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Introduction

Hydrogels, a three dimensional polymer network, may absorb a large quantity of contact liquid. Because of this swelling phenomenon, hydrogels gives a new insight into a model system for the study of a viscoelastic body that is a major topic in polymer physics. In addition to its importance in science, it has many direct applications in the biomedical area, especially in the area of drug and cell delivery.

The concept of drug delivery system in the pharmaceutical area has been investigated using hydrogels as a candidate material. The three dimensional network of hydrogels demonstrated the sustained release of loaded drug [1–3]. Because of presence of a large quantity of water, the swelling transition in response to various stimuli (pH, temperature, light, ionic concentration, metabolites…) is being intensively investigated with respect to the concept of stimulus-sensitive drug delivery [3–7]. In addition, hydrogels have the potential to execute cell delivery, such as pancreatic islet transplantation for diabetes. Transplanted islets are subject to immunologically mediated destruction by both autoimmunity and transplant rejection. Hydrogels can be used as a semipermeable, biocompatible membrane to protect the islets from host immune responses [8–10].

Current interest is focused on the development of nanomedicine platforms in drug delivery and molecular imaging applications. This led to the emergence of nanoscopic therapeutic systems that incorporate therapeutic agents and diagnostic imaging probes (Fig. 1). Studies have shown that this multifunctional nanomedicine improves the therapeutic outcome of drug therapy. To efficiently obtain information on nanomedicine (the drug delivery systems with molecular imaging capability), the nanomedicine should have the reservoir to contain drugs and molecular imaging probes.

Fig. 1.
figure 1

Therapeutic agents and diagnostic imaging capabilities.

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Hydrogels Polymers for Imaging Probes

Poly(vinyl pyrrolidone): Poly(vinyl pyrrolidone) (PVP) is a biocompatible, water-soluble, and nontoxic polymer. Using the hydrogen bonding between the carbonyl groups of PVP and the carboxyl groups of poly(acrylic acid) (PAA) [11] or chitosan [12], various forms of physical gels have been prepared and characterized. PVP gels are utilized as drug delivery systems with the forms of microspheres, nanoparticles, liposomes, and polymer conjugates [13–16].

Self-assembly in aqueous solutions of PVP-block-poly(d,l-lactide) [15], PVP-block-poly(d,l-lactide)-block-PVP, PVP-block-poly(ε-caprolactone)-block-PVP [17], and PVP-block-poly(ε-caprolactone) [18] is a very important property. As a consequence, PVP can be used to form polymeric micelles to deliver the medical drugs or molecular imaging probes.

Poly(vinyl alcohol): Poly(vinyl alcohol) (PVA) is a linear hydrophilic polymer that is nontoxic and biocompatible. Because of intra/intermolecular interactions via hydrogen bonding, PVA forms hydrogels (physical gels). The freeze-thawing method is often used to enhance the mechanical properties [19, 20]. PVA hydrogels are also prepared by chemical crosslinking using irradiation or crosslinkers, such as glutaraldehyde or sodium borate and boric acid [21, 22].

The use of PVA as the base component for hydrogels formation is particularly advantageous, due to the abundance of hydroxyl pendant groups on the PVA chains that can be further substituted with various functional groups. Several research groups have investigated the addition of methacrylate and acrylate pendant groups [23–25], sulfosalicylic acid [26], chitosan [27], hydroxyapatite [28], and alginate [29, 30].

Dextran Hydrogels: Dextran is a polysaccharide consisting of glucose molecules coupled into long branched chains, mainly through 1,6- and some through 1,3-glucosidic linkages. Dextrans are colloidal, hydrophilic, and water-soluble substances that have excellent biocompatibility and hence, they do not affect cell viability. It is susceptible to enzymatic digestion in the body [31]. Dextran has abundant pendant hydroxyl functional groups making it amenable to chemical modification [32–34]. Hydrophobically modified dextrans are used as stabilizers to produce stable hydrophilic poly(styrene) or poly(lactic acid) nanoparticles by the oil in water (o/w) emulsion and evaporation technique [35–37].

