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Healthcare, as a basic human right, has often become the focus of the development of innovative technologies. Technological progress has significantly contributed to the provision of high-quality, on-time, acceptable, and affordable healthcare. Advancements in nanoscience have led to the emergence of a new generation of nanostructures. Each of them has a unique set of properties that account for their astonishing applications. Since its inception, nanotechnology has continuously affected healthcare and has exerted a tremendous influence on its transformation, contributing to better outcomes. In the last two decades, the world has seen nanotechnology taking steps towards its omnipresence and the process has been accelerated by extensive research in various healthcare sectors. The inclusion of nanotechnology and its allied nanocarriers/nanosystems in medicine is known as nanomedicine, a field that has brought about numerous benefits in disease prevention, diagnosis, and treatment. Various nanosystems have been found to be better candidates for theranostic purposes, in contrast to conventional ones. This review paper will shed light on medically significant nanosystems, as well as their applications and limitations in areas such as gene therapy, targeted drug delivery, and in the treatment of cancer and various genetic diseases. Although nanotechnology holds immense potential, it is yet to be exploited. More efforts need to be directed to overcome these limitations and make full use of its potential in order to revolutionize the healthcare sector in near future.
Keywords: nanotechnology, nanosystems, healthcare, cancer, gene therapy, genetic disorders, drug delivery
Nanobiotechnology, a recently coined term, emerged from the blending of molecular biology and nanotechnology. It is a branch of science which revolves around structures or functional materials at the nanoscale, which are produced by employing both physical and chemical methods [1]. In the last thirty years, the discipline of nanotechnology has been a crucial area of research, due to the unique chemical, electrical, optical, biological, and magnetic properties of nanomaterials [2]. Nanotechnology has managed to attract a lot of attention, because it is an established fact that when nanotechnology joins hands with biotechnology, they give birth to a platform which holds immense potential and importance with respect to diversity in applications [3]. Some of these applications include medical imaging, diagnostic kits, diagnostic assays, biological sensors, dentistry, sterilization of medical device surfaces, sunscreens, cosmetics, sports equipment, textiles, environmental cleanup, and gene inactivation [1,4,5]. The development of nanotechnology has provided mankind with some incredible tools that allow the delineation of processes to a degree which was considered to be next to impossible a few years ago [6].
Various types of nanoparticles (NPs), such as metal, metal oxide, semiconductor, organic, and inorganic NPs, have been synthesized in order to exploit their properties. They can be formed via different procedures such as conventional chemical production and green synthesis processes [7]. Associated with toxicity, cost and efficiency, chemically produced NPs pose many problems. Thus, because of their ease of production, low cost and toxicity, bio-inspired NPs hold an edge over traditionally produced NPs [8]. The high cost of raw materials, drug wastage, chemical and physical incompatibilities, clinical drug interactions, and the occurrence of side effects associated with the dose, are the vital limitations of conventional approaches [9].
Generally, NPs range in size from 1 to 100 nm but some exceptions also exist [10]. For example, in medicine, NPs range in size from 5 to 250 nm [11]. There are also some nanosystems that may exceed several micrometers in size, e.g., liposomes. The definition and classification of NPs are continuously evolving as this field is progressing day and night. Adapting the technical and translational information on nanomaterials and nanotechnology from the US National Nanotechnology Initiative and European Commission, the authors feel that it is imperative to mention that the upper size limit of NPs cannot be restricted to 100 nm [12]. In fact, some commercial nanomedicine products are greater than 100 nm, e.g., abraxane (130 nm) and Myocet (180 nm). Therefore, we can limit or specify the range of nanomaterials only on the basis of their sizes [11].
Exceptionally small sizes enable NPs and nanodevices to exhibit novel properties and functions. It should be kept in mind that the small size of NPs gives them another advantage, perhaps their main advantage, which is that they have a very high surface area-to-volume ratio. This may sound trivial but this property actually makes them more reliable and reproducible [13]. In addition, they show enhanced catalytic activity, chemical stability, and thermal conductivity and non-linear optical performance [3]. Various NPs can be developed into nanosystems via modifications in their shape, surface properties, and size to efficiently utilize them in the imaging, diagnosis, and treatment of serious diseases. Controlled released therapy can be provided by means of these functionalized nanomaterials which send drugs to particular sites or tissues [14]. In order to optimize and promote tissue and cell interaction, some factors, such as charge, size, the pattern of nanoscale medical molecules, and shape, need to be modulated and investigated [15].
