Nanomaterials for biomedical applications

Abstract

Nanotechnology involves understanding and manipulating materials normally in the size range of 1 to 100 nm, where particles show completely novel physico-chemical properties from their bulk counterpart. In the last two decades a number of precisely engineered nanomaterials have been developed for diagnostic and therapeutic applications. For diagnostic applications, nanoparticles allow detection at the molecular scale. Some fluorescent nanohybrids can serve the dual purpose of detection and destruction of pathogenic microbes. The specificity and sensitivity of magnetic resonance imaging can be greatly improved by using nanoparticles as contrast enhancement material. As therapeutic agent, they allow targeted delivery and sustained release of drug molecules and they also have huge potential in tissue engineering. Given the vast range of application of nanomaterials in the biomedical domain, this review focuses on the major nanoparticle based diagnostic and therapeutic modalities.

Keywords:

• nanoparticles
• biosensor
• contrast enhancement
• drug delivery
• tissue engineering
• photodynamic therapy

Introduction

With the arrival of micro and nano technologies, fabrication of nanoscale structures and devices with unique phenomena of supramolecular assembly have become recurrent and this has increased their potential for novel intersection in a vast range of fields such as engineering, physics, chemistry, biology, computer science and more. In the last decade a plethora of research has been done on the applications of nanotechnology in biomedical research, involving visualization, manipulation and fabrication of materials on the scale of 100 nm down to 1 nm. Nanoscale materials are increasingly making a major impact on human health, and as they are used progressively more in diagnostic and therapeutic applications they have the potential to revolutionize medicine.

Generally, three groups of nanomaterials are of intense interest: zero-dimensional materials, so-called quantum dots, with variations in shape and diameter, one-dimensional materials, including nanorods and nanowires, and two-dimensional materials, including nanobelts, nanodisks, films, and nanosheets. Herein we will focus on nanoparticles (NPs) that are being used widely in different biomedical applications.

NPs have become popular in biomedical applications due to their ultra small size, extremely high surface area to volume ratio, tunable optical emission, enhanced mechanical properties, and superparamagnetic behavior; in addition they need to be biocompatible and synthesized by simple methods. Nanoscale devices smaller than 50 nm can easily enter most cells, while those smaller than 20 nm can move out of the blood vessels as they circulate through the body. Due to their small size, nanoscale devices can readily interact with biomolecules both extracellularly and intracellularly. By gaining access to most areas of the body, they have the potential to detect disease, monitor cells within the living system and deliver treatment in previously unachievable ways (Patra et al. 2008). Though applications and products dealing with drug delivery systems dominate the nanomedicine market, based on their unique features NPs can be used in areas such as imaging (Park et al.2010), detecting biomolecules (Liu et al. 2004), and tissue engineering (Harrison & Atala 2007).

NPs with dual functionality can be used for diagnostic and therapeutic purposes (e.g. Fe₂O₃-Pt NPs) (Gao et al.2008). NPs can be used as carriers for different drugs. They can be used to reduce toxicity of a therapeutic drug (Vlerken & Amiji 2006), or for pH-triggered drug release (Lim et al. 2011). NPs are currently being used for hyperthermia treatment (Maier-Hauff et al. 2011), and photodynamic therapy (Robertson et al. 2009). For example, AuroShell is a nanoproduct that kills cells at the target tumor site by emitting heat upon absorption of near-infrared (NIR) light (Leung et al. 2013).

This article will review the biomedical application of NPs in some prominent diagnostic and therapeutic fields.

