Synthetic hydrogels as drug delivery systems

A short review on the synthesis and advance applications of polyaniline hydrogels

Conductive polymeric hydrogels (CPHs) exhibit remarkable properties such as high toughness, self-recoverability, electrical conductivity, transparency, freezing resistance, stimulus responsiveness, stretch ability, self-healing, and strain sensitivity. Due to their exceptional physicochemical and physio-mechanical properties, among the widely studied CPHs, polyaniline (PANI) has been the subject of immense interest due to its stability, tunable electrical conductivity, low cost, and good biocompatibility. The current state of research on PANI hydrogel is discussed in this short review, along with the properties, preparation methods, and common characterization techniques as well as their applications in a variety of fields such as sensor and actuator manufacturing, biomedicine, and soft electronics. Furthermore, the future development and applications of PANI hydrogels are also mentioned.

Conductive polymeric hydrogels (CPHs) exhibit remarkable properties for advance technological applications.

Islet Encapsulation: New Developments for the Treatment of Type 1 Diabetes

Islet transplantation is a promising approach for the treatment of type 1 diabetes (T1D). Currently, clinical islet transplantation is limited by allo - and autoimmunity that may cause partial or complete loss of islet function within a short period of time, and long-term immunosuppression is required to prevent rejection. Encapsulation into semipermeable biomaterials provides a strategy that allows nutrients, oxygen and secreted hormones to diffuse through the membrane while blocking immune cells and the like out of the capsule, allowing long-term graft survival and avoiding long-term use of immunosuppression. In recent years, a variety of engineering strategies have been developed to improve the composition and properties of encapsulation materials and to explore the clinical practicality of islet cell transplantation from different sources. In particular, the encapsulation of porcine islet and the co-encapsulation of islet cells with other by-standing cells or active ingredients for promoting long-term functionality, attracted significant research efforts. Hydrogels have been widely used for cell encapsulation as well as other therapeutic applications including tissue engineering, cell carriers or drug delivery. Here, we review the current status of various hydrogel biomaterials, natural and synthetic, with particular focus on islet transplantation applications. Natural hydrophilic polymers include polysaccharides (starch, cellulose, alginic acid, hyaluronic acid, chitosan) and peptides (collagen, poly-L-lysine, poly-L-glutamic acid). Synthetic hydrophilic polymers include alcohol, acrylic acid and their derivatives [poly (acrylic acid), poly (methacrylic acid), poly(acrylamide)]. By understanding the advantages and disadvantages of materials from different sources and types, appropriate materials and encapsuling methods can be designed and selected as needed to improve the efficacy and duration of islet. Islet capsule transplantation is emerging as a promising future treatment for T1D.

Role of Polymer Concentration and Crosslinking Density on Release Rates of Small Molecule Drugs

Over the past few years, researchers have demonstrated the use of hydrogels to design drug delivery platforms that offer a variety of benefits, including but not limited to longer circulation times, reduced drug degradation, and improved targeting. Furthermore, a variety of strategies have been explored to develop stimulus-responsive hydrogels to design smart drug delivery platforms that can release drugs to specific target areas and at predetermined rates. However, only a few studies have focused on exploring how innate hydrogel properties can be optimized and modulated to tailor drug dosage and release rates. Here, we investigated the individual and combined roles of polymer concentration and crosslinking density (controlled using both chemical and nanoparticle-mediated physical crosslinking) on drug delivery rates. These experiments indicated a strong correlation between the aforementioned hydrogel properties and drug release rates. Importantly, they also revealed the existence of a saturation point in the ability to control drug release rates through a combination of chemical and physical crosslinkers. Collectively, our analyses describe how different hydrogel properties affect drug release rates and lay the foundation to develop drug delivery platforms that can be programmed to release a variety of bioactive payloads at defined rates.

Research progress in decellularized extracellular matrix-derived hydrogels

Decellularized extracellular matrix (dECM) is widely used in regenerative medicine as a scaffold material due to its unique biological activity and good biocompatibility. Hydrogel is a three-dimensional network structure polymer with high water content and high swelling that can simulate the water environment of human tissues, has good biocompatibility, and can exchange nutrients, oxygen, and waste with cells. At present, hydrogel is the ideal biological material for tissue engineering. In recent years, rapid development of the hydrogel theory and technology and progress in the use of dECM to form hydrogels have attracted considerable attention to dECM hydrogels as an innovative method for tissue engineering and regenerative medicine. This article introduces the classification of hydrogels, and focuses on the history and formation of dECM hydrogels, the source of dECM, the application of dECM hydrogels in tissue engineering and the commercial application of dECM materials.

