Equilibrium swelling behavior of pH-sensitive hydrogels

Influence of environmental solution pH and microstructural parameters on mechanical behavior of amphoteric pH-sensitive hydrogels

Amphoteric hydrogels contain both ionizable acidic and basic groups attached on the polymer chains, which can change their volume in response to the slight alteration of the surrounding environmental p H. In this paper, a theory of equilibrium swelling of amphoteric p H-sensitive hydrogels which is an extension of the formalism proposed by Marcombe et al. and a new hybrid free-energy density function of amphoteric hydrogels composed of the Edwards-Vilgis slip-link model and the Flory-Huggins solution theory as well as the contributions of mixing the mobile ions with the solvent, and dissociating the acidic and basic groups are presented for the prediction of the influence of environmental solution p H, microstructural parameters and geometric constraints on mechanical behavior. The calculations were modeled on chitosan-genipin gels, and the results were compared to experimental data. Numerical calculations show that the model is able to predict the dependence of swelling on p H and crosslinker qualitatively well and quantitatively close to the experimental data. Each gel shows minimal swelling at low p H but an increase in swelling until a maximum was reached; for most of the p H range, a good fit was achieved except for where the maximum swelling occurs; for experimental data, the maximum swelling appears at about pH = 4 , but for modeled data the maximum swelling appears between pH = 4 and pH = 6 ; each gel swell decreasing with increasing crosslinker concentration was also successfully predicted. The calculated results also show that microstructural parameters and geometric constraints have a significant impact on the mechanical behavior of the amphoteric hydrogels; the gel swells less when the network is more densely entangled and the maximum swelling ratio of the gels under biaxial constraint is only about one-third of the maximum when the gels swell freely. The theory developed here is valuable for the design and optimization of a drug delivery system.

Stimuli-sensitive hydrogels: ideal carriers for chronobiology and chronotherapy

The development of solid-phase peptide synthesis in the early 1960s and recombinant DNA technology in the early 1970s boosted the scientific interest of utilizing proteins and peptides as potential therapeutic agents to battle poorly controlled diseases. While there has been rapid progress in the development and synthesis of new proteins and peptides as potential therapeutic agents, the formulation and development of the associated delivery systems is lacking. The development of delivery systems is equally important due to the problems of stability, low bioavailability and short half-life of proteins and peptides. The main problem in this field is that low stability leads to low bioavailability. In this review we draw attention to chrono-pharmacological drug-delivery systems, which can be used to match the delivery of therapeutic agents with the biological rhythm. They are very important especially in endocrinology and in vaccine therapy. We show that the treatment of hypopituitary dwarfism by administration of human growth-hormone-releasing hormone (GHRH) is more effective when GHRH is administered in a pulsatile manner that exhibits a period characteristic of the patient's circadian rhythm. Here we examine how to design novel chrono-pharmacological drug-delivery systems that should be able to release the therapeutic agents at predetermined intervals.

Light responsive polymer membranes: a review

In recent years, stimuli responsive materials have gained significant attention in membrane separation processes due to their ability to change specific properties in response to small external stimuli, such as light, pH, temperature, ionic strength, pressure, magnetic field, antigen, chemical composition, and so on. In this review, we briefly report recent progresses in light-driven materials and membranes. Photo-switching mechanisms, valved-membrane fabrication and light-driven properties are examined. Advances and perspectives of light responsive polymer membranes in biotechnology, chemistry and biology areas are discussed.

Inverse opals of molecularly imprinted hydrogels for the detection of bisphenol A and pH sensing

Inverse opal films of molecularly imprinted polymers (MIP) were elaborated using the colloidal crystal template method. The colloidal crystals of silica particles were built by the Langmuir-Blodgett technique, allowing a perfect control of the film thickness. Polymerization in the interspaces of the colloidal crystal in the presence of bisphenol A (BPA) and removal of the used template provides 3D-ordered macroporous methacrylic acid-based hydrogel films in which nanocavities derived from bisphenol A are distributed within the thin walls of the inverse opal hydrogel. The equilibrium swelling properties of the nonimprinted (NIPs) and molecularly imprinted polymers (MIPs) were studied as a function of pH and bisphenol A concentration, while the molecular structures of the bulk hydrogels were analyzed using a cross-linked network structure theory. This study showed an increase in nanopore (mesh) size in the MIPs after BPA extraction as compared to NIPs, in agreement with the presence of nanocavities left by the molecular imprints of the template molecule. The resulting inverse opals were found to display large responses to external stimuli (pH or BPA) with Bragg diffraction peak shifts depending upon the hydrogel film thickness. The film thickness was therefore shown to be a critical parameter for improving the sensing capacities of inverse opal hydrogel films deposited on a substrate.

