Journey of Hydrogels to Nanogels: A Decade After

ARTI VASHIST, AJEET KAUSHIK, ANUJIT GHOSAL, ROOZBEH NIKKHAH-MOSHAIE, ATUL VASHIST, RAHUL DEV JAYANT AND MADHAVAN NAIR

1.1 The Journey of Hydrogels

The remarkable invention of crosslinked hydroxyethylmethacrylate (HEMA) hydrogels was a ground-breaking innovation for biomaterials scientists around the globe, which led to the future golden era of research for the next generations. The pioneering work carried out by Lim and Sun in 1980, and later by Yannas and co-workers, showed the potential applications of calcium alginate microcapsules utilised for cell encapsulation as well as the utilisation of the natural polymers collagen and shark cartilage in a hydrogel matrix for dressings for artificial burns. The beneficial aspects of hydrogels for therapeutics led to their clinical use. The ability of these soft materials to provide spatial and temporal control in handling the release of bioactives and various therapeutic interventions is exceptional. Their existence in various forms such as injectable forms, patch or thin film forms, viscous gel forms, and nanocomposite forms of hydrogels make them more desirable for various biomedical applications. Research has been conducted to understand the underlying mechanism for their design so that the drug delivery and release conditions can be modulated. Figure 1.1 demonstrates a histogram showing the immense exponential increase in the research and publication regarding hydrogels that has been done in the past 50 years.

The emerging research on inorganic nanoparticle-based adsorbents, drug delivery carriers and sensor formulations having a three dimensional network has come up with remarkable advantages over conventional nanocarriers and other carriers such as metal oxide nanoparticles, polymeric nanoparticles, liposomes, dendrimers, exosomes, etc. The major advantages of the nanogel-based systems in comparison to nanospheres, that have polymeric dense cores, is that they show the capability to encapsulate diverse therapeutic interventions, proteins, and bioactives (enzymes, DNA/RNA). The first nanogels were synthesized by the promising research group of Kabanov et al. and they developed a chemical crosslink using the polymers poly(ethylene glycol) (PEG) and polyethylenimine (PEI) and used the nanogels for oligonucleotide delivery. Figure 1.2 shows some therapeutic utilisations of hydrogels that are used on a on regular basis for various applications like a substitute for skin application, drug encapsulated hydrogels, hydrogels for burn treatments, sensor applications and many others.

The first physically crosslinked nanogels were reported by the pioneering group of Akiyoshi et al., showing the self-assembly of cholesterol-bearing polysaccharides in water using the self-organization of amphiphilic polymers. The journey of hydrogel research over the following ten years has been commendable in terms of the translation of hydrogels to the market and various other achievements (Figure 1.3). The vast absorbing capacity of hydrogels of physiological fluids and their compatibility towards the cellular environment makes them ideals candidates to be used for various purposes. The era of hydrogels began with the invention of HEMA hydrogels and then various natural and synthetic polymers were exploited in the following years utilising various synthetic approaches. The exceptional swelling capacity and porous structures were utilised by various research groups to translate this hydrogel-based research to clinics. Click chemistry, supramolecular interactions and self-assembly processes were selected to design better hydrogels.

Various biomedical applications were covered utilising bulk hydrogels in diverse forms and certain challenges were raised. The next sections will highlight major limitations imposed by the hydrogels and the upsurge in demand of nanogel-based systems. High quality drug delivery carriers, imaging tools, and diagnostics were in demand with the emergence of biocompatible hydrogel systems.

1.2 Driving Force for Designing the Nanogels

Recently, the realm of nanotechnology and the advances in the design of nano-formulations have come up with innovative biomaterials. These nanoformulations are emerging with advanced features and escaping all the demerits of the bulk hydrogel technology (Figure 1.4). The expensive synthesis of hydrogels is being replaced by cheap, easy and fast synthesis procedures.