Nanoparticle based biosensors

Many kinds of NPs, such as metal, oxide, semiconductor and carbon NPs (CNPs), have been used for constructing biosensors. These NPs play different roles in different sensing systems. The majority of the biosensing applications based on NPs that have been developed so far are based on gold NPs (GNPs), due to their unique optical properties and ease of use with different biomarkers in aqueous solution (Baptista et al. 2008; Sperling et al. 2008; Boisselier& Astruc 2009). An interesting optical property of GNPs is that their optical absorption and scattering peaks can be tuned by varying their size (at around 20 nm range) and shape; in particular these peaks can be put in the NIR optical window (800–1300 nm). Moreover, they are capable of transferring electrons efficiently between different electro-active species and electrode materials. In addition, the light-scattering properties and extremely large enhancement ability of the local electromagnetic field enables GNPs to be used as signal amplification tags in diverse biosensors (Lia et al. 2010). Three main types of GNP based biosensors are used: optical, electrochemical and piezo-electric biosensors. GNPs can act as optical biosensors as these NPs can augment the change of refractive index and enhance electron transfer. Based on these features DNA sensors with GNPs were developed and these were 1000 times more sensitive than those without GNPs (He et al.2000). Electron transfer from redox proteins, particularly redox enzymes, to electrodes has been a subject of extensive research for making bioelectrocatalysts. It was seen that electron transfer rate was 5000 per second in the presence of GNPs (compared with the rate at which molecular oxygen, which is a cosubstrate for the enzyme, accepts electrons), while it was 700 per second without GNPs (Xiaoet al. 2003). Spadavecchia et al. (2013) demonstrated that conjugation of gold nanostructures with thiolated DNA produces an additional plasmonic band, well separated from the localized plasmon resonance band of GNPs. By identifying the appearance of this additional plasmonic band upon hybridization, DNA as low as 200 pM can be detected. In electrochemical biosensors changes in electrical states are the basis for detection. Here GNPs, which have unique catalytic properties, are used as an immobilization platform. GNP based glucose biosensors have a detection limit of 0.18 μM (Andreescu & Luck 2008). A NADH sensor based on GNPs shows 780 mV potential decreases without any electron transfer mediators (Jena & Raj 2006). Piezo-electric biosensors are principally based on changes in mass. Improved sensitivity was obtained using GNP functionalities like biomolecular immobilization and amplification of mass change. An example of such a biosensor is a DNA sensor using GNPs as amplification tags with detection limit of (Liu et al. 2004).

Silver NPs (AgNPs) are of enormous current research attention in the field of novel biosensor development because of their catalytic properties (Sun & Xia 2002; Radet al. 2011). Yanxia et al. (2008) developed a H₂O₂ biosensor based on the direct electrochemistry of hemoglobin (Hb) in Hb–Ag sol on a glassy carbon (GC) electrode. A novel H₂O₂ biosensor based on the direct electrochemistry and electrocatalysis of myoglobin (Mb) immobilized on AgNP doped carbon nanotube (CNT) film with hybrid sol–gel techniques has been reported by Liu and Hu (2009).

Magnetic NPs (MNPs) are also leading to more sensitive, rapid and cost effective biosensors because most biological samples exhibit negligible magnetic susceptibility, and hence the background against which measurements are made is extremely low. MNP based biosensors are being used to detect nucleic acids, enzymes, other proteins, drugs, pathogens and tumor cells with excellent sensitivity (Haun et al. 2011). MNPs have been successfully applied in diagnostic magnetic resonance (DMR). The core principle behind DMR is the use of MNP as proximity sensors that modulate the transverse relaxation time of neighboring water molecules. This signal can be quantified using magnetic resonance imagers or nuclear magnetic resonance (NMR) relaxometers, including miniaturized NMR detector chips that are capable of performing highly sensitive measurements on microliter sample volumes and in a multiplexed format.

Claussen et al. (2012) made a hybrid biosensor, where glucose oxidase is immobilized on a 3D matrix consisting of multilayered graphene petal nanosheets and Pt NPs. This biosensor has exceptional glucose sensitivity.

As CNT has high electrocatalytic activity, a huge number of studies directed at using CNT in electrochemical biosensing are going on. In addition to having small size, nanoscale sensors made from CNTs exhibit higher signal-to-noise ratios in comparison to microelectrodes. A glucose biosensor based on multiwalled CNTs and zinc oxide composite was developed by Palanisamy et al. (2012) and this biosensor can be used for detecting glucose in urine sample with high reproducibility and repeatability.

Nanoparticles as imaging and contrast enhancer agents

Preliminary reports have demonstrated the potential of metal and MNPs as color contrast agents for several techniques such as confocal microscopy, confocal endoscopy, and atomic force microscopy. In tissue and cell studies the addition of GNPs to collagen has been shown to reduce auto fluorescence, an effect attributed to the catalytic effect of the NPs on NADH to NAD conversion (Ivan et al. 2007). This alteration of GNP mediated cellular auto fluorescence might be helpful in using GNPs as a novel class of contrast agents for fluorescence based detection of cancer tissues. Using the surface-enhanced Raman scattering (SERS) effect, the sensitivity of reporter dye molecules, directly attached to the surface of GNPs through Au-S or Au-N interactions, can be greatly enhanced and this hybrid may be used for more sensitive cancer cell imaging (Christiansen et al. 2007; Luet al. 2010; Jiang et al. 2011). Optical coherence tomography (OCT) uses the scattering function of gold nanoshells for in vivo imaging (Agrawal et al. 2006; Adler et al. 2008; Skrabalak et al. 2008).