Elucidation of the chemical and morphological structure of double-network (DN) hydrogels by high-resolution magic angle spinning (HRMAS) NMR spectroscopy

(1)H HRMAS NMR spectroscopy is applied to gain insight into the chemical and morphological structure of double-network (DN) hydrogels, prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(acrylamide) (PAAm). The method enables one to obtain detailed information at the molecular level about the formation of covalent bonds between the two polymer networks through non-reacted double bonds of the cross-linker N,N'-methylene bis(acrylamide) (MBAA). Evidence to the existence of strong hydrogen-bond interactions based on the N-H group of the PAMPS as a hydrogen-bond donor and the C=O group of the PAAm as a hydrogen-bond acceptor is also provided. These findings clarify the origin of the toughening mechanism and the exceptionally strong mechanical properties of DN gels, further supported by microhardness data.

Kinetics of volume change of poly(succinimide) gels during hydrolysis and swelling

Kinetics of hydrolysis induced swelling as well as pure swelling of a novel class of pH responsive biocompatible polymer gels was studied. Poly(aspartic acid) gels were prepared by hydrolysis of chemically cross-linked poly(succinimide) networks. The volume change of spherical gels of different size, measured at constant pH, proceeds in three distinct processes: solvent exchange, hydrolysis and swelling. It turned out that pure swelling as well as the swelling coupled with hydrolysis can be described by the Tanaka-Fillmore-Peters-Candau theory. The relaxation times as well as the cooperative diffusion coefficient of the network chains were determined. It was found that the initial condition of the swelling makes its influence felt neither on the relaxation time, nor on the cooperative diffusion coefficient.

Multiphysics modeling of responsive characteristics of ionic-strength-sensitive hydrogel

A multiphysics model is developed in this paper for simulation of the volume transition mechanism of the smart hydrogel in response to the changes in the ionic strength of bathing solution as an important measure of the ionic concentration of that solution, which is termed the multi-effect-coupling ionic-strength-stimulus (MECis) model. In the present works, the ionic strength is treated as a main stimulus and incorporated into both the ionic convection-diffusion system in the Nernst-Planck flux and the fixed charge density equation characterized by Langmuir isotherm theory. Due to the diffusion and convection, the osmotic pressure is produced by the difference in the ionic concentration between the interior hydrogel and exterior solution, which drives the swelling of the smart hydrogel. The deformation of the ionic-strength-sensitive hydrogel is described by the momentum conservation law, in which the osmotic pressure is a main driving source. Apart from osmotic pressure, however, the repulsive force between the fixed charges is also considered in the mechanical equilibrium equation as another driving source for the swelling of the hydrogel. The simulation is conducted for one-dimensional steady-state problem, and then compared with the experimental data and other theories from open literature. The comparisons demonstrate that the MECis model can simulate well the swelling behavior of the ionic-strength-sensitive hydrogel qualitatively and quantitatively. Probably it is able to predict the responsive characteristics of the bathing solution including the distribution of diffusive ionic concentrations and electrical potential.

Responsive hydrogels for label-free signal transduction within biosensors

Hydrogels have found wide application in biosensors due to their versatile nature. This family of materials is applied in biosensing either to increase the loading capacity compared to two-dimensional surfaces, or to support biospecific hydrogel swelling occurring subsequent to specific recognition of an analyte. This review focuses on various principles underpinning the design of biospecific hydrogels acting through various molecular mechanisms in transducing the recognition event of label-free analytes. Towards this end, we describe several promising hydrogel systems that when combined with the appropriate readout platform and quantitative approach could lead to future real-life applications.