An interconnected macroporous glycidyl methacrylated dextran (Dex-GMA)/gelatin hydrogels scaffold containing microspheres loaded with bone morphogenetic proteins (BMP) has been developed [38]. Microspheres are formed when gelatin was mixed with glycidyl methacrylate dextrans (Dex-GMA); the characteristics of the dextran-co-gelatin hydrogels microspheres can be controlled by the crosslinking density and added substituents to Dex-GMA. Controlled release of bone morphogenetic proteins was observed from 18 to more than 28 days by changing the hydrogels/microsphere ratio.

As a drug delivery system, doxorubicin conjugated dextran nanoparticles have been prepared to improve its therapeutic efficacy in the treatment of solid tumors [39]. In vivo efficacy test of nanoparticles showed faster regression in tumor volume and increased survival time comparing with drug conjugate and free drug.

Chitosan Hydrogels: Chitosan (poly-b(1,4)-d-glucosamine) is a cationic polysaccharide which is obtained by alkaline deacetylation of chitin, the main exoskeletal component in crustaceans. Its molecular weight ranges from 3,000 to 10,000, with a degree of deacetylation from 30 to 95%, depending on the source and preparation method. The amine groups of chitosan are protonated in the acidic conditions (pH < 4). The quality and properties of chitosan products, such as purity, viscosity, deacetylation, and molecular weight, may vary widely because of many factors in the manufacturing process can influence the characteristics of the final product. Chitosan has biodegradability, nontoxicity, biocompatibility, and antifungal activity; chitosan and its derivatives have been studied as biomaterials which are used for drug delivery systems [40] and scaffolds for tissue engineering [41, 42].

Chitosan beads are prepared by simultaneous crosslinking with glutaraldehyde and precipitation in aqueous NaOH [40]. Metronidazole, an antiinfection agent, loaded chitosan beads give faster release at acidic conditions; this pH-sensitive release behavior can be utilized to design targeted delivery system for anticancer drugs.

The differentiation of mesenchymal stem cells (MSCs) and the mass formation of cartilage are possible using an injectable hydrogels composed of copolymer of thermosensitive poly(N-isopropylacrylamide) and water-soluble chitosan. Cartilage formation in the submucosal layer of the bladder of rabbits and the in situ hydrogels system composed of dextran copolymer as a scaffold are being pursued [41].

The reactive amino groups in the backbone of chitosan make it possible to chemically conjugate various biological molecules such as different ligands and antibodies, which may improve targeting efficiency of the drug to the site of action [43, 44]. Chitosan-based polymeric vesicles and niosomes bearing glucose or transferrin ligands for drug targeting have been prepared [43]. Transferrin (TF) coupled to the surface of the polymeric vesicles appears to be accessible to the TF receptor in the A431 cell line. The TF receptors are over expressed on the surface of many proliferating cells and the active targeting of polymeric vesicles for drug/gene delivery can be accomplished.

One of the most useful properties of chitosan is ionic chelation. The strong positive charge of chitosan enables it to bind to negatively charged substrates, such as cholesterol, fats, metal ions, and proteins [45–48]. As a nutritional supplement, chitosan has been reported to reduce lipid absorption in the intestine by binding fatty acids and bile acids and by increasing their excretion [45, 46]. Therefore, oral administration of chitosan inhibits the development of atherosclerosis in individuals with hypercholesterolemia by lowering the serum cholesterol levels.

Alginate: Alginate is a naturally derived anionic polysaccharide, obtained mainly from marine algae; it is widely utilized as a food additive and in drug formulations. Alginate consists of two sugar moieties, 1, 4-linked d-mannuronic acid (M) and l-gluronic acid (G), either block or random sequences [49–53]. Alginate forms complexes with divalent ions, such as Ca2+, Ba2+, and so on [52, 53].

Alginate hydrogels have pH-sensitive swelling transitions that are used in the design of drug delivery systems [54, 55]. Drug release from alginate gels is known to be blocked or sustained at low pH by forming a surface gels cover by deswelling, while drug release is accelerated at neutral pH by the swelling increase [56–59]. Alginate can potentially be used for cell delivery, such as microencapsulation of artificial pancreas [60]. The isolated islets of Langerhans suspended in the alginate aqueous solution are effectively encapsulated in the alginate gels when the solution is treated with divalent cations. For further stabilization of islet-encapsulated alginate gels, the polymer complex (an ionic complex) is usually formed at the surface of alginate gels with polycations, such as poly(l-lysine) [61, 62].

