The paper summarizes the history of the drug/gene delivery domain, pointing on polymers as a solution to specific challenges, and outlines the current situation in the field – focusing on the newest strategies intended to improve systems effectiveness and responsiveness (design keys, preparative approaches). Some recent results of the authors are briefly presented.
Significant human and financial resources have been always invested in medical research and development. Nowadays, the annual growth of sales for medical systems using biomaterials and the development of connected industries is, by far, one of the highest . According to recent healthcare market research reports systems for drug delivery, diagnostics and regenerative medicine are among the global top ten medical device technologies.
In this context, a major contribution to medical technologies comes from chemical engineering research, interfacing bioengineering and materials science. One main research direction is aimed at understanding how materials interact with the human body, i.e., twigging the factors affecting materials biocompatibility and therapeutic efficiency, while another one is dealing with new biomaterials and devices to address unsolved medical problems.
Specific requirements are to be met in the effort towards macromolecular materials with correct engineering, chemical and biological properties for the intended medical application. First, to reduce the costs of the product, off-the-shelf and not custom-made materials are to be used as much as possible. Second, since the developed technologies have to work inside the human body, they have to be safe – that is, biocompatible and sterile. To achieve a targeted action, the materials need high functionality  and have to be highly responsive to specific stimuli, allowing recognitive feedback control, taking nature as a model [3, 4]. At the same time, both the final product and the necessary precursors must be reproducible (no batch to batch variations) . Finally, processing facilities are necessary to mass-produce the materials.
One of the main scientific challenges to overcome in order to meet these requirements is to understand the role of both the nano- and the macrostructure and to finely tune them to achieve new functionalities. This implies addressing a number of key issues. First, the basic elements, intended to create the complex macromolecular structure, have to be designed and synthesized. Second, these building blocks need precisely controlled molecular structures and complementarities to achieve self-assembling and create a structure that follows a certain predetermined hierarchy, in a so-called bottom-up approach. This allows the fine tuning of the material properties by using not only the molecular structure, but also topology and conformation, similar to biological compounds . Another important aspect to be taken into consideration consists in the modification of the properties of macromolecular compounds as a result of nanoscale confinement, such as changed kinetics, structure and organization , as well as that caused by the interaction of interfaces, characteristic of the presence of a nanophase . To reduce financial and time costs of experimentation, the elaboration of theoretical models is also highly desirable. The models are extremely helpful tools, useful in preliminary screenings, prediction of a series of properties and interactions, as well as in identifying possible causes of potential failures . Finally, protocols and technologies, both reproducible and environmentally friendly [5, 10], have to be developed for the manufacturing of the product.
Current research efforts are mainly aimed at designing new and more efficient drug/gene delivery systems [11–13], materials for regenerative medicine  and minimally invasive surgery (i.e., memory shape materials) . Research in advanced materials for areas of convergence of nanotechnology and medicine gained increased importance . More, in times when most of health care costs can be attributed to tissue loss and organ failure , approaches for producing complex tissue structures (3D rapid prototyping) are also of the utmost interest .
To insure improved safety and pharmaceutical efficacy of the delivery systems, increasing patient compliance and satisfaction, drug-delivery technology became nowadays a multidisciplinary science cumulating knowledge from physical, biological, pharmaceutics, biomedical engineering and biomaterials fields. Conventional formulations of former bioactive agent delivery systems included polymers as additives to solubilize and stabilize drugs or as mechanical support for their sustained release. In time, the role of polymers has changed due to strong demands on more efficient and functional bioactive agent delivery vehicles and advancements in polymer science and engineering that gave rise to availability of new, controlled synthetic methods. At present, engineered polymers – polymers with different phenotypes and tailor-made properties – are used for developing advanced drug delivery systems able to provide temporal and spatial controlled delivery of therapeutics in a predictable manner, minimizing side-effects [11–13]. To alleviate the shortcomings of conventional formulations, different polymer-based drug delivery devices such as diffusion-controlled, solvent-activated, chemically controlled (biodegradable), or stimuli responsive/externally-triggered systems were developed. They include synthetic or natural polymers and copolymers with different architectures and form, i.e., micelles, liposomes, polymersomes, polyrotaxanes, dendrimers, polymer particles, capsosomes, gels/hydrogels and interpenetrated polymer networks, molecular imprinting polymers a. s. o.
