last decade in nanotechnology research witnessed an increasing use of artificial intelligence tools (Sacha and Varona, 2013). Current and future perspectives in the nanocomputing hardware development can boost the field of artificial-intelligence-based applications. Moreover, convergence of the two sciences, i.e., nanocomputing and nanotechnology, has the potential to shape research directions and technological developments in medical and information sciences. The great potential of combining nanotechnology and nanocomputing is also shown by hybrid technologies (i.e., nanodevice and biological entities), neuroscience, bioengineering combined with new data representations and computer architectures and a large variety of other related disciplines.

Carbon nanotubes present a broad regenerative spectrum from nerve tissue repair to bone tissue engineering (Newman et al., 2013).

Neuroregeneration and repair. Development of future strategies for tissue repair in order to promote functional recovery after brain damage is one of the main aims of nanotech studies (Fabbro et al., 2012). In this framework, particularly innovative tools are emerging based on CNTs technologies due to their ability to interface with neuronal circuits, synapses and membranes, as well as due to the outstanding physical properties of these nanomaterials. CNTs technology is now applied to the development of devices able to drive nerve tissue engineering, focusing in particular on growth and nerve network reconstruction, neuronal differentiation and nerve tissue repair.

Arslantunali et al. (2014) constructed a nerve conduit from poly (2-hydroxyethyl methacrylate) (pHEMA) that was loaded with MWCNTs. This pHEMA guide was more hydrophobic and more conductive than pristine pHEMA hydrogel when loaded with relatively high concentrations of MWCNTs (6%, w/w in hydrogels). The neuroblastoma cells seeded on pure pHEMA lost their viability when an electrical potential was applied, while MWCNTs carrying pHEMA maintained their viability, demonstrating that MWCNTs are conducting electricity, making them more suitable as nerve conduits. CNTs are instrumental in regeneration and reparation of irreversibly diseased or damaged nerve tissues in the peripheral and central nervous system of the human body (Stankova et al., 2014).

A class of ideal biomaterials for a wide range of regenerative medicine applications is MWCNTs polymer composites because they are hybrid materials combining numerous electrical, mechanical and chemical properties. Using a composite as a substrate to increase electronic interfacing between neurons and micro-machined electrodes (Antoniadou et al., 2011) reported the synthesis and characterization of a novel biomaterial for the development of nerve guidance channels in order to promote nerve regeneration, opening up potential applications for prosthetic devices, neural probes, and brain implants.

Stroke damage repair. Disruption of tissue architecture happens as a result of a stroke. However, in a rat stroke model, amine-modified SWCNTs protect neurons from injury. CNTs used as scaffolds in brain tissues and neural cells have shown promising results, supporting the treatment strategy based on transferring stem cells containing scaffolds to damaged regions of the brain. In rats with induced stroke, protection of neurons and enhanced recovery of behavioral functions were observed for the rats pretreated with amine-modified SWCNTs (Lee et al., 2011a). Also, the amine-modified SWCNTs protected the brains of treated rats, as indicated by the low levels of apoptotic, angiogenic and inflammation markers. In another study, it was shown that CNTs promote recovery from stroke when they are impregnated with neural progenitor cells in subventricular zones. The improvement of stem cell differentiation to heal stroke damage assisted by CNTs was first demonstrated by Moon et al. (2012).

Cancer and brain tumors therapy. It has been shown that the intratumoral delivery of low-dose of free CpG is less effective than immunostimulatory CpG oligodeoxynucleotides conjugated with (CNT-CpG) (Adeli et al., 2012). Moreover, CpG oligodeoxynucleotides conjugated with (CNT-CpG) was shown to induce antitumor immunity that protect mice from subsequent systemic or intracranial (i.c.) tumor rechallenge as well as eradicating i.c. gliomas. Also, it was shown that the treatment of metastatic brain tumors could be more efficient using the same “intracerebral immunotherapy” strategy. Thus, compared to systemic therapy generating antitumor responses that target both brain and systemic melanomas, the intracerebral CNT-CpG immunotherapy is more effective. Moreover, CNTs potentiate CpG.