NPs labeled with fluorescent dyes such as fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC) have wide application in bioimaging (Santra et al.2006; Park et al. 2010), but they have the problem of photobleaching. Semiconductor quantum dots (QDs) such as ZnSe, ZnS and CdS, together with carbon dots, which have excellent photoluminescent properties, are also being used for imaging purposes (Hoshino et al. 2004; Jaiswal et al.2004; Li & Zhu 2013). QD based fluorescence biosensors in particular have become very popular over the past two decades due to their high quantum yield, narrow and tunable emission spectrum and good photostability. Fluorescent NPs (both QDs and fluorescent dye tagged NPs) are being used extensively in both in vitro and in vivo imaging. They have become especially popular for cancer cell imaging (Sage 2004; Xing et al. 2006; Li et al. 2010).

Our research group has developed a ‘glow nanohybrid’ where a highly fluorescent CNP is chemically grafted on surface functionalized ZnO nanorods (Mitra &Chandraet al. 2012). This nanohybrid serves a dual purpose – it can kill bacteria as ZnO has well-known antibacterial property and simultaneously it can detect bacteria because of the presence of highly fluorescent CNPs. For making this conjugate, first ZnO nanorods were functionalized with amine groups followed by attachment of CNPs, activated with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The prepared CNP grafted ZnO nanorods preserved good dispersity along with uniformity in size distribution. This CNP grafted ZnO nanorod could successfully inhibit Staphylococcus aureus and Escherichia coli growth. The unique chemical grafting made the nanohybrid fluorescent, emitting bright green light under UV, and it can effectively detect bacteria under a fluorescent microscope.

In this context the role of MNPs as contrast enhancing agents is very significant. Magnetic iron oxide NPs have been extensively used as magnetic resonance imaging (MRI) contrast agents due to their ability to shorten T2 relaxation times in the liver, spleen, and bone marrow (Na et al. 2009). More recently, uniform ferrite NPs have been successfully employed as new T2 MRI contrast agents with improved relaxation properties. Iron oxide NPs functionalized with targeting agents have been used for targeted imaging via the site-specific accumulation of NPs at the targets of interest. Lai et al. (2012) have developed FITC-incorporated silica-coated FePt NPs that exhibited not only significant T1 and T2 magnetic resonance (MR) contrast abilities but also a fluorescent property. When gadolinium, confined into nanoporous silicon particle, is used as MRI contrast agent, it gives improved T1 relaxivity and ultimately enhanced image contrast is obtained (Ananta et al. 2010). A target-specific MRI contrast agent for tumor cells expressing a high affinity folate receptor using folic acid and polyamidoamine (PAMAM) dendrimer was synthesized by Swanson et al. (2008). Surface modified dendrimer was functionalized for targeting with folic acid (FA) and the primary amines of the dendrimer of remaining terminal were conjugated with the bifunctional DOTA-NCS (2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic) chelator that forms stable complexes with gadolinium (Gd III). The targeted dendrimer contrast NPs can infiltrate tumors and are retained in tumor cells up to 48 hours post-injection. These dendrimer NPs have huge potential for targeted cancer imaging due to the prolonged clearance time, compared with the current clinically approved gadodiamide contrast agent. Jin et al. (2010) have devised an iron oxide and gold coupled core-shell NP, which not only offers contrast for electron microscopy and MRI, but also enables a new imaging mode called magnetomotive photoacoustic imaging, with remarkable contrast enhancement.

MnO NPs have also been recently explored as a new T1 MRI contrast agent (Baek et al. 2010). But the contrast signal of MnO based NPs is weak for a long time in in vivo MRI tracking, and hence further improvement in this regard is needed (Kim et al. 2011).