To expand the scope of delivery systems and to develop realistic alternatives for clinical transfer, the adopted strategies are mainly focused on (1) designing new, more efficient, multifunctional polymer-based formulations, and (2) improvement of delivery techniques. Most desirable, such polymer-based materials with advanced properties must present an appropriate design for efficient on-site delivery of a specific cargo, also insuring exertion of distinct biological functions and achievement of maximum biocompatibility, preferably cost-effective and amenable for industrial scale-up. Significant efficacy enhancement of many therapeutic, imaging and diagnostic protocols [2, 19, 20] arrive from combining several different useful functionalities (such as transporting, specific targeting, intracellular penetration/trafficking ability, process monitoring by contrast loading, bioactivity, feedback control through stimuli-sensitivity) in a single stable construct, affording biocompatibility, biostability and appropriate biodistribution. This is the main strategy to achieve the main goal – an accelerated development of personalized medicine. The challenges appearing from the development and production of such highly controlled and implicitly complex systems brought together polymer chemistry and chemical engineering innovation, as well as achievements from biochemistry, microfluidics, and nanotechnology. In this context, the newest advances in bioactive agent delivery refer to stimuli-responsive polymer systems, polymer therapeutics (polymer-protein, polymer-drug conjugates), polymers capable of molecular recognition or directing intracellular delivery, in situ forming drug delivery systems for local and systemic route delivery (micellar systems, low molecular mass organic gelators, in situ gel forming smart polymeric formulations), systems with improved drug loading (squalene based conjugates, low molecular weight gelators) [11–13]. Green chemistry, controlled/living polymerization methods and nanotechnology approaches are between the frequently used preparation tools [20–22]. Nanomaterials and nanostructured materials are more and more involved in the preparation of new advanced systems – their unique properties (design flexibility, small sizes, large surface/volume ratio) offer important facilities to cross over different barriers (physical, chemical or biological), thus enabling the effective targeting .
An efficient route to achieve new performances and answer to the specific requirements for industry translation seems to be the smart combination strategy applied for (1) materials (natural polymer/synthetic polymer, inorganic/organic components – yielding hybrid/composite materials with improved properties and processing ability), (2) preparative methods (related to covalent/non-covalent bonding, making use of system components chemistry), (3) systems and (4) approaches (intertwining technologies) from apparently different applications domains (implantable drug delivery systems, diagnostics/drug delivery systems, drug or/and gene delivery systems/scaffold, nanocomposites for external in situ triggering).
The high complexity implied by the requests of multifunctionality, controllability and applicability in biomedical area (well-defined structure, composition and physical-mechanical properties, proper biocompatibility, interaction with target cells/tissues, and biodistribution) involves many experiments to monitor a huge number of components (with own dimensional, mechanical, chemical, biological characteristics) and parameters (related to synthesis, purification, storage steps). To facilitate design selection of the optimum system and preparation protocol, the use of in silico tools [9, 23, 24] (statistical methods, artificial neural networks (ANN) and especially molecular simulations) and the development of high-throughput scanning (HTS) methods (such as parallel synthesis)  became a necessity.
According to literature, the multifunctional polymeric systems are useful to strategically deliver drugs, proteins (antibodies, hormones, growth factors, therapeutic enzymes), nucleic acids (DNA, RNA, oligonucleotides), or vaccines, improving the therapeutic index. Some have been already used for cancers and central nervous system diseases, bacterial infections and inflammatory processes [11–13].
Considering the topical results and trends in bioactive compounds release systems development, particulate polymer systems (especially those combining properties like longevity, targetability, intracellular penetration and contrast loading) and implantable devices are the most promising controlled release systems for industrial manufacture in the forseeable future.
Particulate polymer systems (including micro/nanospheres, micro/nanocapsules, micro/nanogels micellar structures, etc.) are by far the most studied polymer materials envisaging biomedical application. Such materials, with finely tuned properties answering the specific requests of application area, can be produced now in a large variety due to the progress of polymer chemistry, polymer colloid physical-chemistry, polymer technology and nanotechnology [2, 11, 19, 20]. More, one may choose from more simple, economic, robust techniques developed in time, which allow the required control on factors and parameters for the large scale production [20, 25–28].