Nanotechnology products have become increasingly useful in healthcare and have led to the advent of novel nanosystems for the diagnosis, imaging, and treatment of various diseases, such as cancer, as well as cardiovascular, ocular, and central nervous system-related diseases [16,17,18]. Nanomaterials integrate well into biomedical devices because most biological systems are also nanosized [5]. In the field of drug delivery, nanosystems offer the precise delivery of drugs to the target tissues or organs with a controlled release and enhanced retention time as compared to conventional techniques. Nano-liposomes are one of the best examples of the nanosystems currently developed for targeted drug delivery to treat various types of cancers and cardiovascular diseases [9,14]. Drug delivery to target tissue, good biocompatibility, and the control of drug flow in the bloodstream are the most significant reasons for the usage of nano-liposomes [9].
Advents in nanomedicines and nanodevices has inspired numerous researchers to look for alternative therapies, as the currently employed methods are limited in terms of earlier detection and treatments. The astonishing properties and applications of various nanomaterials and nanosystems have made them pervasive in the development of technologies to be implemented in the near future. The purpose of this review is to provide readers with information about the most recent applications of nanotechnology in various healthcare sectors in one place. Furthermore, we also critically discuss the limitations, challenges, and future prospects of nanotechnology in allied healthcare systems.
Nanotechnology revolves around some common nanostructures, no matter what field or area of application is concerned. Some of the important ones are nanoparticles, carbon nanotubes, dendrimers, nanoprobes, quantum dots, nano-diamonds, and nanowires ( Table 1 ). Nanoparticles possess unique characteristics and their strikingly small size makes them able to cross microscopic pores and membranes easily. Nanoparticles are broadly classified into five categories, including metal, lipid, ceramic, polymeric, and semi-conductor NPs. Metal NPs are made out of metal precursors. These in particular have unique optoelectrical properties [19]. Ceramic NPs are inorganic and nonmetal NPs are found in amorphous, polycrystalline hollow and dense forms [20]. They are efficient catalysts and help in the photodegradation of dyes and imaging technologies [21]. Semiconductor NPs have properties of both metal and non-metal NPs; hence, they also find applications in numerous fields such as photo-optics and electronic devices [22,23]. Polymeric NPs are organic NPs, which are either matrix particles—that are generally solid which can adhere to molecules to be transported—or are encapsulated within the particle [24]. Lipid NPs contain moieties that are lipid in nature. These are usually spherical in shape and diameters range from 10 to 100 nm. They have a solid lipid core and lipophilic molecules can be transported easily.
Applications of various nanostructures in healthcare sectors.
| Nanostructure | Applications in Health Sector | References |
|---|---|---|
| Nanoparticles | Used as antimicrobials and antifungals; used as sensors, as catalysts, and for imaging in diagnostics | [5,21] |
| Carbon Nanotubes | Used for delivering fibrinogen and bovine protein to cells; serve as vectors for gene delivery; and in the treatment of broken bones, osteoporosis, and breast cancer | [26,27,46,47,48,49] |
| Dendrimers | Used for diagnostic applications, for gene delivery, as anti-bacterial agents, as anticancer drugs, to improve vaccine formulations by acting as carriers of antigens, and in treating ocular diseases. | [32] |
| Nano-Diamonds | Used for the treatment of bone disease by targeted drug delivery (bone regeneration); used in imaging and therapy, in the early detection of cancer, and in the treatment of brain and breast cancers | [40,50,51] |
| Quantum Dots | Useful in diagnostics, real time in vivo bio-imaging, in controlling various diseases, intracellular tracking and therapeutic drug delivery, and to deliver siRNA for RNA interference | [52,53] |
| Nanofilms | Act as useful biological, chemical and nanomechanical sensors in electrochemical devices, used for controlled drug release, used as nanopatches after open surgery to close incisions | [43,44,45,54,55,56,57,58] |
| Liposomes | Used for drug delivery, capable of containing hydrophobic and hydrophilic drugs, protect drugs from chemical and enzymatic degradation, have the ability to encapsulate anti-tumoral drugs, for example, anthracyclines such as epirubicin, daunorubicin, and Dox, etc. | [59,60,61,62,63] |