Nanomaterials in drug delivery

Most research in the field of nanomedicine is on the application of nanomaterials in drug delivery systems (DDS). Application of NPs in drug delivery has the following goals: specific drug targeting and delivery, maintaining biocompatibility during application and therapeutic processes, and faster production (Jong & Born 2008). The release mechanism of drugs, the diffusion coefficient and the biodegradation rate are the main factors which govern the drug release rate. Here, drugs may be bound to the NPs either within the production process of NPs or by adsorption of drugs to NPs. Many of the pharmacological properties of conventional (‘free’) drugs can be improved through the use of DDS. DDS are designed to alter the pharmacokinetics (PK) and biodistribution (BD) of their associated drugs, or to function as drug reservoirs.

Packaging of chemotherapeutics within nanosized formulations like liposomes has been of considerable interest in the past few years (Crommelin & Storm 2003; Metselaar& Storm 2005; Minko et al. 2006) and liposomes are considered to be one of the most successful DDS (Kaneda 2000); a number of liposome-based systems have been approved by the US Food and Drug Administration (FDA) for disease treatment in the clinic. Although due to their small size liposomes can enter the arterioles and epithelial fenestration, drug solubility is an important concern for liposome based DDS. Hydrophilic drugs can be readily entrapped with a high degree of latency within the aqueous interior of liposomes, but neutral hydrophobic drugs or those with intermediate solubilities tend to be rapidly released in the presence of plasma proteins or cell membranes. Fortunately, excellent retention of drugs that are hydrophobic weak bases (such as doxorubicin and vincristine) has been achieved through ‘remote loading’ techniques that rely on pH or chemical gradients across the liposome bilayer to accumulate and retain the drug (Cullis et al. 1997). Doxil was the first liposomal drug formulation approved by the FDA for the treatment of AIDS associated with Kaposi's sarcoma (Northfelt et al. 1998). Other liposomal drugs used in clinical practice today include AmBisome (amphotericin B liposomes), DaunoXome (daunorubicin liposomes), DepoCyt (cytarabine liposomes), and Visudyne (verteporfin liposomes). Attachment of therapeutic molecules to inert polymers decreases drug clearance by the kidneys and by immune recognition. In general, when a drug is associated with a carrier, the drug clearance decreases (the half-life increases), the volume of distribution decreases, and the area under the time–concentration curve increases (Allen & Cullis 2004). Paclitaxel encapsulated in vitamin E TPGS-emulsified poly(D, L-Lactic-c-glycolic acid) (PLGA) NPs showed enhanced cytotoxicity for tumor cells in vitro, and it maintained a sustainable therapeutic potential in an animal model system (Win & Feng 2006). Poly-4-vinylpyridine N-oxide (PVNO) polymer coated micro quartz particles were shown to have efficient distribution without showing any macrophage related toxicity and genotoxicity (Schins et al. 2002; Albrecht et al. 2005). Polyethylene glycol (PEG) coated magnetic NPs of 40–50 nm size are reported to be well taken up by endocytosis (Gupta & Curtis 2004).

The blood–brain barrier (BBB) significantly hinders the passage of systemically delivered therapeutics and the brain extracellular matrix limits the distribution and longevity of locally delivered agents. So the central nervous system (CNS) poses a unique challenge for drug delivery. Polymeric NPs represent a promising solution to these problems. Encapsulation of paclitaxel in cityl alcohol/polysorbate NPs has been reported to enhance drug uptake in rat brain (Koziara et al. 2006). Poly(butylcyanoacrylate) (PBCA) NPs were the first polymer-based NP carrier used to deliver dalargin (a compound with opioid activity) to the CNS (Kreuter et al. 1995). Systemically-administered poly(lactic acid) (PLA) NPs, loaded with breviscapine (a flavonoid), have been shown to penetrate the BBB (Liu et al. 2008). Geldenhuys et al. (2011) have demonstrated that surface modification of paclitaxel-loaded PLGA NPs with glutathione may also improve BBB bypass. Surface functionalization with PEG-PLA NPs with lectins like wheat gram protein, which specifically bind to the sialic acid present in large amounts in nasal cavities, have shown to increase brain uptake enormously (Gao et al. 2006). MnO NP is also found to translocate to the brain through olfactory route (Elder et al. 2006).

Conjugation of specific proteins like antibodies to NP surface has great potential for more specific and immunologically directed targeting (Nobs et al. 2004; Prinzen et al.2007). Peptide coated QDs less than 10 nm in size are being targeted specifically to the lung and tumor vasculature (Akerman et al. 2002).