Traditionally utilised natural polymers with low mechanical strength are being replaced by synthetic polymers and this inhibits the chances of passing on viruses from animal-derived materials. The extensive need for sterilization and the low loading capacity of therapeutics into the hydrogel systems are being replaced by nanoscale gels with exceptionally high loading efficiency. The surgical implantation of the hydrogel device was one of the major limitations of the hydrogel-based drug delivery systems. This drawback was overcome by the invention of injectable hydrogels and nanogels which can be delivered to the humans intravenously or intraperitoneally. The high swelling capacity of the hydrogel imposed limitations such as early degradation, poor mechanical strength, etc. Efforts were made to increase mechanical strength by the addition of nanofillers and modifying the functionality by the addition of hydrophobic groups and thus limiting the water absorption capacity as per the need of the situation. This type of modification was extremely useful to encapsulate both hydrophilic and hydrophobic drugs. Hydrogels were commonly known to be non-adherent and hence, a secondary dressing is needed to secure the hydrogel.

The surface activation procedure was selected to have unique adhesive characteristics for the different cell types in the culture. There is an extensive need to keep the hydrogel moist and thus there are limitations in the shipping and stocking of hydrogels. The other highlighted limitation of hydrogels in stimuli-responsive systems is the low diffusion rate and the limited transduction of signals. This limitation can be overcome by the engineering of the interconnected pores in the polymeric matrix, which results in the formation of capillary networks in the matrix and the lowering of the size of hydrogels, resulting in a significant decrease in diffusion paths. A reduction in the lag time is opted in the induction of the smart responses for biomedical devices like sensors and actuators.

1.3 Transformation from Hydrogels to Nanogels for Imaging

Since the invention of hydrogels, the in vivo fate of the degradable products of hydrogels is very important information needed for designing effective drug delivery systems. Non-invasive imaging of the hydrogels transplanted is the most desirable technique to know the fate of the by-products of the hydrogels. Labelling the hydrogels with contrast agents and labelling with fluorescent dyes have emerged as non-invasive tools for tracking. One interesting study revealed that hydrogels composed of gelatin have been used as multifunctional biomaterials. Hydrogels composed of gelatin crosslinked with lysine diisocyanate ethyl ester (LDI) were subcutaneously implanted in mice. MRI, optical imaging and PET were used to see the degradation of the hydrogel and the interactions with the tissue was studied. The study also showed the MRI images on day one and day 35. The study provided important inputs about the covalent net points with the degradation time and thus was helpful in targeting the modification of hydrogels with reference to the tissue to be replaced. The emerging nanogels had superior features to conventional hydrogels, such as high water content in synergism with the nanosize biocompatibility, making them perfect candidates to be used as imaging probes. The ability to modulate the functionality of nanogels through surface modification has allowed the nanogels to be coated with multiple imaging agents. The high stability of nanogel systems gives them a higher blood circulation time period. Their specificity is far better than the conventional contrast agents used (gold and silver nanoparticles). The well known drawback of the commonly used magnetic resonance (MR) contrast agents such as gadolinium (Gd) and manganese (Mn) is the low circulation time and thus they are cleared from the body and exhibit toxicity aspects. Nanogels overcome these limitations owing to the synthesis procedures and their size. One of the interseting investigations carried out by another research group showed a nanogel comprised of PEGMA, N–(2 aminoethyl) methacrylate. The insertion of Gd(III) was carried out using an isothiocyante derivative of the chelator DTPA.

1.4 Advancements in Medicinal Applications

The nano-regime synthesis of hydrogels produces nanogels, which limits their swelling ability but enhances their mechanical stability, functionality, stimuli-responsiveness and intravenous injection based therapeutic delivery of various biological molecules within the body. However, most macroscopic hydrogels have been used externally for biomedical applications. These nanogels have been considered to be ideal candidates for intravenous delivery of low molecular weight chemotherapeutics such as oligonucleotides, proteins and peptide molecules for specific treatments or during the health recovery process. In other applications, they can also be used as template materials for the synthesis of nanoscopic drug molecules. This not only limits the size of the drug molecule but also the concentration of the drug to be delivered and can even protect the drugs from interaction with physiological fluids at different pH, or various constituents during the passage of the drug within the body from the injected to targeted area. In a way it preserves the pristine activity of the drug until the right time of release or the required stimulus has been received.