Micro- and nanotechnologies for intelligent and responsive biomaterial-based medical systems

Advances in medical treatments of a wide variety of pathophysiological conditions require the development of better therapeutic agents, as well as a combination of the required therapeutic agents with device-integrated biomaterials that can serve as sensors and carriers. Combination of micro- and nano-fabricated systems with intelligent biomaterials that have the ability to sense and respond is a promising avenue for the development of better diagnostic and therapeutic medical systems. Micro- and nano-electromechanical systems (MEMs and NEMs) are now becoming a family of potentially powerful new technologies for drug delivery, diagnostic tools, and tissue engineering. Improvements in micro- and nano-fabrication technologies have enhanced the ability to create better performing therapeutic systems for numerous pathophysiological applications. More importantly, MEMS- and NEMS-based tissue regeneration scaffolds, biosensors, and drug delivery devices provide new opportunities to mimic the natural intelligence and response of biological systems.

Analysis of responsive characteristics of ionic-strength-sensitive hydrogel with consideration of effect of equilibrium constant by a chemo-electro-mechanical model

A multiphysics model is presented in this paper for analysis of the influence of various equilibrium constants on the smart hydrogel responsive to the ionic strength of environmental solution, and termed the multieffect-coupling ionic-strength stimulus (MECis) model. The model is characterized by a set of partial differential governing equations by consideration of the mass and momentum conservations of the system and coupled chemical, electrical, and mechanical multienergy domains. The Nernst-Planck equations are derived by the mass conservation of the ionic species in both the interstitial fluid of the hydrogel and the surrounding solution. The binding reaction between the fixed charge groups of the hydrogel and the mobile ions in the solution is described by the fixed charge equation, which is based on the Langmuir monolayer theory. As an important effect for the binding reaction, the equilibrium constant is incorporated into the fixed charge equation. The kinetics of the hydrogel swelling/deswelling is illustrated by the mechanical equation, based on the law of momentum conservation for the solid polymeric networks matrix within the hydrogel. The MECis model is examined by comparison of the numerical simulations and experiments from open literature. The analysis of the influence of different equilibrium constants on the responsive characteristics of the ionic-strength-sensitive hydrogel is carried out with detailed discussion.

Hydrogels in regenerative medicine

Hydrogels, due to their unique biocompatibility, flexible methods of synthesis, range of constituents, and desirable physical characteristics, have been the material of choice for many applications in regenerative medicine. They can serve as scaffolds that provide structural integrity to tissue constructs, control drug and protein delivery to tissues and cultures, and serve as adhesives or barriers between tissue and material surfaces. In this work, the properties of hydrogels that are important for tissue engineering applications and the inherent material design constraints and challenges are discussed. Recent research involving several different hydrogels polymerized from a variety of synthetic and natural monomers using typical and novel synthetic methods are highlighted. Finally, special attention is given to the microfabrication techniques that are currently resulting in important advances in the field.

Polybasic Nanomatrices Prepared By UV-initiated Photopolymerization

A novel method for synthesizing nanoscale polymer networks that swell in acidic media is described here using photoinitiated emulsion polymerization. These nanomatrices consist of a crosslinked core of poly[2-(diethylamino)ethyl methacrylate] surface grafted with poly(ethylene glycol) (PDGP) with an average diameter of 50-150 nm. Control over mesh size, surface charge, encapsulation efficiency and in vitro biocompatibility was obtained by varying crosslinking density. The ability to image nanomatrices in their dry state using conventional scanning electron microscopy was made possible by increasing crosslinking density. Theoretical calculations of matrix mesh sizes were supported by the encapsulation of both insulin and colloidal gold 2-5 nm in diameter. The ability to sequester and control the aggregation of an inorganic phase confirmed their use as a nanocomposite matrix material. These networks could be used as imaging agents, drug delivery devices or as components of sensing devices.

Novel platforms for oral drug delivery

The aim of this review is to provide the reader general and inspiring prospects on recent and promising fields of innovation in oral drug delivery. Nowadays, inventive drug delivery systems vary from geometrically modified and modular matrices, more close to "classic" pharmaceutical manufacturing processes, to futuristic bio micro-electro-mechanical systems (bioMEMS), based on manufacturing techniques borrowed from electronics and other fields. In these technologies new materials and creative solutions are essential designing intelligent drug delivery systems able to release the required drug at the proper body location with the correct release rate. In particular, oral drug delivery systems of the future are expected to have a significant impact on the treatment of diseases, such as AIDS, cancer, malaria, diabetes requiring complex and multi-drug therapies, as well as on the life of patients, whose age and/or health status make necessary a multiple pharmacological approach.