The main properties considered for a specific use are related to particles size, size distribution, surface area/volume ratio, chemistry, morphology, physical characteristics (mechanical, thermal, optical), self-assembly ability into larger/complex well-defined structures. Polymer particles proved to be useful in the development of BioMEMs (biomedical (or biological) microelectromechanical systems), drug/gene delivery systems, scaffolds for tissue engineering, engineered prosthetics, systems for biological analysis and discovery, protective materials against pathogens, materials for diagnostics and imagistics, devices for high-throughput biomolecule separation etc. Depending on size and their suspension rheology they can be administered to a variety of locations in vivo, as parenteral drug delivery systems or injectable scaffolds. Nanoparticles were found to provide improved therapy through superiority in increasing efficacy, specificity, tolerability, and therapeutic index of the implied drugs, and seem to represent the next generation of drug delivery systems [2, 19, 20]. Due to their nanoscopic size scale and large surface area/volume ratio they exhibit unique physical and biological properties, highly advantageous when used in drug delivery systems, such as (1) facile circulation in the body and easy penetration and absorption through tissues and cells, easily circumventing the biological barriers (i.e., tight intestinal barrier, blood–retinal and blood–brain barrier), (2) high drug loading capacity, (3) increased contact area monitoring adhesion to tissue, and (4) long shelf-life (increased suspension stability) . Monodisperse polymer particles have self-assembly ability in dispersion forming ordered 3D structures, i.e., colloidal crystals, materials that revealed interesting properties (i.e., optical characteristics similar to synthetic opal and photonic crystals) . They can be used as templates for porous membranes and scaffolds. Hybrid polymer nanoparticles including ferromagnetic materials become superparamagnetic below 20 nm, being often used as multifunctional nanocarriers manifesting under an applied magnetic field both therapeutic effect (drug release targetability in conjunction with localized heating) and ability to act as a contrast agent for magnetic resonance imaging .
The polymers used as constituents of particles designed to deliver bioactive agents in vivo must fulfil requirements such as absence of toxicity, non immunogenicity, biodegradability (to non toxic, non immunogenic products) or ability to avoid uncontrolled accumulation, ability to be processed in particulate form with appropriate characteristics for the drug/gene delivery goal. This is why they include a limited number of natural and synthetic biodegradable polymers, their derivatives and copolymers, the last aiming to improve functionality and performance. The loading efficiency highly depends on the composition, molecular weight, functional groups of the polymer and its ability to develop specific interactions with the drug or other system components, as well as on particulate system morphology. The bioactive compounds may be entrapped inside during the micro/nanoparticle production or adsorbed/bonded on the surface of preformed micro/nanoparticle. Their release involves hydration and diffusion through the polymer material, its erosion/degradation and combination of erosion and diffusion process, or desorption/ dissociation processes, the release rate being highly dependent on the physico-chemical properties of the polymers and drugs and on the environmental.
Recent progress of the domain is underlined by the development of (1) multifunctional particulate systems , (2) new or improved preparation approaches , and (3) complex systems with new/enlarged functionality resulting through self-assembly in 2D or 3D higher ordered structures [21, 29] or by combination with devices from other biomedical areas .
Multifunctionality is often achieved by designing bioinspired materials and combining materials from different classes. (Multi)stimuli-responsive, multicompartmental, and magnetic micro/nano polymer particles are among the most known results of such strategies .
Considering the used raw materials common polymer micro/nanoparticles preparation methods include (1) dispersion of preformed polymers, (2) heterogeneous polymerization of monomers, and (3) gelation or coacervation of hydrophilic polymers, making use of specific interactions. The most known preparation strategies may be classified in  (1) two steps procedures that include emulsification – solvent evaporation/solvent diffusion and salting out methods, in situ polymerization (by emulsion/miniemulsion/nanoemulsion/microemulsion/ interfacial polymerization, and multiple emulsion method), (2) one step procedures which allow the obtaining of (nano)spheres with narrow size distribution by methods based on (nano)precipitation of a polymer, (3) efficient one step procedures, using mild conditions such as methods based on self-assembling of the macromolecules.
When self-assembling of interactive species (at least one macromolecular) is applied, the systems may contain  (3a) polymers with complementary functions able to interact (i.e., corresponding to different ionic charges – polyelectrolyte complex/polyplex formation, or to neutral associating species – cyclodextrins and appropriate guests), (3b) polymer chains and a low-molecular compound able to crosslink them covalently or by non-covalent interactions, (3c) linear chains able to collapse by intramolecular covalent  or non-covalent  crosslinking, yielding well-defined single chain polymer nanoparticles.
Last years other methods, based on multidisciplinary approaches, gained increased interest : microfluidics assisted generation of simple oil-in-water (o/w) and double (DE) water-in-oil-in-water (w/o/w) emulsions , supercritical fluid technology , particle replication in non-wetting templates (PRINT) , and the bioinspired methodology using superhydrophobic substrates to produce hydrogel and polymeric particulate biomaterials .
The choice of appropriate preparative methodology by comparatively considering the advantages and drawbacks in relation to the requirements of the specific application, and the use of computer-guided process to ensure batch-to-batch repeatability, are throughput tools for a rationale design of the product and preparation technology.