Pluronics: Pluronic is a triblock copolymer of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO, Poloxamers, Pluronics). Because of its ­nontoxicity and ability to form a gels, it is widely used in the pharmaceutical area [63–65]. For example, at 20% (w/v), aqueous solutions of poloxamer 407 form hydrogels at body temperature [66].

Poloxamer 188 (PEO80–PPO27–PEO80, molecular weight: 7,680–9,510) is used in ­intravenous injections and oral formulations and Poloxamer 407 (PEO101–PPO56–PEO101, mole­cular weight 9,840–14,600) is used in ophthalmic solutions. The sol–gel phase diagrams of poloxamer 188 and poloxamer 407, as a function of concentrations, are shown in Fig. 2.

Fig. 2.
figure 2

Sol–gel phase diagram with different concentration of poloxamer 188 and poloxamer 407.

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In addition to the enhancement of the bioavailability of low-solubility drugs in oral solid dosage forms, Pluronics are used as an emulsifier, solubilizer, dispersant, and wetting agent in the preparation of solid dispersions [67, 68]. When these polymers are bound to the surface of nanospheres by the hydrophobic interaction of the PPO chains, the hydrophilic PEO chains stretch into the surrounding medium creating a steric barrier [69, 70]. This barrier prevents or restricts the adsorption of plasma proteins onto the particle surface decreasing recognition by liver and spleen macrophages [71, 72].

The adsorption of these surfactants is the most widely used procedure to modify the surface characteristics of the primitive carriers; the incorporation of these copolymers into the particles during the manufacturing process has become a significant alternative strategy.

Poly(Ethylene Glycol) (PEG) and Its Copolymers

Poly(ethylene glycol) (PEG) is a neutral, water-soluble, and nontoxic synthetic polymer approved by the FDA for internal use and inclusion in a variety of foods, cosmetics, and drug delivery systems. For prolong blood circulation time, PEG is used to modify nanoparticles to avoid uptake by the reticuloendothelial system (RES). This is important in the design of effective therapeutic systems for injectable delivery and for the controlled drug delivery [73–75].

Modifying the polymer composition, particularly, the middle block composition, the block length, and the block ratio, produced a new generation of PEG–(poly(l-lactic acid-co-glycolide acid))–PEG (PEG–PLGA–PEG) triblock copolymers. The sol–gel transition temperature can be controlled by changing the repeating units of PEG–PLGA–PEG triblock copolymers, such as the PLGA length. As the hydrophobic block (PLGA) length is increased, a stronger shear stress is required to make the gels system. Increasing the PEG length of a PEG–PLGA–PEG triblock copolymer shifts the thermo-phase diagrams to higher temperatures [76, 77].

In situ gels formation in vivo was first made by subcutaneous injection of PEG–PLGA–PEG triblock copolymer aqueous solutions into rats [78]. Based on this phenomenon, paclitaxel-loaded biodegradable polymeric micellar system using low molecular weight and biodegradable amphilic diblock copolymer and monomethoxy PEO2000-Poly(d,l-Lactide)1750 micelles (Genexol®-PM) were published by Kim et al. [79, 80]. In Phase I human trials, micellar encapsulation of paclitaxel allowed safer administration of high doses of paclitaxel.

Poly(N-isopropylacrylamide) (PNIPAm)

Poly(N-isopropylacrylamide) (PNIPAm) is one of the most widely used thermosensitive polymers. PNIPAm has a hydrophilic amide group and a hydrophobic isopropyl group. The linear PNIPAm chain undergoes a rapid dehydration of the hydrophobic isopropyl groups in aqueous solution at its lower critical solution temperature (LCST) of around 32–34°C in water due to its coil-to-globule transition [81–85]. The potential of PNIPAm for drug delivery system [86–89] and cell engineering [90–92] has been well documented. For example, hybrid block and graft copolymers of PNIPAm containing phosphocholine [93, 94], poly(d,l-lactide) [95], and alginate [96, 97] have been successfully synthesized and well characterized as biomaterial candidates.