Filamentous Phages as Building Blocks for Bioactive Hydrogels

Filamentous bacteriophages (bacterial viruses) are semiflexible proteinous nanofilaments with high aspect ratios for which the surface chemistry can be controlled with atomic precision via genetic engineering. That, in addition to their ability to self-propagate and replicate a nearly monodisperse batch of biologically and chemically identical nanofilaments, makes these bionanofilaments superior to most synthetic nanoparticles and thus a powerful tool in the bioengineers' toolbox. Furthermore, filamentous phages form liquid crystalline structures at high concentrations; these ordered assemblies create hierarchically ordered macro-, micro-, and nanostructures that, once cross-linked, can form hierarchically ordered hydrogels, hydrated soft material with a variety of physical and chemical properties suitable for biomedical applications (e.g., wound dressings and tissue engineering scaffolds) as well as biosensing, diagnostic assays. We provide a critical review of these hydrogels of filamentous phage, and their physical, mechanical, chemical, and biological properties and current applications, as well as an overview of limitations and challenges and outlook for future applications. In addition, we present a list of design parameters for filamentous phage hydrogels to serve as a guide for the (bio)engineer and (bio)chemist interested in utilizing these powerful bionanofilaments for designing smart, bioactive materials and devices.

Thermoresponsive Hydrogels and Their Biomedical Applications: Special Insight into Their Applications in Textile Based Transdermal Therapy

Various natural and synthetic polymers are capable of showing thermoresponsive properties and their hydrogels are finding a wide range of biomedical applications including drug delivery, tissue engineering and wound healing. Thermoresponsive hydrogels use temperature as external stimulus to show sol-gel transition and most of the thermoresponsive polymers can form hydrogels around body temperature. The availability of natural thermoresponsive polymers and multiple preparation methods of synthetic polymers, simple preparation method and high functionality of thermoresponsive hydrogels offer many advantages for developing drug delivery systems based on thermoresponsive hydrogels. In textile field applications of thermoresponsive hydrogels, textile based transdermal therapy is currently being applied using drug loaded thermoresponsive hydrogels. The current review focuses on the preparation, physico-chemical properties and various biomedical applications of thermoresponsive hydrogels based on natural and synthetic polymers and especially, their applications in developing functionalized textiles for transdermal therapies. Finally, future prospects of dual responsive (pH/temperature) hydrogels made by these polymers for textile based transdermal treatments are mentioned in this review.

Thermoresponsive Hydrogels and Their Biomedical Applications: Special Insight into Their Applications in Textile Based Transdermal Therapy

Various natural and synthetic polymers are capable of showing thermoresponsive properties and their hydrogels are finding a wide range of biomedical applications including drug delivery, tissue engineering and wound healing. Thermoresponsive hydrogels use temperature as external stimulus to show sol-gel transition and most of the thermoresponsive polymers can form hydrogels around body temperature. The availability of natural thermoresponsive polymers and multiple preparation methods of synthetic polymers, simple preparation method and high functionality of thermoresponsive hydrogels offer many advantages for developing drug delivery systems based on thermoresponsive hydrogels. In textile field applications of thermoresponsive hydrogels, textile based transdermal therapy is currently being applied using drug loaded thermoresponsive hydrogels. The current review focuses on the preparation, physico-chemical properties and various biomedical applications of thermoresponsive hydrogels based on natural and synthetic polymers and especially, their applications in developing functionalized textiles for transdermal therapies. Finally, future prospects of dual responsive (pH/temperature) hydrogels made by these polymers for textile based transdermal treatments are mentioned in this review.

pH responsive N-succinyl chitosan/Poly (acrylamide-co-acrylic acid) hydrogels and in vitro release of 5-fluorouracil