Self-assembly is one of the approaches used not only to obtain nanoparticles in a simple, efficient and environmental friendly way, but also to achieve multifunctionality in nanoparticulate systems and/or to use them to construct new, complex architectures, miming hierarchical structures principle from biological materials. Especially when envisaging when envisaging biomedical uses, mild preparation conditions are imposed, biopolymers or bioactive compounds being implied. Biomimicry and the use of biopolymers in the formulations designed for biomedical area or cosmetics became a main trend of the last years. Capsosomes (multicompartmentalized polymer capsules [38, 39]) and multilayered polymer capsules [40, 41] are examples of applying polymer self-assembly to improve the performances of delivery systems. Along with the advance in preparative methodology, the development of such materials is the effect of the increasing need of strategies for encapsulation/release of multiple active substances (with distinct kinetics) in a single carrier towards therapeutic applications of synergistic combinations of actives.
As bioinspired systems, capsosomes are subcompartmentalized polyelectrolyte capsules using liposomes, obtained via the sequential deposition of interacting polymers onto particle templates (layer-by-layer technique, LbL) followed by core removal . They combine the advantages of the two co-assembled components – the semipermeability of the shell allowing communication with the environment, and the possibility of triggered drug co-release due to nanosized subcompartments confined within the micron-sized structurally stable scaffold. Versatile materials may be developed by engineering the building blocks of such liposome/polymersome based multicompartmentalized assemblies, to integrate specific permeability, stability, stimuli responsiveness, biocompatibility, and targeting abilities in the design of artificial cells/organelles, microcontainers for controlled capture/release, and microreactors .
Multi-layered micro/nanoparticles are results of research efforts to overcome problems inherent to single-layered particulate systems for actives delivery. They provide improved stability and a large variety of drug delivery kinetics and profiles, being driven by drug-polymer affinity, may be adjusted by encapsulation of multiple drugs within alternate layers with well-designed properties. Particle morphology, layers configuration, and drug distribution can be tailored. Simple, economical, reliable and versatile one- or two-steps fabrication methods were developed over time, based on conventional or modern approaches [40, 41]. LbL deposition process, based on polymer self-assembling ability, is one of the most used alternatives for engineering polymer particles surface to increase functionality, stability, loading efficiency or to achieve multiple actives loading [42, 43]. The technique may be combined with the specific reactivity of the included polymer to obtain multilayered micro/nanoparticles with tuned properties, creating new advantages for practical application. As an example, biocompatible, biodegradable multilayered micro-/nanoparticles with a porous core, containing biopolymers (atelocollagen or/and dimethylsilanediol hyaluronate) crosslinked with a poly(ε-caprolactone) reactive derivative and different coating shells, were obtained by combining a modified double emulsion method with LbL technique (Fig. 1), making use of specific reactivity of surface polymer layer and the ability of involved polymers to develop electrostatic/hydrogen interactions .
Depending on formulation and reaction conditions, nature and number of deposited polymer layers, the polymer particles present different, tuned chemistry, degradation rate, and dimensional characteristics (Dn in the range of 30–500 nm). The final coating consists of poly(ε-caprolactone), hyaluronan or a cationic polymer (poly(L-lysine) (PLL) or polyethylenimine (PEI) grafts). Preliminary data confirmed their usefulness in drug/gene delivery .
Making use of recent advances in controlled polymerization, strategies to obtain new, versatile polymeric vehicles in the nanometer range in mild conditions were developed based on the predictable self-assembly ability of block copolymers with tuned structural properties. These allow the preparation of nanostructured molecular assemblies (polymeric micelles, preprogramed functional nanoparticles) with well-defined size, shape, morphology, interaction properties, by appropriate design in terms of component sequence chemistry, block length and architecture and intelligent choice of solvents, factors controling phase separation in solution [3, 21]. The resulted nanomaterials may be dotted with one or more specific superficially exposed bonding sites with key functionalities, or may be able to respond to different stimuli, giving rise to dinamic self-assembly towards more complex, precisely ordered superstructures by a co-assembly process controlled by specific interactions and triggered by external-stimuli. This provides a versatile route for the preparation of advanced active substances-delivery systems, able to create preprogrammed encapsulating structures with controlled time, dose and site delivery profile.
On the other hand, interesting 2D or 3D micro-/nanostructured materials (membranes, hydrogels with optimised properties for storage/delivery of actives or (injectable) scaffolds with tuned characteristics) may be easily designed making use of the self-assembling ability of micro-/nanoparticles [29, 45, 46]. As an example, highly ordered porous materials obtained through the self-assembling of thermosensitive microgel particles (Fig. 2) (size distribution ∼1.1, Dn=27–200 nm) with a ternary composition based on poly(N-isopropylacrylamide)/poly(2-alkyl-2-oxazoline)/poly(2-hydroxyethyl methacrylate) (PNIPAAm/PROZO/PHEMA) copolymers were found to present a high response rate as compared to that of materials based only on PNIPAAm . The high responsiveness was related to composition (hydrophobe/hydrophile balance, thermosensitive units content) and to morphological features resulting from the self assembling facilitated by the dimensional characteristics of polymer particles and H-bonding ability of the components. The experimental data pointed on the possibility to control the hydrogels swelling-deswelling behavior and protein loading/release by formulation and synthesis conditions.