The copolymers that include the LCST block and a hydrophilic block, such as PEG–PNIPAm copolymers, form micelles above the LCST of PNIPAm, with PNIPAm block ­forming a micelle core [98]. Block copolymers consisting of the LCST block and a hydrophobic block, such as poly(N-isopropylacrylamide)-poly(methyl methacrylate) (PNIPAm–PMMA), form micelles below the LCST, with PMMA block forming a core and PNIPAm block forming a shell [99].

Molecular Probes for Imaging

With the advances in imaging technology, the importance of molecular imaging probes has increased for precise diagnosis. The visualization of the cellular function and the follow-up of the molecular process in living organisms without surgical operation are facilely carried out. Some of the techniques used for noninvasive imaging in diagnosis medicine are listed in Table 1.

Table 1. Noninvasive imaging in medical application
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Gold Nanoparticles

One of the most interesting aspects of gold nanoparticles is that their optical properties are varyingly dependant on the particle size and shape. Bulk gold looks yellow in reflected light, but this characteristic changes to orange, through several tones of purple and red, as particle size is reduced to ∼20 nm. These effects are the result of changes in the so-called surface plasmon resonance (SPR) [100].

Gold nanoparticles are usually prepared by reduction in a boiling sodium citrate solution [101]. The formation of gold nanoparticles appears as a deep wine red color and the UV absorption in the aqueous media at around 520 nm. Functionalization of gold nanoparticles (gold surfaces) with molecules containing thiol (-SH), which has a high affinity for gold atoms is commonly used. A number of biosensors are designed based on this phenomenon.

The gold nanoparticles are biocompatible and nontoxic in vivo [102, 103]. However, plasma proteins and salts in the blood nonspecifically adsorb onto the surface of gold nanoparticles, this often causes aggregation; therefore, the direct use of gold nanoparticles in vivo can lead to clearance from the bloodstream due to uptake by the reticular endothelial system (RES) (Kupffer cells of the liver) [104–107]. Therefore, gold nanoparticles used in vivo are usually surface modified with PEG [104].

Magnetic Nanoparticles

Magnetic nanoparticles are manipulated under the influence of a magnetic field and are commonly composed of magnetic elements such as iron oxide (superparamagnetic iron oxide (SPIO) and ultrasuperparamagnetic iron oxide (USPIO)) and gadolinium compounds. Because of difficulties in recognizing tumors from normal tissues by magnetic resonance imaging (MRI), patients are often injected with a contrast agent, such as iron oxide nanoparticles or gadolinium chelates.

Nanoparticles are prepared by either coprecipitation [108–110], high-temperature decomposition [111–113], or microemulsion [114, 115]. Coprecipitation is a facile and convenient way to synthesize iron oxides (Fe3O4 or γ-Fe2O3) from aqueous Fe2+ and Fe3+ salt solutions by the addition of base at room temperature or at raised temperatures. The stability is maintained by electrostatic and repulsive interaction between counter-ions. The size, shape, and composition of the magnetic nanoparticles mainly depend on the type of salts used, the Fe2+/Fe3+ molar ratio, reaction temperature, the pH, and ionic strength of the media [116–118].

The high-temperature decomposition (>200°C) of an organic iron precursor in the presence of hydrophobic ligands, such as oleic acid, is typically used; the hydrophobic ligands form a dense coating around the nanoparticles, thereby avoiding their aggregation.

Microemulsions are used to obtain relatively small particles (high surface area) with well controlled properties; water-in-oil microemulsions are usually used to produce iron oxide nanoparticles. The type and concentration of surfactant [119, 120], type of oil [121] and alcohol [122–124], droplet core size [125], and the speed of microemulsion mixing [126] all play an important role in the formation of iron oxide nanoparticles by microemulsion techniques.

Iron oxide nanoparticles have been approved for clinical use, especially for MRI, for example, Endorem® (diameter 80–150 nm, Advanced Magnetics) and Resovist® (diameter 60 nm, Schering) for liver/spleen imaging [127–129].