Carbon nanotubes (CNT) are nanosized, seamless tubes made out of graphite sheets. They have open terminal parts that are closed by fullerene caps. They have the highest mechanical strength out of all natural materials. They are efficient absorbers of magnetic radiation, along with providing the efficient conduction of heat and having catalyzing properties. Their properties are dependent on their purity, length and diameter, special surface area, and amorphous carbon. Carbon nanotubes are included in the fullerene nanotube family and have a rather cylindrical configuration. CNTs also include buckyballs, which are spherical and cylindrical in shape [25]. CNTs are widely employed in modern healthcare systems because they have the potential to overcome hindrances that were previously impossible to address. They can cross partially permeable cell membranes very easily, using a mechanism that is still unclear. They can carry small organic drugs, proteins, peptides, nucleic acids, antibiotics, etc., to precise locations. These small molecules can be either covalently attached, adsorbed, or encapsulated in these CNTs [26]. They can carry protein less than 80 KDa that can be bound either covalently or non-covalently. These are taken up by cells via endocytosis. CNTs also have applications in X-ray imaging [27]. A CNT solution was placed in a laser infrared beam, which was able to heat CNTs up to 158 °F in 2 min. Cells containing CNTs are not destroyed by laser beams since they can absorb near-infrared waves. These lasers can effectively kill cancer cells [28].
Dendrimers are naturally biodegradable nanopolymers. They are macromolecular nanostructures having a 3D globular shape due to the presence of many branched layers. Their small size (1–10 nm), globular structure, and the fact that they can penetrate through cell membranes due to their lipophilic nature make them ideal systems for use in healthcare for gene and drug delivery purposes [28,29,30]. A dendrimer structure consists of three major components—the core made of an atom or a multifunctional molecule, repetitive branching units covalently bound to the core, and many functional groups present at the terminal of the branching units [31,32]. Dendrimers interact with drugs through physical and chemical interactions. The physical interactions (encapsulation of the drug) are due to the presence of empty internal cavities, which bind the drug molecules through hydrophobic interactions [33,34,35]. The chemical interactions occur either through electrostatic interactions (due to the presence of ionizable functional groups in dendrimers) or through covalent bonding [36]. For covalent binding, the dendrimer surface is first mixed with active moieties such as poly-ethylene glycol (PEG) or p-amino benzoic acid, etc. After this, the drugs can successfully conjugate with the dendrimers through covalent bonding [32,37].
Nano-diamonds (NDs) are nanostructures consisting of a single diamond crystal with carbon in the sp 3 configuration. Their particle size is approx. 4–5 nm. NDs are very hard and chemically inert and they have high thermal conductivity and bio-compatibility [38]. They have a tunable surface and a large surface area to which drugs and genes can easily conjugate. The fluorescence produced by NDs makes them useful as imaging probes for diagnostic purposes [39,40]. All these properties of NDs are actually due to the combined characteristics of diamonds and NPs [40]. The structure of NDs consists of two major components—(1) the inner diamond core, with carbon atoms in the sp 3 configuration; and (2) the outer graphitic shell (carbon atoms in the sp 2 configuration), with functional groups on the terminal of dangling bonds [41]. Techniques used for the synthesis of NDs include the detonation of explosives, high temperature, high pressure, and the chemical vapor deposition method [22].
Quantum dots are synthetic nanostructures ranging in size between 1.5–10 nm. their semi-conductor nature allows them to transport electrons. When UV light passes through them, the electrons in the QDs are excited, and when these excited electrons move back to their ground state, they emit light. QDs emit light of different colors depending upon their size [42]. QDs made from heavy metals such as cadmium are very toxic and carcinogenic; therefore, they cannot be widely used in the health sector. However, graphene and carbon QDs are safe and stable and have wide scope in the health sector [43].