Hydrogel nanoparticles in drug delivery

Hydrogel nanoparticles have gained considerable attention in recent years as one of the most promising nanoparticulate drug delivery systems owing to their unique potentials via combining the characteristics of a hydrogel system (e.g., hydrophilicity and extremely high water content) with a nanoparticle (e.g., very small size). Several polymeric hydrogel nanoparticulate systems have been prepared and characterized in recent years, based on both natural and synthetic polymers, each with its own advantages and drawbacks. Among the natural polymers, chitosan and alginate have been studied extensively for preparation of hydrogel nanoparticles and from synthetic group, hydrogel nanoparticles based on poly (vinyl alcohol), poly (ethylene oxide), poly (ethyleneimine), poly (vinyl pyrrolidone), and poly-N-isopropylacrylamide have been reported with different characteristics and features with respect to drug delivery. Regardless of the type of polymer used, the release mechanism of the loaded agent from hydrogel nanoparticles is complex, while resulting from three main vectors, i.e., drug diffusion, hydrogel matrix swelling, and chemical reactivity of the drug/matrix. Several crosslinking methods have been used in the way to form the hydrogel matix structures, which can be classified in two major groups of chemically- and physically-induced crosslinking.

Metal-chelating affinity hydrogels for sustained protein release

Affinity hydrogels based on poly(ethylene glycol) diacrylate and a metal-ion-chelating ligand, glycidyl methacrylate-iminodiacetic acid, have been developed to systematically decrease protein release rates from hydrophilic tissue engineering scaffolds formed in situ. In the current work, tunable and sustained release of a model protein, hexa-histidine tagged green fluorescence protein (hisGFP), is accomplished by judiciously increasing ligand:protein ratio or replacing low-affinity nickel ions with high-affinity copper ions. Agreement between theoretical predictions of a reaction-diffusion model and experimental measurements confirm metal- ion-mediated sustained protein release from these affinity hydrogels is governed by equilibrium protein-ligand binding affinity (dissociation constant, Kd) as well as protein-ligand dissociation kinetics (protein debinding rate constant, k off). The former dictates the release rate in the early period of protein release while the latter determines the long-term sustained release effect. While metal-ion affinity binding has been widely used for various purposes including protein purification and surface patterning, this is the first report describing its application in systematically controlling protein release from hydrophilic PEG networks suitable for cell encapsulation. By using ligands with proper binding kinetic constants (Kd and k off), localized protein delivery can be sustained over clinically relevant timescales while maintaining a favorable environment for cell encapsulation and viability.

Synthesis and swelling properties of novel pH-sensitive poly(aspartic acid) gels

Chemically cross-linked poly(aspartic acid) (PASP) gels were prepared by the hydrolysis of poly(succinimide) (PSI). The latter was prepared by thermal polycondensation of aspartic acid. The PSI chains were cross-linked by natural amines and amino acid derivatives such as putrescin, spermine, spermidine, lysine and cystamine to obtain biodegradable, biocompatible, amino acid-based hydrogels. The volume of the synthesized unhydrolyzed PSI gels changes abruptly at a well-defined pH that results in ring opening, while the hydrolyzed gels show a volume phase transition around the pK values of PASP. The unidirectional stress-strain behavior of the gels as well as the dependence of equilibrium swelling degree on the pH was carefully studied and the most important network parameters were determined by a modified version of the Brannon-Peppas-Peppas theory.