Gadolinium is also an FDA approved contrast agent for MRI. Gadolinium, or gadodiamide, provides greater contrast between normal tissue and abnormal tissue in the brain and body. Because of their paramagnetic properties, solutions of organic gadolinium complexes and gadolinium compounds are used as intravenous radiocontrast agents to enhance images in medical MRI. After it is injected into a vein, gadolinium accumulates in the abnormal tissue with bright (enhanced) images on the MRI. With the administration of MRI contrast agents, the relaxation times T1 and/or T2 of a proton in the vicinity of an agent change, thus ­generating image contrast (bright/dark) (Fig. 4) [130].

Fig. 3.
figure 3

Schematic description for molecular structure of poly(N-isopropylacrylamide) (PNIPAm) and drug release mechanism in response to temperature (T) changes.

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Fluorescence Dyes

Optical fluorescence depends on the inherent property of fluorophores, such as fluorescein isothiocyanate (FITC) and FITC derivatives, cysteine, cyanine dye (cydye), and Indicynine green dye (ICG) are used for fluorescence imaging (Fig. 5) [130].

Fig. 4.
figure 4

T1 and T2 relaxation processes [130].

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Fig. 5.
figure 5

Various used fluorophores in biological imaging.

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Fluorescein isothiocyanate (FITC) is used in several biological applications, such as fluorescent-labeled antibodies and molecules that are taken up by cells or organelles. Usually, the energy from an external source is absorbed by the fluorophores injected or accumulated at the tumor site.

Microbubbles

Ultrasound contrast agents, which consist of a hydrophobic gas (microbubbles) and a stabilizing shell, have enabled clinical contrast echocardiography due to their enhanced stability in circulation. Moderate intensity ultrasound assisted by encapsulated microbubbles has been used in in vitro and in vivo targeting drug delivery via a process called “sonoporation.” Ultrasound imaging is used to molecularly target microbubbles to the liver [131], breast [132], and prostate tumors [133].

These advances have created interest in ultrasound as a molecular imaging modality. Ultrasonic imaging of molecular targets associated with angiogenesis [134–137], thrombosis [138], and inflammation is being used [139, 140].

There are two types of bubbles that are related to sonoporation process: free bubbles and encapsulated microbubbles. Free bubbles are usually cavities filled with air, other gases, or gas vapor from surrounding liquid. However, due to their instability, free bubbles are usually encapsulated in biocompatible polymers as microbubbles for the ultrasonic imaging of angiogenesis [136, 141–146].

Quantum Dots

In general, the quantum dots are prepared in the organic solvent at high temperatures between 180 and 310°C, depending on the ligands and solvents employed in the preparation.

Quantum dots are nanoscale semiconductor crystals composed of Group II B (Transition metal)-Group VI A compounds (CdTe, CdS, CdHg, ZnS) or Group III A-Group V A elemental groups (InAs, InP, GaAs). A noble class of inorganic fluorophores is gaining widespread recognition as a result of their exceptional photophysical properties. Both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller (Fig. 6) [147, 148].

Fig. 6.
figure 6

Fluorescence spectra depending on the size of quantum dots [147, 148] (Blue fluorescence can be emitted from small particles of approximately 2 nm in diameter, green from ∼3 nm particles, yellow from ∼4 nm particles, and red from large particles of ∼5 nm. The wavelength of the excitation light is 365 nm).

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Quantum dots have broad excitation spectrum; therefore, different-colored quantum dots can be activated by using a single source laser at the same time, making them extremely attractive in multiplexing studies [149–151]. For biological imaging applications, quantum dot materials are chosen based on size, optical properties, and toxicity. The emission wavelength should be in a region of the spectrum where blood and tissue absorb minimally but detectors are still efficient, approximately in the near-infrared (700–900 nm).

In spite of these attractive features the use of quantum dots in the biomedical application has been limited due to their hydrophobic character; now hydrophilic surface ligands, such as mercaptoacetic acid [152, 153] and polyethylene glycol (PEG), are used to increase their stability in aqueous media and to reduce the nonspecific adsorption. However, quantum dots capped with these small molecules are easily degraded by hydrolysis or oxidation of the capping ligands [153]. Heavy metal ions, such as Cd2+, that can escape from the quantum dot matrix are cytotoxic and cause biocompatibility concerns [154, 155].

Molecular Probe/Polymer Composite Systems

Metal nanoparticles used in the biological imaging applications, such as gold and iron oxide, are easily cleared from the body because of biofouling of metal nanoparticles in the body. To overcome this limitation, the polymers used in the fabrication of hydrogels are utilized to stabilize the metal nanoparticles as molecular probe/polymer composite systems.

