INTRODUCTION

Materials that have one or more dimensions lower than 100 nm are considered nanomaterials.¹ To be more specific, in 2011 the European Commission defined a nanomaterial as follows:

“A natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm-100 nm.”²

Nanomaterials have great research and development/product development potential in medical applications. Some of these applications include DNA/RNA nanotechnology, diagnosis by molecular imaging, biosensing, nanomedicine, and nanocarriers for drug delivery.³ A considerable number of nanomaterials have been developed, produced, and utilized for these application fields, such as nanohydrogels, chitosan/starch/cellulose nanoparticles, graphene (GR)/GR oxide (GO) nanosheets, iron oxide nanoparticles, gold nanoparticles, cerium oxide nanoparticles, and carbon nanotubes/nanoparticles.

Nanomaterials exhibit extraordinary optical, electronic, and/or mechanical properties when compared with their greater scaled forms. They can differ in color, conductivity, reactivity, surface area to volume ratio, and surface tension from macro forms. Due to this, nanomaterials have attracted the attention of scientists for their potential utilization in vaccines, drug development, and drug delivery.⁴ Over many years, many nanomaterials have been adopted as nanocarriers, i.e. nanohydrogels, oil-in-water (O/W) emulsions, liposomes, and nanoparticles based on synthetic polymers or natural macromolecules.⁵ The very first studies were conducted by Couvreur et al.⁶ and Kreuter and Speiser.⁷ in the late 70s, where the team exploited polymeric nanocapsules as lysosomotropic carriers and adjuvants.

Drug nanocarriers usually serve two main purposes: targeted drug delivery to specific tissue, organ, or cells and controlled drug release. The foundation of drug delivery is based on biocompatible nanoparticles or nanocapsules and targeting molecules. Biocompatible materials are selected and incorporated to enhance the hydrophilicity of hydrophobic carrier systems or drugs. Targeting molecules are generally antibodies or avidin/biotins that directly target tissue, organs, or cells. Drug release features of nanocarrier systems are provided by the environmentally sensitive structure of the carrier. Controlling drug release ensures paramount therapeutic effect by releasing the delivered drug with high efficiency in the targeted area and preventing any healthy tissue damage that could be caused by some drugs such as chemotherapy agents.⁸ Nanocarriers that have been designed from polymer-based nanoparticles are solid colloidal particles that are approximately 10-500 nm in size.⁴ Drug incorporation into nanocarriers is based on 5 methods: dissolution, entrapment, adsorption, attachment, or encapsulation.⁹ Herein a brief review of nanocarrier systems is given. A summary of the literature including easily manipulated popular nanomaterials that have been adopted as nanocarriers (nanohydrogels, chitosan (CS) nanoparticles, GR/GO nanocarriers, and solid lipid nanoparticles) is given. Nanohydrogels and CS nanoparticle derivatives are the most heavily rotated amphiphilic nanocarrier materials. GR/GO nanomaterials are favored nanocarriers since they are present in a wide range of carrier designs. Finally, solid lipid nanocarriers (SLNs) are currently the most promising and novel lipophilic drug carriers.¹⁰

NANOHYDROGEL CARRIERS

Nanohydrogels can be defined starting with the descriptions of macro-scaled hydrogels. Hydrogels are three-dimensional hydrophilic polymer chain networks that are crosslinked. These networks can consist of natural or synthetic polymers and display swelling behavior when introduced to water or physiological fluids. Moreover, they are able to revert to their initial state when removed from the presence of water/biological fluids.^11,12,13 Due to this unique behavior, hydrogels have gained attention and been adopted in biomedical applications such as drug delivery, drug release, and vaccine design.¹⁴

Drug delivery and drug release system designs that utilize hydrogels have been and are still considered appealing in medicine due to their crosslink-controlled pore structures. Moreover, physiochemically, hydrogels are very similar to the extracellular matrix of the human body. With also a very high content of water, hydrogels are known to have very high biocompatibility. A main disadvantage is their viscosity, which created an alternative solution: nanohydrogels. These submicron particles made excellent drug carriers that could easily be extruded through an injector needle. In addition, decreasing size ensures an increase in surface area that provides further bioconjugation.^11,15