Nanofilms consist of polymeric sheets with a large surface area and a thickness of relatively few nanometers (10–100 nm) [44]. Multiple oppositely charged layers are assembled together to form multilayered yet ultra-thin biofilms. Layers are deposited one by one for deposition. Various methods are used for the deposition of individual layers, including fluidic assembly, electromagnetic deposition, spin coating, and emersion [45].
Liposomes are spherical vesicles made up of one or more lipid bilayers with an aqueous compartment in between them [42,43]. They are found in a variety of sizes, starting from as small as a few nanometers, and can be as large as several micrometers [44]. They are capable of entrapping various substances, including hydrophilic and lipophilic agents. Therefore, they are also considered to be the most efficient drug delivery system. Another reason for this is because their composition is very similar to the cellular membranes found in the body, which helps with drug delivery in vitro. Their large size also enables them to deliver a high quantity of drugs [45]. The major domains of healthcare in which nanotechnology-mediated nanosystems are playing their positive role are summarized in Figure 1 .
Schematic presentation of applications of various nanosystems in allied healthcare sectors.
Gene therapy is a procedure to replace a defective gene in the DNA (which is responsible for causing a disease) with a normal gene. The gene is usually inserted into the stem cells using a vector [64]. Stem cells have long life and a self-renewal ability; therefore, they are the most suitable targets for gene therapy [65]. The vector used should be highly specific and efficient in releasing the gene or genes of variable sizes. It should not be recognized as an antigen by the host immune system. The vector must have the ability to express the inserted gene throughout the life of that organism [66,67]. When the gene is correctly inserted into the cells, it inhibits and corrects the functions of the mutated gene and induces the normal functioning of cells [68,69].
Viral vectors have been used for years in gene therapy and are still being used. They can take over the host metabolic machinery for the synthesis of proteins that are coded by their DNA. Furthermore, their insertion in the host genome is very stable, and the transduced cells cause the long-term expression of the transgene. These are the properties that make them suitable for gene therapy [67,68]. Some common and efficient viral vectors include lentivirus, retroviruses, adenoviruses, etc. [67,69,70,71,72,73,74,75,76]. However, there are many risks associated with the use of viral vectors. These include the generation of an immune response, inflammation, and the occurrence of off-target changes in the host body. If the virus triggers the immune response, it not only makes the therapy less efficient but when the same virus enters the body the second time (with the desired gene inserted into its DNA), a secondary immune response occurs, which would rapidly kill the virus, making it impossible to use the same virus for gene therapy [77,78,79,80,81]. Inflammation caused by viral vectors can sometimes be very dangerous, as reported in a recent study in which a leukemic patient died when given a high dosage of adenovirus for gene therapy [82]. Virus virulent genes are deleted prior to therapy, which also compromises the integration and infection ability of viral vectors. Insertional mutagenesis can be life-threatening too, because sometimes these viruses (mostly retroviruses) insert DNA into the tumor-suppressing gene or the oncogenes, activating them to cause tumors in the host body. The selection of appropriate viruses for different body cells is another difficulty in the field of gene therapy. Moreover, viruses can also go through genetic changes with the passage of time, which can lead to other complications in the body [83]. These are some major concerns relating to viral gene therapies, and therefore these methods are not encouraged, and the world is now moving towards the use of nanostructures for gene therapy.
Gene therapy using non-viral nanostructures is safe, as compared to therapy using viral vectors. They are also much less oncogenic and rarely trigger immune responses. Their preparation is much easier than that of viral vectors. There is no risk of virus recombination and no limit on the size of the gene to be loaded. NPs are one of the many nanostructures that are used for non-viral gene delivery. The presence of a positive charge, small size, and high surface-to-volume ratio enables them to penetrate deep into the membranes, thus making them ideal vectors for gene delivery [84,85,86]. The major nanosystems used in gene therapy are shown in Figure 2 .
Graphical representation of various nanosystems used in gene therapy.
One of the ways in which gene therapy treats many diseases is through gene silencing. Various diseases, such as autoimmune disorders, cancers, and viral infections, can be treated by silencing the expression of genes [87]. RNA interference using small interfering RNA (siRNA) has been used for gene silencing. SiRNA is a 21–25 nucleotide long double-stranded RNA molecule. It forms a complex with RNA-induced silencing complex (RISC) in the cytoplasm and targets the directed complementary mRNA molecule, thus silencing its expression [88,89].