Mesoporous silica NP has also become a multifunctional delivery platform that has been shown at both in vitro and in vivo levels to be capable of delivering chemotherapeutic agents to a variety of cancer cell types (Lee et al. 2009; Meng et al. 2010; He et al. 2011). The major advantage of using a mesoporous silica based carrier is that its internal ordered pore structures may be tailored to host differently sized drug molecules and its release profile can be predetermined. Mesoporous silica NP based drug carriers have been shown to be able to incorporate a high dosage of drugs in the internal pore system owing to their large internal surface areas and pore volumes as well as their tunable pore size. This carrier allows effective and protective packaging of hydrophobic and charged anticancer drugs for controlled and on demand delivery.

Silica coated iron oxide NPs, covalently functionalized with light-responsive azobenzene derivatives, have been recently reported as novel target delivery systems (Turcheniuk et al. 2013). This composite has unique magnetic targeting and stimuli-responsive release properties. The cargo molecules inside the pores cannot diffuse out because of the blocking by high density azobenzene chains. But visible light irradiation causes trans/cis isomerization of azobenzene chains and it helps in release of guest molecules out of the pores. Phosphonic acid has strong affinity for iron oxide NP through the formation of bidendate Fe-O-P bonds. This NP can further be functionalized with rhodamine B isothiocyanate, propargyl folate and paclitaxel (Das et al. 2011). These trifunctional NPs could selectively target and induce apoptosis to folate-receptor overexpressing cancer cells with higher efficacy in comparison with the free drug. Iron oxide NPs can also be used for delivery of chemotherapeutics such as gemcitabine (Lee et al. 2013).

Our research group has developed surface functionalized porous ZnO nanorods for cancer cell specific targeted drug delivery (TDD) (Mitra & Bano et al. 2012). Here porous ZnO nanorods were prepared by a simple route and had very high surface area of with uniformly distributed pores of 15 nm. These were then functionalized with 3-aminophosphonic acid followed by folic acid to yield folate conjugated porous ZnO nanorods (ZnO-FA). Its extremely high surface area and uniformly distributed pores on its surface made the nanocarrier suitable for high drug loading (88%) of the anticancer drug doxorubicin (DOX). A pH triggered drug release was observed with minimum release in pathophysical conditions. In vitro efficacy of DOX loaded ZnO-FA (ZnO-FA-DOX) was evaluated against breast cancer cells MDA-MB-231 (Figure 1). This nano-hybrid was much more effective than DOX alone or ZnO-FA. Moreover, the targeted scaffold was functionalized with a pendant –NH₂ group and was covalently bonded with the fluorescent dye RITC for cellular uptake and imaging studies in MDA-MB-231 cells (Figure 1). In vivo acute and intravenous toxicological evaluation in murine model system complemented the biocompatibility of ZnO-FA-DOX in TDD. So this porous ZnO based nanocarrier has great potential in TDD.

Figure 1. Surface functionalized mesoporous ZnO nanorod mediated cancer cell specific targeted delivery. Porous ZnO nanorods were functionalized with 3-aminophosphonic acid followed by folic acid. The nanocarrier was then loaded with anticancer drug doxorubicin. Efficacy of doxorubicin loaded Zno nanorods was shown in breast cancer cells MDAMB-231. Its biocompatibility was evaluated in a murine model system.

Nanoparticles in tissue engineering, hyperthermia and photodynamic therapy

Engineered NPs which are capable of translocation through the bloodstream and can reach specific targets are now the subject of large numbers of studies in the field of biomedicine, because they are useful in diagnostics and drug delivery to specific targets; also such NPs are now also being developed for treating diseases. This makes them attractive even at the clinical level.

Nanotechnology in tissue engineering

Bone is a natural composite of hierarchically arranged collagen fibrils, hydroxyapatite (HAp) and proteoglycans. Hence, various tissue engineering strategies have adopted composite scaffold approaches to closely mimic the bone in structure and composition. NPs including calcium triphosphate and HAp have been fabricated into porous 3D scaffolds for bone repair or regeneration purposes (Khan et al.2006). This method allows bone composition to be mimicked; also the incorporation of nanoceramics enhances the mechanical strength of bones. Researchers are now evaluating the use of polymer or ceramic matrices containing single multi-walled CNTs to improve the mechanical properties of the artificial bony structure (Harrison & Atala 2007). CNTs offer excellent properties, such as high tensile strength, high flexibility, and low density that can be exploited to develop more successful orthopedic implant materials.