Nanogels, in the range of 10-100 nm size, are small enough to be used as sys­temic drug carriers. For designs that include clearance of nanogel carriers by kidney filtration the diameter is lower than 10 nm. Drug release to tissue, organs, or cells is through the meshes of nanohydrogels, which are typically between 5 and 100 nm in size.¹⁶ Mesh sizes in environmentally dependent designs such as temperature- and pH-sensitive ones change with the stimuli according to the crosslink bond concentration that forms or breaks.¹⁵ Regulating the breakages of crosslinking bonds that form the initial mesh size of the carriers will provide control of drug release acceleration. Other designs include utilization of the swelling capacity of nanohydrogels.¹⁷ As swelling continues, mesh sizes increase and gradually release the encapsulated drug.¹⁵

Nanohydrogel carriers that are environmentally dependent include designs sensitive to pH, temperature, electric field, light, enzyme, calcium, glucose, redox, etc.¹⁸ In this paper, some of these designs are summarized according to their sensitivity features as below. From this summary, it can be stated that as nanohydrogel carriers there are several popular materials that are prominent when compared with others. In Table 1, materials that receive the greatest attention from scientists are listed.

Temperature-sensitive nanohydrogel carriers

Temperature-sensitive nanohydrogel carriers are systems that exhibit swelling behavior that is dependent on temperature changes and are a widely studied field.¹⁹ A temperature-sensitive drug-release design was reported by Ichikawa and Fukumori²⁰ in 1999. The design consists of a water-soluble hemostatic drug core inside a thermosensitive poly[N-isopropylacrylamide (NIPAAm)] nanohydrogel containing an ethyl cellulose shell. Ichikawa and Fukumori²⁰ stated that the mentioned shell could change and revert to its initial size with temperature changes between 30°C and 50°C in water and that nanohydrogels exhibit positive thermosensitive swelling. The drug release rate is reported to be not only temperature dependent but also nanohydrogel concentration dependent.²⁰ A very recent study introduced thermosensitive 5-fluorouracil (5-FU; a chemotherapeutic drug employed for solid tumor treatments) containing methyl cellulose (MC) nanohydrogels for decreased side effects of chemotherapy. In this 2018 study Dalwadi and Patel²¹ produced MC nanohydrogels by a tip probe-sonicator method from MC hydrogels. 5-FU release depends on both temperature and its biodegradability. Within 48 h the drug is released in the injected area, preventing a cytotoxic drug burst in a very large area as in conventional chemotherapy.²¹

pH- and/or ionic-strength-sensitive nanohydrogel carriers

pH and/or ionic strength sensitivity allows nanocarriers’ mesh size to be manipulated according to the environmental pH. Elsaeed et al.²² synthesized poly(NIPA-co-AAC) nanohydrogels by inverse microemulsion polymerization method in 2010. On average, the diameter of these nanohydrogels is reported to range between 60 and 80 nm. The team delivers a possible drug release methodology that is pH dependent through poly (NIPA-co-AAC) nanohydrogel by characterizing its swelling behavior between the pH values of 4.00 and 8.00 (ionic strength=0.4). That study shows that the nanohydrogels’ swelling capacity increased with environmental pH.²² In an earlier study, in 2004, Dufresne et al.²³ reported pH-sensitive poly (N-isopropylacrylamide) derivative copolymers or poly(alkyl(meth)acrylate) diblock copolymers were produced as indomethacin (a nonsteroidal anti-inflammatory drug), fenofibrate (a drug for treating abnormal blood lipid levels), and doxorubicin (DOX) and aluminum chloride phthalocyanine carriers. PNIPAM copolymers were stated to be synthesized by free radical polymerization while the poly[alkyl(meth)acrylate] diblock copolymers were synthesized by atom transfer radical polymerization. The team carried out both in vitro and in vivo assays. Dufresne et al.²³ refer to the PNIPAM derivatives as a potential safe alternative to Cremophor^®EL, a common carrier for various poorly water-soluble drugs. Furthermore, poly[alkyl(meth)acrylate] derivative [polyethylene glycol (PEG)-b-(EA-co-MAA)] nanoparticles were stated to be excellent carriers for hydrophobic drugs that could be used orally. The carrier system is reported to exhibit dissociation behavior with increasing pH.²³