This technique can be very useful if the problem of their stable delivery into the cytosol (they become unstable in physiological fluids) and limited intracellular uptake are resolved [90]. This problem can be resolved by using some vector system. Viral vectors are very risky to use, as mentioned earlier. However, non-viral NPs have been used to overcome these limitations [91]. For example, one of the over-expressed proteins in cancer cells is the RhoA protein. Anti-RhoA siRNA was encapsulated in chitosan-coated polyisohexylcyanoacrylate (PIHCA) NPs. When these NPs were administered to mice infected with breast cancer, they showed 90% tumor inhibition with no toxic effects [92].
In rheumatoid arthritis, tumor necrosis factor-α (TNF-α) plays a role in the release of cytokines and thus causes chronic inflammation. A nanocomplex, thiolated glycol chitosan (TGC) polymer loaded with poly-siRNA, was targeted to TNF-α, which proved to be very efficient in curing rheumatoid arthritis. The inhibition in bone erosion and a reduction in inflammation was also observed in mice in that study [93]. These are just a few examples; there are several other studies in which nanostructure-based complexes have been effectively used to deliver siRNA, thus treating various diseases.
In another way, genetic materials (RNA, DNA, siRNA) can be encapsulated or conjugated with NPs for efficient gene delivery [84,94,95,96]. The most efficient way to attach genes with NPs is through the formation of DNA-NP complexes. These complexes are formed by means of the electrostatic bonding between them. For this, the surface charge on the NPs is made positive, which then binds strongly with negatively charged nucleic acids. Liposomal and polymeric and many other nanostructures use this mechanism of gene transfer [85,97,98,99,100]. The encapsulation of genetic material in NPs protects them from enzymatic digestion when they are targeted into the cells. It also protects them from phagocytosis by monocytes [94]. Due to the advantageous aspects of nano-based gene therapy, research is in process on large scale to develop new strategies for its implementation in the healthcare sector.
Nanovectors have great potential in target-specific drug delivery for the treatment of various diseases. Targeted drug delivery is important, especially if the solvents of hydrophobic drugs are toxic. If these solvents are released somewhere else other than the target cell, they may enter the blood stream or other body fluids and contaminate them. Nanostructures allow the continuous controlled release of drugs in desired amounts. Specific and localized drug delivery also reduces drug doses. The small size of NPs allows them to penetrate deep into the tumor cells, and thus they can be useful in improving cancer treatments [94].
The NPs used for drug delivery must contain some important components, including a particle core, an outer biocompatible protective layer and a linking molecule for increased bioactivity (it attaches the core of NPs to bioactive molecules because of the reactive compounds present at both of its ends). Nanovectors are modified before drug delivery and this modification includes coating with ligands such as peptides, folic acid, and antibodies. Ligands are attached to NPs so that they can bind specifically to targeted sites to enhance the specificity even more [16,95,101,102,103,104,105,106]. It is essential to attach more than one ligand because if only one ligand is attached, there is a possibility that it may bind to receptors present in places other than on the targeted site. In addition, tumor cells are usually overexpress, i.e., they have more than one type of surface receptor [17].
Since nanovectors possess unique properties and various modifications can be performed during drug loading, scientists are now moving towards the implement of nanotechnology-based nanosystems for efficient targeted drug delivery with the aim of curing various serious diseases. Some examples of targeted drug delivery using nanovectors are discussed in the following sections.