A novel type of nanoprobe employs SWCNTs as a photosensitizer for application in cancer cell imaging and therapy. Ou and Wu (2013) developed a nanoprobe based on SWCNTs and a fluorescent photosensitizer pyropheophorbide (PPa) for use in cancer cell imaging and therapy in vitro. Phospholipids bearing polyehylene-glycol modified SWCNTs which provide an interface for the conjugation of PPa were prepared by ultrasonication. The polyehylene-glycol modified SWCNTs were then conjugated with PPa using covalent functionalization methods to construct a SWCNTs-PEG-PPa nanoprobe. The functionalization of SWCNTs was evidenced by UV-vis absorption spectra and fluorescence spectra. Imaging cancer cells with SWCNTs-PEG-PPa nanoprobe was confirmed using two cancer cell lines via laser scanning confocal microscope tests, and killing cancer cells with SWCNTs-PEG-PPa was demonstrated using cytotoxicity tests. This indicated that the resulting SWCNTs-PEG-PPa nanoprobe has great potential to be a potent candidate for cancer imaging and therapy. Also, CNTs-polymer interactions play a key role in cancer therapy (Adeli et al., 2012).

One of the major challenges for nanotechnology deals with the diagnosis and treatment of BBB-related dysfunctions involving stroke, brain tumors and cancer. Tight junction (TJ) barriers protect the CNS. These barriers are located in three main locations inside CNS: the brain endothelium, the arachnoid epithelium, and the choroid plexus epithelium (Figure 3, Abbott et al., 2006). BBB consists of endothelial cells connected by close fitting junctions that separate the flowing blood from the brain extracellular fluid. Therefore, BBB controls the entrance of biomolecules into the brain and protects the brain from many common bacterial infections. However, the BBB presents a few limitations for nanomedicine approaches. For instance, due to the presence of BBB, the drug delivery to the brain area for tumor therapy or other neurodegenerative diseases such as Alzheimer's is a crucial challenge. The majority of diagnosed brain tumors are currently treated with surgery, radiation, and chemotherapy; nanoscience and technology could be a promising solution to this challenge. There are several comprehensive reviews on the interaction of BBB with nanomaterials that focus on various methods to transfer nanomaterials across BBB (Chen and Liu, 2012; Khawli and Prabhu, 2013; Krol et al., 2013).

Figure 4 (Chen and Liu, 2012) presents the main, well-recognized, transport pathways across BBB, which are commonly used for carrying solute molecules. Among all the pathways shown in Figure 4, the “g” route is the most suitable for drug delivery via liposomes or nanoparticles. A summary of the conventional methods used for BBB permeability assessment is given in Stam's work (Stam, 2010).

Different approaches and routes possible for transport of drugs across the BBB as summarized in Table 1. Biocompatible nanomaterials such as nanoparticles, liposomes, and supramolecular aggregates are promising drug carriers since their size can be tuned to fit the BBB transport. In addition, their surfaces can be functionalized to facilitate their transport through the BBB. It should be mentioned that the cytotoxicity of NPs must be precisely monitored, using various well-recognized methodologies (Mahmoudi et al., 2010, 2011a; Mao et al., 2013), to ensure their biocompatibility. The surface functional groups enhance the BBB permeability by various mechanisms such as adsorptive-mediated transcytosis and receptor-mediated transcytosis. As an example, Lactoferrin is a receptor located on cerebral endothelial cells that facilitates the transport of NPs across BBB by receptor-mediated transcytosis (Qiao et al., 2012).

In addition to the brain tumors which require drug delivery to the brain, there are some diseases which are related to dysfunctional BBB (Azhdarzadeh et al., 2013). The BBB diseases are epilepsy, Meningitis, Alzheimer's disease, multiple sclerosis (MS), brain abscess, Neuromyelitisoptica, Progressive Multifocal Leukoencephalopathy (PML), late-stage neurological trypanosomiasis (Sleeping sickness) and De Vivo disease (Mahmoudi et al., 2011b, 2012a; Amiri et al., 2013a). Nanomaterials show promising results for treatment of these diseases. Some of the recent nanomaterials investigations in BBB-related disease are summarized in Table 2.