There has been significant progress in the last few decades in addressing the biomedical applications of polymer hydrogels. Particularly, stimuli responsive hydrogels have been inspected as elegant drug delivery systems capable to deliver at the appropriate site of action within the specific time. The present work describes the synthesis of pH responsive semi-interpenetrating network (semi-IPN) hydrogels of N-succinyl-chitosan (NSC) via Schiff base mechanism using glutaraldehyde as a crosslinking agent and Poly (acrylamide-co-acrylic acid)(Poly (AAm-co-AA)) was embedded within the N-succinyl chitosan network. The physico-chemical interactions were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and field emission scanning electron microscope (FESEM). The synthesized hydrogels constitute porous structure. The swelling ability was analyzed in physiological mediums of pH 7.4 and pH 1.2 at 37°C. Swelling properties of formulations with various amounts of NSC/ Poly (AAm-co-AA) and crosslinking agent at pH 7.4 and pH 1.2 were investigated. Hydrogels showed higher swelling ratios at pH 7.4 while lower at pH 1.2. Swelling kinetics and diffusion parameters were also determined. Drug loading, encapsulation efficiency, and in vitro release of 5-fluorouracil (5-FU) from the synthesized hydrogels were observed. In vitro release profile revealed the significant influence of pH, amount of NSC, Poly (AAm-co-AA), and crosslinking agent on the release of 5-FU. Accordingly, rapid and large release of drug was observed at pH 7.4 than at pH 1.2. The maximum encapsulation efficiency and release of 5-FU from SP2 were found to be 72.45% and 85.99%, respectively. Kinetics of drug release suggested controlled release mechanism of 5-FU is according to trend of non-Fickian. From the above results, it can be concluded that the synthesized hydrogels have capability to adapt their potential exploitation as targeted oral drug delivery carriers.

Mineralized poly(lactic acid) scaffolds loading vascular endothelial growth factor and the in vivo performance in rat subcutaneous model

The functionalization of degradable polymeric scaffolds with therapeutic molecules such as vascular endothelial growth factor (VEGF) is a key strategy to gain better regenerative ability of damaged bone tissue by stimulating vascularization and tissue perfusion. Here, we combined VEGF with poly(lactic acid) (PLA) porous scaffold, after modifying the PLA surface with calcium phosphate (CaP) mineral. The mineralized PLA scaffold (mPLA) showed more effective loading capacity of VEGF than the PLA without mineralization as well as profiled sustainable release of VEGF for up to a couple of weeks. The VEGF-loaded mPLA scaffold presented significantly improved proliferation of primary endothelial cells for up to 7 days, with respect to the scaffold without the VEGF loading. The performance of the engineered scaffold was assessed after subcutaneous implantation in rats for 4 weeks. Histological results showed favorable tissue compatibility of both the mPLA scaffolds (with and without VEGF loading), as characterized by infiltration of inflammatory cells, formation of fibrous capsule, and ingrowth of fibroblasts into the matrices. Immunohistochemical staining of the von Willebrand Factor revealed significantly improved formation of neo-capillaries in the VEGF-loaded mPLA. Based on this study, the strategy of VEGF loading onto mineralized PLA scaffold is considered beneficial for gaining improved vascularization of the polymeric scaffolds, suggesting potential applications for bone tissue engineering.

Bupivacaine-loaded comatrix formed by albumin microspheres included in a poly(lactide-co-glycolide) film: in vivo biocompatibility and drug release studies

Bupivacaine-loaded comatrix, formed by bupivacaine-loaded microspheres included in a poly(lactide-co-glycolide) film, was assayed for the controlled release of the drug 'in vivo'. The comatrix, with 66.37 microg of bupivacaine, signifying a dose of 265.5 microg/kg, was subcutaneously implanted in the back of rats. Maximum plasma bupivacaine concentration was 147.6 +/- 5.0 ng/ml 95 h after the device implantation, and the drug was detected in plasma for 17 days. The half-life time of bupivacaine improves by more than 50 times with regard to that of the drug administered in a solution by intraperitoneal injection. After 15 days of implantation the comatrix was included in a thin fibrous capsule and degradation of the microspheres was observed. The histological studies show good biocompatibility of this comatrix. After 50 days the comatrix was degraded and its remains were almost indistinguishable from the surrounding tissue. Small number of microspheres was observed and they were surrounded by conjunctive tissue. Nerve packets and small blood vessels were also observed in the periphery of the implant.

In vitro swelling studies and preliminary biocompatibility evaluation of acrylamide-based hydrogels

In this in vitro study, the biocompatibility of acrylamide-based hydrogels such as acrylamide/crotonic and acrylamide/itaconic acid, prepared by using gamma-rays, has been investigated against some biochemical parameters of human serum. The swelling kinetics of acrylamide/crotonic and acrylamide/itaconic acid hydrogels are investigated in distilled water and physiological saline solution and some swelling and diffusion parameters have been calculated. For the analysis of biocompatibility, acrylamide/crotonic and acrylamide/itaconic acid hydrogels are incubated in 10 different human sera for 24 h and their biocompatibilities with some biochemical parameters have been investigated. No significant difference in values before and after the test procedures have been found. It is therefore concluded that acrylamide/crotonic and acrylamide/itaconic acid hydrogels are biocompatible.