Controlled drug delivery systems have represented a major interest of researchers in the last two decades [4, 11–13]. However, although these systems were fashionable for a long period of time, they are not appropriate for all kinds of medical disorders because they are based on the release of a certain amount of drug over a short or long time interval in a predictable manner.
Nevertheless, there are medical situations where such an approach is not a satisfactory one. These include the administration of insulin for patients with diabetes mellitus, nitrates for patients with angina pectoris or antiarrhithmics for patients with heart rhythm disorders, etc. Therefore, significant progress has been made in the development of “intelligent” devices for optimizing drug delivery. They are able to recognize changes in the parameters of normal physiological functions and to act to address them. Most of “intelligent”/self-regulating drug delivery systems are based on stimuli-sensitive polymers. These are a group of polymeric materials that undergo a sharp phase transition to small changes of temperature, pH, light, ionic strength [11–13, 47], etc. Among them, polymers sensitive to temperature and pH are largely used – they exploit temperature/pH changes of the human body occurring when normal physiological conditions are disturbed. These changes act as triggering agents causing the release of the drug.
The literature shows a wide range of temperature-sensitive polymers for biomedical applications – those from poly(N-substituted acrylamide family) [poly(N-isopropylacrylamide), poly(N-isopropylmethacrylamide), poly(N,N-diethylacrylamide)], polymers with amphiphilic balance [(poly(ethylene oxide)-b-poly(propylene oxide) (Poloxamer) and related copolymers] and some biopolymers .
Among them, PNIPAAm is the most used in drug delivery because in aqueous solution it possesses a sharp phase transition [lower critical solution temperature (LCST)] at 32 °C . At a temperature below the LCST, the polymer is hydrated and soluble whereas above the LCST the water of hydration is disrupted, the polymer becomes hydrophobic and precipitates.
However, the physiological fluids are rich in salts and in these conditions the phase transition temperature of the polymer becomes lower than that of the human body . To increase polymer LCST to values close to that of the human body, NIPAAm is usually copolymerized with hydrophilic monomers , but the comonomers, even in very small amounts, significantly reduce or even annihilate the thermosensitivity of the copolymer . Nevertheless, NIPAAm copolymers with relatively high percentage of hydrophilic monomers preserving the thermosensitive properties were obtained. These copolymers contain comonomers such as acrylamide , N,N-dimethyl acrylamide , N-hydroxymethyl acrylamide , hydroxyethyl acrylamide . The inclusion of PROZO sequences in graft and block copolymers of NIPAAm was found to modulate the LCST value, as well as temperature responsiveness .
Most thermosensitive copolymers with applications in controlled delivery of drugs are used as crosslinked three-dimensional hydrogels [57, 58]. In contrast to linear copolymers, the phase transition of the hydrogel covers a wider temperature range, depending on crosslinking degree , hydrogel size  and porosity . The phase transition temperature of the hydrogel [“volume phase transition temperature” (VPTT)] does not coincide with the transition temperature of the linear copolymer, being slightly lower  or higher  than LCST. Crosslinked hydrogels swell under the VPTT and collapse above it (Fig. 3).
The most important characteristic of a thermosensitive hydrogel is the rapidity of the volume change when the temperature slightly changes below and above the VPTT.
Due to their reduced swelling and collapsing rates [61, 62], the usual thermosensitive hydrogels have limited biomedical applications. Basically, the swelling and collapsing rates depend on solvent diffusion within/out of the hydrogel. Solvent diffusion is usually a time-consuming process, depending on the size  and porosity  of the hydrogel. The smaller is the dimension of the hydrogel, the faster is the response rate to the temperature change. In addition, a porous structure circumvents the formation of the “skin” at the surface of the hydrogel, improving the flexibility of the polymeric network. Moreover, porous structures allow water to be absorbed or expelled by convection, a process much more rapid than diffusion. Several literature reports describe the preparation of porous hydrogels [46, 65, 66]. High porosity poly(N-isopropylacrylamide-co-acrylamide) microspheres (Fig. 4) were prepared by crosslinking the acrylamide groups in the presence of NIPAAm oligomers (porogens) .