Several “ignored” factors exist at the nano-bio-interface such as the effects of protein corona, cell “vision,” gradient plasma concentration, and temperature (Laurent et al., 2012; Mahmoudi et al., 2012b; Amiri et al., 2013b; Ghavami et al., 2013). In order to have high-yield NP delivery to the brain environment, these crucial hidden factors should be carefully considered. The protein corona is a tightly formed layer of proteins at the surface of nanomaterials at their entrance into the biological fluids (such as blood plasma) (Monopoli et al., 2012). Thus, the biological species (e.g., cells) interact with the corona-coated NPs, rather than the pristine surface-coated one. In this case, in vitro models evaluating NPs for brain-related diseases should use the corona-coated NPs to reflect the real in vivo situation (Mahmoudi et al., 2012c, 2013a,b, 2014). As mentioned earlier, engineering the surface of drug carriers with functional groups to enhance BBB targeting and transport is one of the main approaches to reach therapeutic agents to the brain environment (as confirmed by in vitro BBB models) (Ragnaill et al., 2011). However, the protein corona may cover the designed functional groups and significantly reduce the ability of NPs to cross through cell barriers (Laurent and Mahmoudi, 2011; Mirshafiee et al., 2013; Salvati et al., 2013). Thus, in order to design NPs with high BBB cross-ability yield, one should control the corona composition in vivo. As the composition and the structure of protein corona depend on the chemistry and the physics of the nanostructured materials (e.g., shape, size and distribution, crystallinity, surface composition, surface functional groups, surface roughness/smoothness, and surface charges), one can tune these characteristics to have desired proteins in the corona composition. For instance, association of apolipoprotein-E in the corona composition could enhance NPs transport across BBB (Wagner et al., 2012). Another important but ignored matter is the effect of temperature on the protein corona. Slight temperature changes may cause considerable variation of protein corona composition at the surface of NPs (Ghavami et al., 2012; Amiri et al., 2013a; Mahmoudi et al., 2013c, 2014). The mean body temperature varies slightly for healthy individuals (mainly in the range of 35.8–37.2°C) but will vary in different parts of the body in different circumstances.

For instance, in the case of disease, the body temperature may have significant variations (e.g., it can reach to 41°C in the case of fever). Therefore, one can expect to have different corona composition at the surface of the injected NPs for different individuals, leading to the various therapeutic effects. In order to have high-yield therapeutic results, the body temperature of the individuals must be tightly monitored/controlled. Additionally, local temperature changes near the surface of NPs (e.g., by laser activation of plasmonic NPs) can change the composition of protein corona (Mahmoudi et al., 2014). Therefore, one can expect that the potential changes in the protein corona following hyperthermia or laser treatment of magnetic and plasmonic NPs may change in vivo toxicity or biodistribution in clinical applications.

By far the best therapy is to prevent damaging/degenerative effects of any kind. For example, rare earth NPs prevent retinal degeneration (Chen et al., 2006b). It is believed that in blinding diseases such as macular degeneration or retinitis pigmentosa, as well as stroke, Alzheimer's, atherosclerosis, diabetes and other disorders, these NPs could efficiently inhibit cell death. Therefore, a unique technology for multiple diseases can be created by using NPs as a novel strategy to direct therapy for various disorders. Other applications of NPs are as follows:

Improvement in cerebrovascular dysfunction following traumatic brain injury. Cerebrovascular dysfunction that is characterized by a decrease in cerebral blood flow (CBF) is a critical factor that worsens after traumatic brain injury (TBI). To improve cerebral dysfunction, a new class of antioxidants (nontoxic carbon particles) based on poly(ethylene glycol)-functionalized hydrophilic carbon clusters (PEG-HCCs) has been developed. This was demonstrated in a mild TBI/hypotension/resuscitation in rat when administered during a clinical relevant event: the resuscitation (Bitner et al., 2012). A concomitant normalization of superoxide and nitric oxide levels was also noticed. This is highly relevant for patient health improvement under clinical conditions requiring resuscitative care as well as in circumstances of stroke and organ transplantation.