Cardiovascular diseases cause millions of deaths around the world [18]. Various treatments have improved the survival rate of patients with heart diseases but none of them has achieved complete cardiac regeneration, especially for patients after cardiac infarction [107]. Stem cell therapy can be used for therapeutic angiogenesis [108]. Introducing anti-apoptotic and pro-angiogenic genes into the genetically engineered stem cells can prolong their rate of survival and increase their paracrine secretion [109,110]. Viral vectors cannot be used to deliver genes to stem cells as they cannot carry large gene volumes and have immunogenic effects. Bio-compatible NPs are efficient in transferring genes to stem cells. Various nanostructures can be used for delivering genes to stem cells. Liposomes are one of the best contenders for gene delivery as they can prevent the non-specific binding of genes and protect them from degradation [111,112]. Polymers show improved specificity for targets and higher efficiency [113]. In one study, chitosan alginate NPs were used to deliver growth factors to placental cells. The continuous release of growth factors improved the functioning of cardiac tissues at the site of myocardial infarction [114]. NPs also have the potential for tracking and monitoring stem cells. Superparamagnetic iron oxide nanosystems (SPIONs) are made to enter the cells by attaching to cell surfaces. These cells are then internalized by endocytosis [115]. Quantum dots can also be used for monitoring the living cells for a long time [116,117].
Hypertension is a disease that gives rise to many problems, including myocardial infarction, heart failure, stroke, increased blood pressure, and damage to many body organs, including the eyes, kidney, brain, etc. [118]. Many antihypertensive drugs have been used to treat this, but various problems are associated with the use of these drugs, including their short half-life, low bioavailability, poor solubility in water, unwanted side effects, and many more. Targeted drug delivery using nanostems has been effectively performed in order to solve these problems [119]. Nanocarriers that have been used so far for treating hypertension include lipid carrier NPs, solid lipid NPs, polymeric NPs, liposomes, and nanoemulsions [120]. These are just a few examples, but nanotechnology has very promising applications in treating many other cardiovascular diseases through non-viral stem cell-based therapies. Further studies on the effects of nanovectors in the cardiovascular system of a living model need to be performed before they can be safely used in humans.
The efficient delivery of drugs in the eye is an enormous challenge because of the presence of complex barriers and elimination mechanisms in the eye. The various barriers present include the tear film, the ocular surface epithelium, and the internal blood–aqueous and blood–retinal barriers. NPs are, however, able to overcome these barriers because of their small size and highly variable surface properties. They can efficiently transport the drug to the targeted site with no toxic effects. Most of the NPs are biodegradable, which means they do not require surgical removal after they have delivered the drug [121,122].
Anterior eye diseases, such as cataracts, conjunctives, keratitis, dry eye, corneal injury, etc., are usually treated using eye drops but the corneal barrier causes drugs to have poor bioavailability. However, nanosystems can increase the bioavailability by prolonging the retention time of the drug on the surface of the eye and improving the penetration of the drug [123]. On the other hand, posterior eye diseases in the choroid and retina include retinoblastoma, glaucoma, choroidal neovascularization, macular degeneration, and posterior uveitis. Eye drops are not usually effective in treating these diseases, so interocular injections are performed, which leads to many unwanted side effects [124]. However, nanosystems have improved the delivery of drugs to the posterior portion of eye and the various nanosystems used for this purpose include nanovesicles, nanoimplants, NPs, and hydrogels [123].
Brain diseases can be treated efficiently if we can overcome the issue of the blood–brain barrier (BBB). The BBB is a boundary between circulating blood and the neural tissues of the brain. The presence of the BBB is the major hurdle in the treatment of brain diseases because it does not allow the drugs to enter the central nervous system (CNS) and maintains homeostasis in the brain. Any disturbance to the BBB causes neuro-inflammatory and neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, etc., but even a damaged BBB does not allow drugs to enter the brain [125,126]. However, various types of NPs can cross the BBB and so can efficiently deliver drugs to damaged areas of the brain. NPs use organic and inorganic materials as a core to penetrate the BBB. Inorganic materials include silica, molybdenum, cerium, iron, and gold, whereas organic materials that can be used include PLA, PLGA, and trehalose. The distinct features by which NPs are able to treat neurodegenerative diseases are their small size, high drug loading ability, and efficient imaging performance (particularly for inorganic NPs). Some NPs themselves show some therapeutic efficacy, i.e., showing antioxidant properties, inhibiting Aβ aggregation, and reducing ROS levels [125].
NPs, when conjugated with ligands, show the best performance by interacting with BBB receptors at low density. NPs can adopt multiple pathways in order to cross the BBB [127]. The proposed pathways which NPs can use to cross the BBB are shown in Figure 3 . The main pathways include