Composite poly(vinyl alcohol) beads for controlled drug delivery

A new method of preparing composite poly(vinyl alcohol) (PVA) beads with a double-layer structure has been developed, which involves a stepwise saponification of suspension polymerized poly(vinyl acetate) (PVAc) beads and subsequent stepwise cross-linking of the PVA core and shell with glutaraldehyde. This process results in PVA beads with thin, highly cross-linked outer shells and lightly cross-linked inner cores of different degrees of cross-linking. In addition to structural characterization of the polymer based on equilibrium swelling measurements, the kinetics of water swelling and drug release from these beads were studied at 37 degrees C using acetaminophen and proxyphylline as model drugs. The results show that the outer shell functions as a rate-controlling membrane upon increasing its cross-linking ratio, X, above 0.47. This aspect is reflected in the observed diffusional time lags and constant-rate regions during swelling and drug release. Based on observed time lags, the diffusion coefficient of water through the outer PVA shell with a high cross-linking ratio of X = 0.5 is estimated to be at least six times higher than that of acetaminophen and proxyphylline. In addition, drug diffusion coefficients in the lightly cross-linked PVA core appear to be at least 10 times larger than that in the highly cross-linked outer shell. At lower shell cross-linking ratios (X less than 0.4), the diffusional time lags appear to be absent and the diffusion profiles are apparently first-order (Fickian) in nature.

Transdermal delivery of theophylline to premature infants using a hydrogel disc system

1. Preterm infants show incompletely developed skin with reduced barrier function. The possibility of transdermal delivery of theophylline from hydrogel discs swollen with choline theophyllinate has been investigated. 2. Drug loaded hydrogel discs 2 cm2 in area were applied to the abdomen and occluded. Serum theophylline concentrations were measured in twenty-one infants of less than 31 weeks gestation. 3. Therapeutic concentrations were achieved in 18 individuals, and maintained for up to 15 days after repeated application of discs. A correlation between maximum serum drug concentration and transepidermal water loss, gestation and birthweight was demonstrated.

Correlation between mesh size and equilibrium degree of swelling of polymeric networks

A correlation is established between the mesh size of a swollen cross-linked network and its equilibrium polymer volume fraction, for hydrogels isothermally swollen up to semidilute, dilute, or concentrated conditions. The exponent of this correlation decreases as the water concentration of the hydrogel decreases. Experimental results with hydrogels of poly(vinyl alcohol) and poly(2-hydroxy ethyl methacrylate) are used to verify the new scaling laws.

Temperature dependence of non-Fickian water transport and swelling in glassy gelatin matrices

The effect of temperature on the swelling kinetics of glassy gelatin matrices exposed to water was studied. The movement of two distinct and characteristic swelling boundaries was measured directly using an optical microscope. Swelling rate constants associated with these moving boundaries demonstrated Arrhenius behavior over the temperature range of 15 to 40 degrees C. The apparent activation energy for non-Fickian water transport into the gelatin glassy core was determined to be 8.1 kcal/mol, and 3.5 kcal/mol was found for the outer expansion of the swelling gelatin network due to water sorption. These findings are compared with activation energies for other solvent-glassy polymer systems, and possible reasons for the unexpectedly low value for non-Fickian water transport in the glassy gelatin solid are considered.

Thermal behavior of poly hydroxy ethyl methacrylate (pHEMA) hydrogels

The freezing and melting behavior of water in poly hydroxy ethyl methacrylate (pHEMA) hydrogels of different cross-linker and water contents was investigated in relation to the glass transition temperature (Tg) of the gels. After prolonged cooling at -15 degrees C a constant amount of 1.7 mol water per monomeric unit did not freeze, regardless of both the cross-linker and the water content of the gels. At this water content and temperature, pHEMA gels were below their Tg, and the water molecules were prevented from diffusing to the ice crystals formed in the gel. Therefore, the inability of part of the water in pHEMA gels to freeze is not a thermodynamic phenomenon but is caused by kinetic factors.