Primary brain tumors: diagnosis and treatment. Since glial tumors seem to be able to create a favorable environment for the invasion of neoplastic cells into the cerebral parenchyma when they interact with the extracellular matrix via cell surface receptors, the prognosis in patients affected by primary brain tumors is still very unfavorable. The major problem for drug delivery into the brain is due to the presence of BBB as discussed above. NP systems can represent ideal devices for delivery of specific compounds to brain tumors, across the BBB. The results described by Caruso et al. (2010) shed light on the emerging novel applications of NP systems in diagnosis and treatment of primary brain tumors, and also on the NP systems as drug delivery carriers in brain tumor diagnosis and therapy.

Applications of boron-enriched nanocomposites in cancer therapy. Nanocomposites have stirred much attention due to their applicability in cancer therapy. In particular the isotope ¹⁰B, has a unique ability to absorb a slow neutron in order to initiate a nuclear reaction with release of energetic Li-particles that was not observed in the carbon analogs. The nuclear capture reaction principle has been successfully applied in radiation therapy and further used in boron neutron capture therapy (BNCT). Thus, BNCT may be applied as a promising treatment for malignant brain tumors and in other types of cancer, regardless of the limitation in neutron sources (Yinghuai and Hosmane, 2013). Recent research demonstrated that such development of “boron-based therapeutic nanomaterials” based on BNCT agents holds promise for cancer therapy.

Electroless deposition can be used to create nanoelectrodes in templates that generally speaking are not conductive. Electroless deposition can be used to create nanotubes or nanowires by coating the nanopore wall or by filling up the pore with the material of interest. Nanocables with radial junctions can also be produced (e.g., Au/Te, Ku et al., 2005; Vidu et al., 2006). Slow electroless plating (no mass transfer limitations) allows for a uniform metallic film, where the metal deposition occurs uniformly at the pore walls creating hollow metallic nanotubes inside the pores (Hou et al., 1998; Bergquist et al., 2001; Bercu et al., 2004; Yuan et al., 2004a,b,c).

In recent years, electrochemical nanotechnology (nanoelectrochemistry) has become a key technology due to the scale up potential and low energy consumption. Typically, a decrease in the size of an electrode causes changes in the diffusion layer from linear to spherical form. For multiscale nanostructures such as nanotubes, nanofibers, and nanocables, it is important to know which characteristic length scale, nm or μm, governs the deposition process (Lebedev et al., 2005). For the μm scale, diffusion limitations can be important if the surface deposition processes are relatively fast.

Electrochemical template synthesis is mainly used to create arrays of nanoelectrodes (Wirtz et al., 2002a,b; Wirtz and Martin, 2003; Ku et al., 2004; Quach et al., 2010; Vidu et al., 2012). Both axial and longitudinal growth of nanocables with p-n junctions can be produced. Figure 5 illustrates the process of creating nanocable structures using a combination of electroless and electrochemical deposition. There are several advantages associated with nanoelectrode arrays, which usually display a small potential drop. This behavior makes possible electrochemical measurements at low electrolyte concentrations. The small size of the nanoelectrodes array maintains a steady-state current and has a high ratio of signal to noise. This property is mainly used in sensors, where the sensitivity of the device could increase more than 100 times. Furthermore, one of the many advantages for preparing multilayered nano-sized materials is that the electrodeposition can be performed at room temperature, which is very important for systems in which undesirable interdiffusion occurs between the adjacent layers.

Electrochemical deposition can also be used to synthesize conductive polymer nanotubes and nanowires, such as polypyrrole, polyaniline, or poly(3-methylthiophene). In this case, the pore walls are favorable sites for nucleation and growth, resulting in polymeric tubules. Various polymeric structures such as thin-walled tubules, thick-walled tubules or solid fibrils can be obtained by simply controlling the polymerization time (Brendel et al., 1997; Cepak et al., 1997; Demoustier-Champagne et al., 1998, 1999; Demoustier-Champagne and Legras, 1998; Demoustier-Champagne and Stavaux, 1999).