Contrast agents for Computer Tomography (CT) are based on iodinated small molecules because, among nonmetal atoms, iodine has a high X-ray absorption coefficient. However, iodinated compounds have very short imaging times due to rapid clearance by the kidney. Therefore, gold nanoparticles are used as they have a higher atomic number and X-ray absorption coefficient than iodine [156, 157]. However, gold nanoparticles also showed the rapid clearance by biofouling [158]. Gold nanoparticles can be combined with polymers containing thiol (-SH), which has a high affinity for gold atoms. Numerous modifications have been made based on this chemical nature of gold nanoparticles and this has led to several kinds of biosensors.

Poly(ethylene glycol)-SH (PEG-SH) can be design with CT contrast agents; the formation of PEG-coated gold nanoparticles enhances antibiofouling capability [159]. The X-ray absorption coefficient in vitro indicates that the attenuation of PEG-coated gold nanoparticles is 5.7 times higher than the iodine-based CT-contrast agent Ultravist in in-vivo animal test using rat.

The anionic character of gold nanoparticles stabilized with citrate attracts macromolecules with cationic character (positively charged polymers), such as chitosan and poly(ethyleneimine) (PEI). Through this electrostatic interaction gold nanoparticles/polymer composite systems are formed.

Multilayer film composites of gold nanoparticles and chitosan are constructed using layer by layer assembly [160]. The formation of the multilayer film was verified by UV–Vis Spectrometry, Atomic Force Microscopy, and Electrochemical Impedance Spectroscopy, and applied to nanodevices.

Gold nanoparticles protected/stabilized by biotinylated PNIPAM were prepared via a thiol anchoring end-group. The introduction of a biotin at the free chain-end of the stabilizer is to induce the supramolecular assembly containing gold nanoparticles via complexation with avidin in water [161].

As shown in Fig. 7, the gold nanoparticles stabilized by biotinylated PNIPAm demonstrated the nanostructure organization at the supramolecular level by biotin/avidin complexation in response to the biochemical species in the aqueous media, which can be utilized in the design of biosensors.

Fig. 7.
figure 7

Gold nanoparticles stabilized by biotinylated PNIPAM before and after the addition of avidin [161].

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Iron Oxide Nanoparticle/Polymer Composite Systems

Iron oxide nanoparticles have been evaluated as an MRI contrast agent for the liver and the spleen. However, the applications are still subject to many limitations such as size monodispersity, magnetization, stability, nontoxicity, biocompatibility, injectability, and the short blood half-life of magnetic nanoparticles for in vivo applications. To overcome these limitations, a variety of biocompatible polymeric materials, such as PVP [162], Pluronic [163], dextran [164], chitosan [165], poly(d,l-lactid-co-glycolide) [166], and ε-caprolactone [167], have been employed as coating materials for MRI contrast agents.

Magnetic nanoparticles composites are prepared with Fe3O4 as core and chitosan as polymeric shell [168]. Chitosan and Fe3O4 aqueous suspensions are mixed in appropriate proportions using reverse-phase suspension crosslinking. The saturated magnetization of composite nanoparticles shows the characteristics of superparamagnets. The decrease in the saturated magnetization is related to the increased amounts of polymer incorporated in the polymer-coated magnetite suspension.

Similarly, sonochemistry can be employed to prepare iron oxide-loaded chitosan nanoparticles [165]. The magnetic Fe3O4 nanoparticles have been prepared by coprecipitation. Ferric chloride hexahydrate (FeCl3·6H2O) and ferrous chloride tetrahydrate (FeCl2·4H2O) are mixed with ammonium hydroxide (NH4OH) under irradiated ultrasonic waves. The ferrofluid, made of iron oxide nanoparticles and chitosan, is sprayed on the surface of the alkali solution (NaOH/ethanol/water, 4/30/66, w/v/v) to form iron oxide-loaded chitosan nanoparticles. These nanoparticles were injected into the left kidney of a rabbit and T2-weighted MR images of the kidney were obtained. The iron oxide-loaded chitosan nanoparticles enhanced contrast of the T2-weighted MR images.