1.5 Conclusions

Nanogels have outshined other methods in the area of drug delivery for cancer, tuberculosis diagnosis and treatment, tissue engineering, stimuli-responsive gene and drug delivery, imaging and drug delivery to relatively less explored areas of humans, i.e. the brain. The ability of polymeric nanogels to cross the blood–brain barrier is due to their capability in reversing surface characteristic, i.e. from hydrophobic to hydrophilic or vice versa, or pertaining to fixed surface polarity, which has caused an upsurge in research in the nanotheranostics of neurological disorders. So, the designing of nanogels has to be carried out with great care to exploit the specific properties of the components for developing responsiveness towards temperature, osmotic pressure, pH, and differences in diffusion coefficients.

This chapter describes the journey of hydrogels, which began with the components involving simple biopolymers, towards the development of innovative nanogels, i.e. smart hydrogel particles with improved properties. The scientific advancements in hydrogel technologies promoted them as the material of choice to develop the next generation therapy and devices for health care. The advancement in the use of nanogels in biomedical applications such as imaging agents, drug delivery agents, drug nanocarriers, biosensor developments, and scale-up research for clinical trials are discussed descriptively in the following chapters of this book. We believe that nanogel research at both the fundamental and applied level needs more attention and promotion. Once in practice, nanogels based therapies and devices will be more effective, safe, and cost-effective resulting in affordable personalized health care management.

Design and Engineering of Nanogels

ANUJIT GHOSAL, SHIVANI TIWARI, ABHIJEET MISHRA, ARTI VASHIST, NEHA KANWAR RAWAT, SHARIF AHMAD AND JAYDEEP BHATTACHARYA

2.1 Introduction

The effectiveness of treatments of various ailments via therapeutic delivery of drugs or molecules, such as nucleic acids, proteins, specific hormones, etc., has been dependent on the carrier platform or vehicle for delivery. A targeted and on-demand delivery of medicinal molecules has been desired by researchers, doctors and patients. The major difficulties associated with drug carriers or drug vehicles is the concept of material development. A vehicular structure which can inertly support the drug, i.e. one which does not dilute the activity of the biologically-active molecule by itself reacting to the drug or restricting any changes due to environmental reactions, is preferred (Scheme 1). Further, the pivotal points of concerns are:

(i) inherent toxicity of the material

(ii) reduction in the efficacy of drug due to presence of plasma

(iii) instability

(iv) existence of various intracellular barriers such as the reticuloendothelial system (RES) whose functioning is in the immune system's defence against foreign bodies, in lysosomal enzymatic degradation, in various membranes (plasma membrane, blood barriers, nuclear membrane), in endosome entrapment, etc.

Polymeric nanogels, having a nano-regime structure, are considered to be one of the solutions for all the above-mentioned concerns and some not mentioned difficulties in drug delivery therapeutics. Now, the utility of these nanogels can be explored by designing, developing and engineering nanogels with predefined structures (spherical, cylindrical, flakes), interactions (electrostatic, physical, covalent), different types of polymers (biodegradable or non-biodegradable, synthetic, natural, semi-synthetic), responsiveness to stimuli (pH, temperature, electric-magnetic fields) and other specifications.

The constituents required for the synthesis of gels vary based on the size of the gel to be prepared. As in the case of bulk or macro-gels, monomers, cross-linkers and activators are generally employed. However, microgels are obtained by emulsion polymerization or copolymerization of monomers, but they can also be formed without the use of surfactants during emulsion by using sonication in place of conventional stirring. The nano polymeric hydrogels can be prepared by the basic synthesis mechanism of precipitation, dispersion, and emulsion polymerization, solvent diffusion, spray drying, salting out, molecular imprinting (MIT) and milling processes. All these general synthesis methodologies can further be differentiated on two broad bases:

(i) Bottom-up approaches

(ii) Top-down approaches

In the former case of synthesis, in situ reactions of the monomeric species in the nano range resulted in the formation of a nanogel. Templates like surfactants, micelles, vesicles, nano-fluidics, etc. play an important role in the shaping of the final nanogel. However, in the latter case, already prepared or pre-existed bulk material gets converted into the nano range by milling, cutting, shredding, spray drying, etc. Alternatively, in top-down approaches, bulk materials are processed in a controlled way to obtain nanomaterials.