Chronobiology, drug delivery, and chronotherapeutics

Biological processes and functions are organized in space, as a physical anatomy, and time, as a biological time structure. The latter is expressed by short-, intermediate-, and long-period oscillations, i.e., biological rhythms. The circadian (24-h) time structure has been most studied and shows great importance to the practice of medicine and pharmacotherapy of patients. The phase and amplitude of key physiological and biochemical circadian rhythms contribute to the known predictable-in-time patterns in the occurrence of serious and life-threatening medical events, like myocardial infraction and stroke, and the manifestation and severity of symptoms of chronic diseases, like allergic rhinitis, asthma, and arthritis. Moreover, body rhythms can significantly affect responses of patients to diagnostic tests and, most important to the theme of this special issue, medications. Rhythmicity in the pathophysiology of disease is one basis for chronotherapeutics--purposeful variation in time of the concentration of medicines in synchrony with biological rhythm determinants of disease activity--to optimize treatment outcomes. A second basis is the control of undesired effects of medications, especially when the therapeutic range is narrow and the potential for adverse effects high, which is the case for cancer drugs. A third basis is to meet the biological requirements for frequency-modulated drug delivery, which is the case for certain neuroendocrine peptide analogues. Great progress has been realized with hydrogels, and they offer many advantages and opportunities in the design of chronotherapeutic systems for drug delivery via the oral, buccal, nasal, subcutaneous, transdermal, rectal, and vaginal routes. Nonetheless, innovative delivery systems will be necessary to ensure optimal application of chronotherapeutic interventions. Next generation drug-delivery systems must be configurable so they (i) require minimal volitional adherence, (ii) respond to sensitive biomarkers of disease activity that often vary in time as periodic (circadian rhythmic) and non-periodic (random) patterns to release medication to targeted tissue(s) on a real time as needed basis, and (iii) are cost-effective.

Chemomechanical Polymers as Sensors and Actuators for Biological and Medicinal Applications

Changes in the chemical environment can trigger large motions in chemomechanical polymers. The unique feature of such intelligent materials, mostly in the form of hydrogels, is therefore, that they serve as sensors and actuators at the same time, and do not require any measuring devices, transducers or power supplies. Until recently the most often used of these materials responded to changes in pH. Chemists are now increasingly using supramolecular recognition sites in materials, which are covalently bound to the polymer backbone. This allows one to use a nearly unlimited variety of guest (or effector) compounds in the environment for a selective response by automatically triggered size changes. This is illustrated with non-covalent interactions of effectors comprising of metal ions, isomeric organic compounds, including enantiomers, nucleotides, aminoacids, and peptides. Two different effector molecules can induce motions as functions of their concentration, thus representing a logical AND gate. This concept is particularly fruitful with effector compounds such as peptides, which only trigger size changes if, e.g. copper ions are present in the surroundings. Another principle relies on the fast formation of covalent bonds between an effector and the chemomechanical polymer. The most promising application is the selective interaction of covalently fixed boronic acid residues with glucose, which renders itself not only for sensing, but eventually also for delivery of drugs such as insulin. The speed of the responses can significantly increase by increasing the surface to volume ratio of the polymer particles. Of particular interest is the sensitivity increase which can be reached by downsizing the particle volume.

Transient acid exposure increases sevelamer HCl phosphate binding**Preliminary results were presented at the 37th Annual Meeting of the American Society of Nephrology, St. Louis, MO, October 2004

The patient-to-patient variability in the dosing of sevelamer HCl for phosphate control led us to test whether the binder's transient exposure to acidic environments (such as the stomach) might alter the compound so as to change its subsequent binding capacity in the more alkaline small intestine. We hypothesized that an acid milieu could either increase the reactive sites (protonated amine groups) or make the polymer more hydrophilic (hydration and swelling allowing more phosphate to reach those sites). Eight hundred milligrams of Renagel tablets were exposed to pHs 1, 2.3, and 7 (n = 7 each acidity level) for 1 h. NaCl was added to keep ionic strength the same. Measured by atomic emission phosphate uptake after 3 h at pH 7 was, respectively, 3.13 +/- 0.21, 2.72 +/- 0.35, and 1.85 +/- 0.46 mequiv./g (p = 0.0006, pH 1 vs. pH 7). Semi-automated computerized image analysis was then performed to measure swelling of the particles. We constructed a glass continuous-flow cell that allowed stationary particles and real-time photography. Using digitized optical measurements there was no difference (p > 0.8) between the swelling after 1 h of pH 1 or 7 solutions (60.2 +/- 14.8% vs. 59.5 +/- 9.8% increase in diameter). Our findings support the importance of transient acid exposure in enhancing phosphate binding, due to increased protonated sites rather than by more swelling. Patients with acquired or pharmacologic achlorhydria would not benefit from this unexpected in vivo reaction. Possibly manufacturing sevelamer with a higher degree of protonation or administering it with appropriately acidic vehicles or beverages remains to be investigated.