Recently, active localizing imaging probes (gold nanoparticles and metal nanoparticles) in tumor tissue were accomplished by conjugating target specific molecules, such as folic acid [169], RGD peptide [170], or integrins [171] (Fig. 8).

Fig. 8.
figure 8

Scheme of active cellular targeting [170].

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The surface modification of iron oxide nanoparticles with folic acid was carried out to improve receptor binding and the efficiency of cellular internalization of nanoparticles [169]. To evaluate the targeting specificity of the nanoparticle–PEG–folic acid (NP–PEG–FA) conjugate to tumor cells, the uptake of the nanoconjugate by HeLa cells was compared with that by human osteosarcoma MG-63 cells (folate receptor negative cell line). Human osteosarcoma MG-63 cells express very low levels of the α and β forms of the folate receptor. The level of nanoparticle conjugate uptake by HeLa cells ranged from twice to as much as ten times that by MG-63 cells. Concomitant with this nanoparticle uptake, the T2-weighted MR phantom image showed a significant increase in the negative contrast enhancement of the HeLa cells compared with that of the MG-63 cells.

Quantum Dot/Polymer Composite Systems

Fluorescent semiconductor nanocrystals or quantum dots provide a new class of biomarkers that could overcome the limitations of organic dyes as in vitro and in vivo imaging probes. Despite of their advantages as a molecular probe, the semiconductor core of quantum dots has raised concerns regarding heavy metal cytotoxicity. In fact, quantum dots are cytotoxic due to cadmium oxidation and the leaching of heavy metal ions [171–173]. As quantum dots applications broaden in biotechnology research, it is important to consider these potential hazards and develop novel approaches to avoid toxicity, such as encapsulation or polymer coating, to form a protective insulating material or wide band gap semiconductor structurally matched with the core material.

The formation of quantum dot/polymer nanocomposites involves strong noncovalent interactions, such as hydrogen bonding, ionic attraction [174–176], and physically entrapping quantum dots into particles formed by emulsion polymerization [177] or sol–gel synthesis [178].

Quantum dot-encapsulated nanoparticles are noncytotoxic during long-term incubation with viable cells in the absence of light exposure, which makes them appropriate for cell monitoring and drug delivery [179, 180]. The quantum dots were conjugated with various molecules and proteins, such as myosin VI, transferin, and kinesin; when these bioconjugated quantum dots were present, receptor-mediated endocytosis occurred and the luminescent quantum dots enabled the investigation of cellular uptake pathways and detection within cells due to the bright fluorescence of the colloids. Since the quantum dots have broad excitation properties for all colors, multiple colors can be efficiently excited simultaneously with one light source, such as blue-violet filtered light or a 405 nm or 488 nm laser [181, 182].

Microbubble/Polymer Composite Systems

Ultrasound contrast agents are widely used to image perfusion and have potential for drug and gene delivery, where therapeutic release is initiated by local sonication [183–192]. Microbubble-loaded and lipid-based contrast agents have a self-assembled shell that provides a flexible, protective membrane around a perfluorocarbon gas core. In the diagnosis, these agents have been successfully used in the measurement of blood volume and flow in cardiology and radiology [192, 193].

Lipid-based microbubbles are usually stabilized with ligand and/or polymer molecules before bubble production, and the stabilized lipids are self-assembled into a shell with exposure to the aqueous medium. The approach for these lipid-stabilized contrast agents (diameters ∼1–10) utilizes the lipid with PEG or PEG/ligand to specifically bind to a preferred target site [194].

Drug Delivery System with Molecular Imaging Capability

The development of noninvasive imaging technology (MRI, CT, PET, and Ultrasound) that integrates drug delivery systems with medical imaging is an important technology. A drug loaded with an imaging probe will enable real-time, targeted monitoring of drug delivery with medical imaging devices and to quantify drug uptake at the site as well as monitor the response to the therapy.

Yuk recently used composite gold nanoparticles, for the delivery of an anticancer drug; the ionic interaction between the gold nanoparticles and chitosan to form the composite nanoparticles loaded with paclitaxel [195]. Considering the optical property of gold nanoparticles, the gold nanoparticles/chitosan composite was utilized as a drug delivery system with molecular imaging capability (Fig. 9) [195].