Yamashita et al. (2009) investigated the effects of rough and smooth surfaces of ceria and yttria stabilized zirconium dioxide, which is promising as a dental implant material. Tan et al. (2011) created a heparin/chitosan NP immobilized decellularized bovine jugular vein scaffold to increase the loading capacity and allow for controlled release of vascular endothelial growth factor (VEGF). This was successfully used to reconstruct dog pulmonary and right ventricle with potential regeneration capability.

Hyperthermia

NP mediated hyperthermia treatment (killing of tumor cells by generating slightly increased temperature) is principally based on delivery of MNP through a magnetically governed processes to the target area (Corchero & Villaverde 2009). These NPs can be activated either by using radiofrequency (RF) pulses or infrared light to release heat and kill the target tissue. Super paramagnetic iron oxide NP is currently being used in hyperthermia applications in vivo (Maier-Hauff et al.2011). The surface plasmon resonance of metal NPs is another property which is used for irreversible thermal ablation of tumors (Chen et al. 2010). Optically activated gold nanoshells are reported to reduce tumor size in both mouse and human xenograft models of triple-negative breast cancer, without a concomitant increase in the percentage of cancer stem cells (Atkinson et al. 2010). Although hyperthermia treatment using NPs has certain disadvantages as most NPs have a high specific absorption rate, research is currently going on in this particular field to increase efficiency.

Photodynamic therapy

Photodynamic therapy is used to destroy tumor cells and is based on concentration of singlet oxygen (¹O2) as part of ROS production in localized area (Takahashi et al. 2002; Oberdanner et al. 2005). Here photosensitizers are activated with appropriate excitation wavelength to produce ROS, thereby killing target cells. Some NPs, which can produce ROS upon excitation (Lee & Kopelman 2011), are now being used in photodynamic therapy. Conventionally UV light is used to excite photosensitizers. As such types of excitement cannot penetrate deep inside the tissue, a new generation of photodynamic therapy has been developed using NIR wavelengths as excitation energy. The penetrating capability of NIR is much deeper than conventional UV excitation. Another advancement introduced in this area is the introduction of a newer class of NPs, namely upconverting NPs (UCNPs), which on efficient encapsulation of photosensitizer molecules can perform better. For example, Guo et al. (2010) developed mesoporous silica coated onto sodium yttrium fluoride upconversion nanocrystals to form a core-shell structure and then loaded with the photosensitizer zinc (II)-phthalocyanine (ZnPc) into the porous silica. On irradiation with 980 nm, NPs emitted visible light that activated ZnPc molecules in the silica shell through resonance energy transfer showing a strong photodynamic cell destruction effect on MB49 cells. Another novel class of multifunctional NPs (MFNPs) based on UCNPs with combined optical and magnetic properties was shown to be useful in multimodality imaging and therapy (Cheng et al.2011, 2012).

Outlook

The unique and superior properties of NPs have shown strong potential for rapid development of nanotechnology based biomedical applications. A large number of nanoscience researchers are trying to exploit the exciting, novel properties of nanomaterials to use them in the field of biomedical sciences worldwide. There are already some NP based products are available in the market and many are in the pipeline for commercialization. This review article focused on some of the major biomedical applications (diagnostics, drug delivery and photodynamic and hyperthermia mediated therapy) of nanomaterials.

NPs have already revolutionized the field of biosensing. As far as the field of therapeutics is concerned, the currently approved NP systems have obviously improved the therapeutic index of drugs. Sometimes some surface functionalized NPs, e.g. glycan functionalized diamond NPs, can themselves be used as an antibacterial agent (Barraset al. 2013). But next generation NPs will have targeting ligands such as antibodies, peptides, or aptamers tagged to them and they will not only improve the efficacy of the drug but also reduce their side effects. Many multifunctional NPs have been developed which are capable of targeting, imaging and therapy. It is expected that in the future many more optimally designed NPs will enter the clinic. But until now there has not been sufficient and comprehensive data on the long-term biotoxicity profile of most NPs. The bio-distribution and release kinetics of the NPs from biological systems should also be investigated in detail. Collaborative, multidisciplinary research endeavors are still needed for more successful application of nanomaterials in biomedical science.