The microspheres display a very rapid swelling/collapsing rate, related to their porosity, when the temperature changes around body temperature. This swelling/shrinking process has been used for the development of pulsatile drug delivery systems. The drug is released by diffusion when the hydrogel is in the swollen state (i.e., below the VPTT)  and no steric interactions occur (Fig. 5). Above the VPTT, the matrix is in the collapsed state and obstructs the drug release. However, even in shrunken state, these three-dimensional microgels are to some extent permissible for small molecule drugs . Another major disadvantage of these systems is that a large amount of drug is mechanically expelled during the collapsing cycles and a firm “on-off” release mechanism is not achieved (Fig. 5) .
To circumvent these limitations, microgels with pendant thermosensitive arms were synthesized by grafting semitelechelic thermosensitive oligomers with well-defined molecular weights on pullulan microspheres . In these systems the release of drugs is not controlled by swelling/collapsing process but by the extension/contraction of the pendant arms that acts as a valve. Below the LCST, the thermosensitive arms are in the extended state and stop drug release, while above the LCST the thermosensitive arms collapse, allowing drug release.
pH-sensitive polymer-based drug delivery systems were developed to exploit the variation of pH in different compartments of the human body. The pH changes from 1.0–3.0 in stomach to 4.8–8.2 in duodenum, 7.0–7.5 in colon, 7.3–7.4 in the blood and 5.5–6.5 in extracellular tumors. Usually, pH-sensitive polymers are natural or synthetic macromolecules containing weakly acidic or basic functional groups . Such delivery systems may control the release of drug either by dissolution of the linear polymers when reaching a region with a certain pH  or by swelling/collapsing of a three dimensional crosslinked network .
The linear pH-sensitive polymers are usually used for coating tablets, capsules, or pellets protecting the active drug from gastric acid and releasing it in the small intestine. Methacrylic acid copolymers are the most frequently used .
Different dual pH and temperature responsive polymer systems were also developed over time . The most known pH-sensitive copolymer, containing methacrylic acid and methyl methacrylate units, ionizes and dissolves above pH=5.5 (Eudragit®, Röhm Pharmaceuticals, Darmstadt, Germany) [71, 72]. Comparatively, NIPAAm copolymer with methacrylic acid and methyl methacrylate shows interesting pH- and temperature-sensitive properties [69, 73]. The copolymer is not soluble in gastric juice at pH=1.2 and 37 °C, but rapidly solubilizes in intestinal fluids (pH=6.8 and pH=7.4) at the same temperature. The copolymer was solubilized together with cellulose acetate butyrate in an acetone/methanol mixture and transformed in microspheres by solvent evaporation method. The examination by thermal and optical techniques established that the pH- and temperature-sensitive copolymer precipitates separately from cellulose acetate butyrate during solvent evaporation and forms small microdomains in microspheres. Vitamin B12, taken as model drug, was released in small amounts at pH=1.2 and 37 °C. However, at pH=7.4, a large amount of drug was released due to the dissolution of the copolymer.
Crosslinked pH-sensitive hydrogels control the release of drugs due to their swelling/ collapsing properties at different pH. Generally, polyacid hydrogels are relatively unswollen at low pH, since the acidic groups are in protonated form and hence unionized. With increasing pH, the polyacid hydrogel reaches the ionized state, swells to a higher degree and the drug is released by diffusion. On the opposite, polybase hydrogels swell at low pH because the basic groups are ionized. With increasing pH, the hydrogel collapses since the basic groups are not ionized and forms hydrogen bonds. Fundueanu et al.  developed a new multinucleated system for colonic delivery of tetracycline – the so-called “self-propelled drug release system”. Sulfopropyl dextran microspheres were first loaded with tetracycline and then encapsulated in cellulose acetate butyrate microcapsules. When the microspheres reach the intestine, the drug starts to be disclosed by the competitive ions of the intestinal fluid and the swelling degree of microspheres increases. The swelling creates a pressure against polymeric microcapsules and therefore the thin polymer film surrounding the microspheres is broken. In this way, the microspheres escape and the drug starts to be released. The higher is the amount of released drug the greater is the swelling degree of the encapsulated pH-sensitive microspheres and the pressure generated by their additional swelling will break more resistant walls. Thus, a self-propelled drug release system is achieved, and the drug is released even from compartments with the thick walls.
Magnetic nanoparticles show certain properties sustaining their use in biomedical applications, such as the ability to be guided through magnetic fields, the ability to generate thermal energy in alternating magnetic field, or to modulate the spin–spin relaxation time of water molecules surrounding nanoparticles in magnetic resonance imaging.