Fig. 9.
figure 9

Schematic description of gold/chitosan composite nanoparticles [195].

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The oleic acid (OA)-Pluronic (F-127)-coated iron oxide nanoparticles were formed with high doses of water insoluble doxorubicin [163]. Because of drug partitions into the OA shell, the surrounding iron oxide nanoparticles and the Pluronic anchor at the water–OA interface which significantly increased the solubility (dispersity) of the doxorubicin. Neither the formulation components nor the drug loading affected the magnetic properties of the core iron oxide nanoparticles and sustained release of doxorubin was observed 2 weeks under in vitro conditions. The nanoparticles in this study showed an enhanced intracellular drug retention, comparing with free drug in the aqueous solution, and a dose-dependent antiproliferative effect in breast and prostate cancer cell lines.

Doxorubicin-loaded superparamagnetic iron oxide (SPIO) nanoparticles were made using polymeric micelles with cRGD attached onto the surface of polymeric micelles for efficient targeting to tumors [196]. Amphiphilic block copolymers of maleimide-terminated poly(ethylene glycol)-block-poly(d,l-lactide) [MAL-PEG-PLA, Mn = 7,200, Mn(PEG) = 3,200] and methoxy-terminated poly-(ethylene glycol)-block-poly(d,l-lactide) copolymer [MPEG-PLA, Mn = 6,400, Mn(PEG) = 2,000] were used to form micelles with cRGD attached to the surface through a thiol-maleimide linkage. The cRGD on the surface of polymeric micelle targeted the delivery of doxorubicin to αvβ3-expressing tumor cells. The in vitro MRI and cytotoxicity of the αvβ3-specific cytotoxic response of these multifunctional polymeric micelles were observed by ultrasensitive MRI.

To combine contrast-enhanced ultrasound tumor imaging with targeted drug delivery is a challenging task [197–199]. Rapoport et al. developed novel ultrasound-sensitive multifunctional nanoparticles composed of nanoscale polymeric micelles that function as drug carriers and nano- or microscale echogenic bubbles that combine the properties of drug carriers, enhancers of ultrasound-mediated drug delivery with long-lasting ultrasound contrast agents [200, 201]. In their study, perfluoropentane (PFP) nanoemulsions dispersed in a solution of polymeric micelles were produced by introducing an aliquot of a sterilized PFP into a micellar solution of a copolymer which was subsequently subject to sonication to produce cavitation. Biodegradable diblock copolymers poly(ethylene oxide)-block-poly(lactide) and poly(ethylene oxide)-block-poly(caprolactone) were used to form polymeric micelles with doxorubicin as the drug model. The copolymer-stabilized PFP nanoemulsion systems undergo nanodroplet/nanobubble conversion in vivo, accumulate locally in the tumor tissue and coalesce into larger, highly echogenic microbubbles, which provide long-lasting ultrasound contrast in the tumor while maintaining effective levels of doxorubicin at the tumor site.

The visualization and monitoring of transplanted islets using iron oxide nanoparticles covered with a modified dextran was carried out by incubating the Islets with magnetic nanoparticles consisting of a superparamagnetic iron core covered with a modified dextran coating [202]. The MRI showed a marked decrease in signal intensity on T2-weighted images at the implantation site in the left kidney as compared with the right kidney (implanted unlabeled islets). Thus, in vivo detection of transplanted human pancreatic islets using magnetic resonance imaging (MRI) that allowed noninvasive monitoring of islet grafts in diabetic mice in real time is now possible [202].

Summary

The unique feature of hydrogel-based drug delivery systems with molecular imaging capability involves loading a therapeutic agent into polymer network (hydrogels) surrounding molecular imaging probes. Although understanding and demonstrating the combination of hydrogels containing therapeutic agents with molecular imaging probes has been performed successfully, there remains the challenge for efficient application of this technology to diagnosis and therapy. The realization of hydrogels/molecular imaging probe composite systems on the nanoscale and the optimized drug release in response to the diagnosis is an important step. In the near future, this integrated smart system will open many potential opportunities for the effective therapeutic delivery and monitoring as well as molecular imaging probes for noninvasive procedures in early detection of disease.