CHITOSAN NANOCARRIERS

Chitin is a long-chain polymer derivative [poly (b-(1-4)-N-acetyl-D-glucosamine)] of glucose with significance as the raw material of CS nanocarriers (CSNs). When chitin is deacetylated up to about 50%, it transforms into CS, which has a linear backbone linked through glycosidic bonds.^24,25 CS’s efficient bio-adhesiveness and permeabilization capacity make it one of the most popular nanocarrier materials amongst other hydrophilic polymers.²⁶ Moreover, CS is a nanocarrier that has a high loading efficiency of drugs. Based on the protonation of -NH₂ at the C-2 position of the D-glucosamine repeat, one of the most important characteristics of CS is its solubility in aqueous acidic media as given in Figure 1.²⁴ Thus, CS nanocapsules provide an effective solution for the delivery of hydrophobic drugs.²⁷ All the mentioned features of CS nanoparticles make it an excellent nanocarrier material.

Moreover, CS exhibits pH-sensitive behavior due to the percentage of its acetylated monomers and their distribution in the chains.²⁸ This behavior is utilized for controlled drug release by scientists. A common example for this is drug delivery to tumor cells and controlling release since the pH of tumor cells is significantly lower than that of healthy cells.²⁹ A summary of the literature that features CSNs as drug delivery systems is provided in Table 2 in chronological order. Production methods for CS carriers differ however, the most common method used being ionotropic gelation, which is based on the capability of polyelectrolytes to crosslink in the presence of counter ions.³⁰

As can be seen in Table 2, Fernández-Urrusuno et al.³¹ proposed the use of CS nanoparticles as potential drug carriers for transmucosal delivery in 1999. In their design the team loads insulin into CS nanoparticles to be given nasally to conscious normoglycemic rabbits. It is reported that there was a 40% reduction in the serum glucose levels.³¹ Aktaş et al.³⁴ reported the use of PEG-grafted CS nanoparticles as peptide drug carriers. They observed nanoparticle formation through intermolecular hydrogen bonding in an aqueous solution. The incorporation and release of insulin were dependent on the degree of introduction of the PEG chain on CS and observed sustained release phenomenon over time.^52,53 Pérez-Álvarez et al.⁵¹ reported one of the most recent studies in this field revealing the state of art in 2019. Their work exploits the designed CSN as a great candidate for polyoxometalate delivery into tumoral cells. CSN production is achieved by dissolving low molecular weight CS in 1% (v/v) acetic acid solutions for crosslinking in inverse microemulsion medium, which results in the attainment of nanometric CS gel particles. Utilizing the pH-sensitive characteristics the team managed to inhibit cytotoxic drug release.⁵¹

GRAPHENE AND GRAPHENE OXIDE NANOCARRIERS

Professor Andre Geim and Professor Kostya Novoselov made a groundbreaking disclosure by finally discovering a production method for GR in 2004. The research was outstanding since it had not been possible previously to produce a single layer of graphite (carbon atoms with sp2 bonds in the shape of honeycomb). Later, GR became known as the basic building block of graphitic materials such as spherical nanoparticles that are also known as 0D fullerenes, 1D carbon nanotubes, and 3D graphite.^{54,55,56,57,58}

Following the discovery, scientists began to reveal GR’s unique characteristics provided by its submicron dimension and the π-conjugation in its structure. GR is revealed to exhibit extraordinary thermal, mechanical, and electrical properties.⁵⁷ Further research provided a better understanding of the physical and chemical structure of GR’s surface, which has created interest in medical and pharmaceutical technologies as well as other fields of science. GR is researched and utilized for nanoscaffolds, chemical/biosensing, imaging, drug delivery and controlled drug release.⁵⁹ In the area of nanomedicine and nanocarriers, GR and its composites are important due to its large surface area where every single atom is exposed on the surface (2600 m² g^-1), layer number, lateral dimension, surface chemistry, and purity.^60,61,62 Hereby, GR could be considered a superior candidate as an ideal nanocarrier with the mentioned characteristics that allows a high drug load capacity.⁵⁸

One of the most popular derivatives of GR is GO, GR with oxygen-containing functionalities (epoxide, carbonyl, carboxyl, and hydroxyl groups). GR and GO have a major difference that affects their drug delivery performance when used as nanocarriers: GO is highly hydrophilic, whereas GR is hydrophobic so that it requires surface modifications for use in biological fluids. Thus, any nanocarrier design that uses GR should take into consideration the possible impurities and negative effects such as cytotoxicity.^61,63 This leads researchers to gravitate towards GO-containing designs rather than GR nanocarrier designs.