To handle magnetic nanoparticles, especially in the biomedical area, requires the understanding of magnetic properties of materials. These are related to electron velocity and orientation which means that in fact all materials display a magnetic behavior, characterized by specific physical quantities. The magnetic moment is a quantity that commonly occurs in the description of materials in terms of magnetization. Only ferri- and ferromagnetic materials are interesting for biomedical applications, particularly for targeted drug delivery. When a ferri- or ferromagnet is sufficiently small, it acts like a single magnetic spin (superparamagnetism), exhibiting no tendency to agglomerate and being of interest in transport and drug delivery . Surface properties of magnetic nanoparticles are very important because they very strongly influence the solubility/dispersibility of magnetic cored nanoconjugates in various chemical media and lead to different environmental compatibilities for biomedical applications.
The most commonly magnetic nanoparticles used in biomedical applications are superparamagnetic magnetite (Fe3O4) and maghemite (Fe2O3) due to their facile synthesis and biocompatibility. Different formulations based on these magnetic materials have been approved by the FDA for in vivo applications such as biodetection and bioseparation , targeted drug delivery , magnetic resonance imaging , and hyperthermia treatment of cancers .
Magnetic nanoparticles are commonly prepared by aqueous co-precipitation of iron salts, sol-gel reaction, micro-emulsion method, solid-state reaction by grinding inorganic iron salts, polyol-mediated preparation, chemical decomposition of iron pentacarbonyl Fe(CO)5 . Cobalt nanoparticles show also very good magnetic properties, but the high toxicity of the metal is an important drawback. One way to prevent the toxicity of cobalt ions could be their inorganic encapsulation, i.e., in silica . Pure metal particles such as iron, cobalt or nickel should not be used directly in vivo because they oxidize easily and release +2 or +3 charged metal ions with undesired toxic effects on living organisms . In contrast to these “pure” metal nanoparticles, iron oxide magnetic nanoparticles coated and stabilized with hydrophilic polymers have been found to be stable in physiological conditions, without obvious toxic effects. Moreover, it was found that these particles are identical in size and dimension with those present in human tissues . Pharmacokinetic studies of magnetic nanoparticles have proven that inside the body magnetite nanoparticles are taken up by the cells of reticuloendothelial system and transported to lysosomes, where they oxidize at low pH and then are recycled. Tests performed with radiolabeled 59Fe have shown that up to 60 % of the iron is recovered in the red blood cells within 20–40 days . In turn, the inhalation of nanoparticles in humans is toxic and may cause asthma, inflammation and potentially even cancer, because particles smaller than 100 nm are not able to be exhaled, being almost completely retained in alveoli .
For in vivo biomedical uses the surface of nanoparticles with ferro-, ferri- and superparamagnetic properties have to fulfil certain conditions to reach biocompatibility, colloidal stability, to increase their circulation time in the bloodstream, and to become useful for diagnosis. The most used strategy to overcome these problems is coating the magnetic nanoparticles with a polymeric shell that protects the core from oxidation, allows the formation of stable aqueous suspensions and further functionalization/modification for targeted applications. Thus, core-shell magnetic nanoparticles must possess multifunctional properties for specific biomedical application. The progress in advanced nanotechnology and in material and engineering sciences through optimizing the synthesis and derivatization brought significant improvements in magnetic polymer nanoparticles production. For example, magnetic nanoparticles carrying peptides, antibodies, aptamers, drugs, genes and fluorescent dyes have been discussed in the literature .
Consequently, to date magnetic polymer nanoparticles were successfully used for the following purposes: (1) drug or gene transport and release ; (2) medical imaging as contrast agents in MRI ; (3) hyperthermia treatment, when body tissues are exposed to slightly higher temperatures which lead to a local inhibition of cell function, inducing death of cancer cell or thermoresponsive drug release ; (4) magnetic separation of biological compounds .
Induced hyperthermia in magnetic nanoparticles relies on the thermal effect produced when they are placed in alternating current magnetic field. This heat production can be used in cancer therapy for eradication of tumor cells – temperatures above 56 °C lead to thermoablation . Another approach to hyperthermia undergoes using large biocompatible plastic microspheres incorporating maghemite nanoparticles. Thus designed particles have proven the embolization of blood vessels supplying tumors .
The use of magnetic polymer nanoparticles in chemotherapy offers an increased concentration of highly effective anticancer drugs in tumor tissue, with minimal and shorter systemic toxic effects. The drug release from magnetic particles commonly occurs passively, by desorption from the matrix. The main driving forces are pH, osmolarity and concentration gradient between particles and blood or tissue. As examples of chemotherapeutic magnetic polymer nanoparticles with positive results one can mention the chitosan nanoparticles loaded with oxantrazole , solid lipid nanoparticles loaded with methotrexate  or polyalkylcyanoacrylate nanoparticles filled with dactinomycin . Studies on a promising antitumor drug delivery system based on heparin-magnetite nanoparticles loaded with a carboxylic antraquinone derivative (Rhein) were reported . Citotoxicity tests have shown that the viability of tumor hepatocytes (HepG2 cell line) is significantly reduced in this case.