However, more recent developments have suggested that a more sophisticated architecture of the NCs arrangement cannot be achieved using commercial templates. More complex NCs or NWs configurations for certain applications in neuroscience can use other technologies to create particular nanostructured arrays for nanoelectrodes, such as nano-imprinting. This is particularly important in designing new nanoelectronics for applications in neuroscience.

Carbon nanotubes (CNTs) array chips are now used for non-invasive measurement of action potentials, real-time concentration of dopamine, and postsynaptic potentials. Suzuki et al. (Suzuki et al., 2013) developed MEA chips of planar CNTs that can measure both the electrophysiological responses (such as action potentials and field postsynaptic potentials) and release of the dopamine neurotransmitter. These CNTs-MEA chips have been fabricated directly on the microelectrode surfaces by electroplating an indium-tin oxide. Chronoamperometric measurements based on such CNTs-MEA chips detected dopamine concentration at nanomolar level with high temporal resolution and a 100-fold better signal to noise ratio. MEA chips may be useful for various applications such as drug screening and toxicity, in vitro stem cell differentiation, synaptic plasticity, or pathogenic processes associated with stroke, epilepsy, Alzheimer's and other neurodegenerative diseases.

MEAs using MWCNTs have the advantage of decreased physical size of microelectrode with increased impedance and decreased charge-transfer capability (Gacem et al., 2013). To decrease impedance, the effective surface area for recording of the electrode needs to be increased. With a steam-plasma treatment the surface of MWCNTs becomes converted from super-hydrophobic to super-hydrophilic. This hydrophilic property is attributed to the OH bonding on the surface of MWCNTs. A multi-walled MEA was employed to record neural signals from a lateral giant cell of an American crayfish. This electrode type allows the separation of neural signals with their distinct shapes for long-term recordings and improved recording performance.

Recent neural probes based on silicon (Du et al., 2011) employed nanofabricated, high-density electrical leads that can read out multichannel data. MEA uses an application-specific integrated circuit (ASIC) to intensify signals, multiplexing functions and band-pass filtering. Multiplex high density devices with a fully integrated low noise, 64-channel system can perform high spatial resolution extracellular measurements and weighs just 330 mg (Du et al., 2011). These on-chip multiplexers allow recordings with “substantially fewer external wires than the number of input channels.” The combination of ASICs and nanofabricated probes that is both “minimally invasive and highly scalable” (Du et al., 2011) was employed for carrying out large-scale, high-density electrophysiology in small animals.

Similarly, Viventi et al. (2011) integrated “ultrathin and flexible silicon nanomembrane transistors” into a MEA, enabling “dense arrays” of thousands of amplified and multiplexed sensors to be connected with fewer wires. This system was employed to record in cat the spatial properties of brain activity in vivo, including patterns of activity like sleep spindles, single-trial visual evoked responses or electrographic seizures. These developments might provide diagnostic and therapeutic brain-machine interface devices.

A new kind of electrode is based on “structurally controlled nanowires,” for neurophysiological measurements in vivo (Xie et al., 2010). This electrode has a sensing part made of a thin metal layer deposited on epitaxial grown GaP nanowires. Suyatin et al. (Xie et al., 2010) realized the first functional NWs-based electrode. The team also has successfully tested the electrode by in vivo recordings in the cortex of rat in multiple brain implantations. This kind of electrode with a controllable geometry of nanowires now can be further used for the investigation of many in vivo functional properties in nanostructured neuronal interfaces.

Current methodologies permit the simultaneous, long-term non-invasive recordings of extracellular field potentials, but not the sub-threshold synaptic potentials that are generated by single cells. Because intracellular recordings of the electrophysiological properties (sub-threshold action potentials, synaptic potentials and membrane oscillations) can be acquired only by sharp (or patch) microelectrodes, these recordings may be limited to very short durations and single cells at a time. An emerging approach in a number of laboratories is based on the merging of the advantages of extracellular microelectrode arrays with those of intracellular microelectrodes (Spira and Hai, 2013).