In Table 3, a summary of GR/GO nanocarrier designs is given. As can be seen, Hummer’s method for production is the most popular choice, where graphite oxidative exfoliation is applied with NaNO₃. Although Hummer’s production method is usually opted for rather than other complicated methods, over the years it can be seen that nanocarrier designs have evolved into more complex systems that apply chemotherapy and photothermal therapy for treating cancer.

In 2008, Liu et al.⁸⁷ published a study that demonstrates PEG-functionalized GO nanocarriers used as a noncovalent physisorption chemotherapy drug delivery system. The team reveals that the nanocarriers have an adequate in vitro cellular uptake capacity.⁸⁷ A very recent study by Bullo et al.⁸⁸ examined the state of the art in GO nanocarriers. GO is reported to be synthesized by Hummer’s method. GO is modified with PEG for higher biocompatibility and loaded with two chemotherapeutic drugs: protocatechuic acid and chlorogenic acid. The carrier is then coated with folic acid to target cancer cells since tumor surface membranes have a greater number of folate receptors. The final size of the nanocarrier system is stated to be 9-40 nm with a median of 8 nm. The team reveals that drug release of this design took more than 100 h, which ensures a steady therapeutic effect.⁸⁸

SOLID LIPID NANOPARTICLES

Nanocarriers designed with a polymer foundation have a certain advantage in terms of the wealth of possible chemical modifications, including the synthesis of block and comb polymers.⁸⁹ Designs that use SLNs exploit this advantage by combining the advantages and avoiding the disadvantages of other colloidal carriers.

Lipids are defined as molecules that are hydrophobic or consisting of both hydrophilic and hydrophobic parts that are insoluble in water and soluble in organic solvents.⁹⁰ IUPAC gave the following further detailed definition in 1995:

“A loosely defined term for substances of biological origin that are soluble in nonpolar solvents. They consist of saponifiable lipids, such as glycerides (fats and oils) and phospholipids, as well as nonsaponifiable lipids, principally steroids.”⁹¹

SLNs are developed by researchers as a substitute colloidal carrier with a spherical morphology for drug delivery and drug release.⁵ SLNs have an average size of between 150 and 300 nm but could reach up to 1000 nm according to the surfactant used during production and are composed of roughly 0.1-30 (% w/w) solid fat.⁹² Size and solid to liquid fat ratio affect the long-term stability, drug-loading capacity, and drug-release behavior of SLNs.⁹³ As mentioned, SLNs have several favored assets such as low to no toxic effect on healthy tissue and ease of production in greater units of production, ability to load both lipophilic and hydrophilic therapeutic agents, and high drug load capacity.⁵ The most common use of SLNs as nanocarriers is for oral drug delivery. Other than this example, several drugs have been loaded using SLNs for drug delivery, such as doxorubicin and idarubicin,⁹⁴ thymopentin,⁹⁵ and camptothecin.⁹⁶

DISCUSSION AND CONCLUSION

Nanocarriers provide researchers with a highly applicable alternative method for targeted drug delivery and controlled drug release. The first and foremost reason that nanocarriers have become such a great focus in pharmaceutical technologies is that nanomaterials demonstrate extraordinary characteristics when compared with their larger scaled forms. These characteristics are summarized in this review as color, visible light, reactivity, surface area to volume ratio, conductivity, and surface tension. A variety of these carriers are more popular due to their high biocompatibility, ensuring greater efficacy especially in cancer treatments. Successful applications have not only ensured a greater focus on therapeutic development but also created a new solution available in the pharmaceutical market. In this paper, nanocarrier materials that have gained the most attention in drug delivery and release are summarized under the titles of nanohydrogels carriers, CSNs, GR and GO nanocarriers, and SLNs. Besides these nanomaterials there are also a great number of different nanocarrier designs that are not included in this review, such as gold nanocarriers,⁹⁷ starch and/or cellulose nanocarriers,⁹⁸ cerium oxide nanocarriers,⁹⁹ and carbon nanotube incorporated nanocarriers.¹⁰⁰ It is clear that, with further information gathered on nanocarriers for drug delivery and the current state in the development process of these nanomaterials, there is a high possibility to deliver better treatment to patients desperate in need of efficient treatment strategies.

Conflict of interest: No conflict of interest was declared by the authors.