However, some requirements of magnetic drug delivery systems still need to be fulfilled to include magnetic particles in routine clinical practice. One of the challenges consists in generating magnetic fields able to promote high concentrations of drug within the body. Another issue is related to achieving a homogeneous distribution of particles, given that blood flow in the region of interest may vary in broad ranges. A requirement not to be neglected is the optimization of magnetic nanoparticles synthesis in terms of magnetization and size distribution.
Gene therapy can be defined as a treatment strategy to overcome medical disorders determined by absent or mutated genes, by delivering an adequate genetic payload into the cell nucleus to achieve the expression of the deficient gene product. The therapeutic gene must be maintained into cell nucleus, replicated and passed on through cell divisions .
Gene therapy requires the use of a vector which can transport, protect and release the genetic information in cell nucleus. The ideal gene vector must ensure the transport and sustained release of the intact therapeutic gene in the targeted cells nucleus by avoiding the interactions with non-specific cells, plasma proteins and various components of the extracellular matrix . In addition, the vector must ensure cell entry, endolysosomal escape, cytoplasmic trafficking and the unpacking of the genetic component. Upon extra- and intracellular handling, the vector and its constituents must not exhibit an immunogenic or cytotoxic effect.
In terms of in vivo transfection efficiency, viruses (i.e., retroviruses, adenoviruses and adeno associated viruses) are the most effective gene vectors. In spite of their transfection effectiveness viral vectors are immunogenic, cytotoxic and in some cases exhibit insertional mutagenesis which translates in the disruption of a tumor suppressing gene or the activation of an oncogene  and subsequent malignant transformation of the cells . By comparison, non-viral vectors such as liposomes, proteins, polycationic polymers and others do not exhibit these side effects, are easier to handle/manufacture and can be functionalized to meet the specific requirements of the application. Thus, an ideal non-viral vector for gene therapy should be a theranostic system fulfilling at least the following conditions: low toxicityis, ability to complex DNA, ability to penetrate cell membrane and to release plasmid inside the cell.
The synthetic gene vectors, such as those based on cationic lipids and polymers, have the ability to complex DNA/RNA molecules via electrostatic interactions between the negatively charged phosphate backbone of the nucleic acid and the positively charged amino groups of the synthetic carrier. In addition, the cationic residues ensure the endolysosomal release of the vector/DNA complex due to the proton-sponge effect . After DNA/vector complex entry into the cell via endocytosis, the basic secondary and tertiary amine groups express their buffering ability in the acidic environment of the endosome. The protonation of these groups determines an influx of counter ions (Cl-), the osmotic pressure is increased and the endosome is ruptured releasing the complex in the cytosol . The large number of amino groups of the vector affords the conjugation of various components such as ligands for cell targeting and receptor mediated endocytosis , peptides for DNA compaction , endosome disruption  and nuclear import signals  or hydrophilic segments (PEG) to minimize non-specific interaction with extracellular proteins .
Common synthetic vectors able to interact with plasmid DNA to form nano-sized polyplexes and to pass through the cell membrane are based on (1) cationic lipids (DOTMA, DOTAP, etc.), comprised of a cationic head group and a hydrophobic tail, when DNA is encapsulated inside the liposome, (2) linear or branched polyamine polymers (PEI, PLL, PAMAM, etc.), (3) dendritic structures bearing amine groups, and (4) core-shell nanoparticles whose shells contain biopolymers or synthetic polyamine polymers .
Recently, fullerene derivatives gained increased interest as multifunctional systems in drug/gene delivery . Using conjugates with high water solubility based on fullerene (as a core) and polyethyleneimine (as a shell) , transfection efficiencies equivalent to the results reported in literature were obtained .
Drug and gene controlled delivery are application areas with pivotal role for the research and academic community, as well as for industry, generating new niche markets. In this context the current research objectives are related to the production of tailored polymer materials, with clinical application capabilities, i.e., engineered to exert distinct biological functions, implying biocompatibility, multifunctionality as well as appropriate form/architectural features, giving rise to specificity and high responsiveness. Nowadays, improvements and development of new polymer based materials design and methodologies imply multidisciplinarity. This desiderate may be achieved thanks to innovation coming out from macromolecular chemistry conjugated with the combination of preparation approaches and tools taken from nanotechnology, biomimetics, tissue engineering, polymer science, microfluidics and computer assistance/modeling.
This work was supported by project PN-II-ID-PCCE-2011-0028, contract 4/30.05.2012.
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