Drug release from hydrogel devices with ratecontrolling barriers

Composing On-Program Triggers and On-Demand Stimuli into Biosensor Drug Carriers in Drug Delivery Systems for Programmable Arthritis Therapy

Medication in arthritis therapies is complex because the inflammatory progression of rheumatoid arthritis (RA) and osteoarthritis (OA) is intertwined and influenced by one another. To address this problem, drug delivery systems (DDS) are composed of four independent exogenous triggers and four dependent endogenous stimuli that are controlled on program and induced on demand, respectively. However, the relationships between the mechanisms of endogenous stimuli and exogenous triggers with pathological alterations remain unclear, which results in a major obstacle in terms of clinical translation. Thus, the rationale for designing a guidance system for these mechanisms via their key irritant biosensors is in high demand. Many approaches have been applied, although successful clinical translations are still rare. Through this review, the status quo in historical development is highlighted in order to discuss the unsolved clinical difficulties such as infiltration, efficacy, drug clearance, and target localisation. Herein, we summarise and discuss the rational compositions of exogenous triggers and endogenous stimuli for programmable therapy. This advanced active pharmaceutical ingredient (API) implanted dose allows for several releases by remote controls for endogenous stimuli during lesion infections. This solves the multiple implantation and local toxic accumulation problems by using these flexible desired releases at the specified sites for arthritis therapies.

Development of Hormonal Intravaginal Rings: Technology and Challenges

Intravaginal rings (IVRs) are minimally invasive polymeric devices specifically designed to be used for the sustained and prolonged release of various type of drugs such as hormones. One of the benefits of using topical drug delivery systems (e.g., IVRs) is the fact that systemic drug delivery may cause drug resistance due to elevated drug levels. Topical drug delivery also provides higher concentrations of the drug to the target site and has fewer side effects. In addition, when a drug is administered vaginally, the hepatic first-pass effect is avoided, resulting in higher absorption. Contraception and treatments for specific diseases such as endometriosis and hormone deficiencies can be improved by the administration of hormones via an IVR. This article aims to classify and compare various designs of commercially available and non-commercial hormonal IVRs and to analyze their performance. Current challenges affecting the development of IVRs are investigated, and proposed solutions are discussed. A comprehensive search of publications in MEDLINE/PubMed and of commercial product data of IVRs was performed, and the materials, designs, performance, and applications (e.g., contraception, endometriosis, estrogen deficiency and urogenital atrophy) of hormonal IVRs were thoroughly evaluated. Most hormonal IVRs administer female sex hormones, i.e., estrogen and progestogens. In terms of material, IVRs are divided into 3 main groups: silicone, polyurethane, and polyethylene-co-vinyl acetate IVRs. As regards their design, there are 4 major designs for IVRs which strongly affect their performance and the timing and rate of hormone release. Important challenges include reducing the burst release and maintaining the bioavailability of hormones at their site of action over a prolonged period of administration as well as lowering production costs. Hormonal IVRs are a promising method which could be used to facilitate combination therapies by administering multiple drugs in a single IVR while eliminating the side effects of conventional drug administration methods. IVRs could considerably improve womenʼs quality of life all over the world within a short period of time.

Dissolution of epoxy thermosets via mild alcoholysis: the mechanism and kinetics study

Thermoset dissolution based on degradable bond or exchange reaction has been recently utilized to achieve thermosetting polymer dissolution and recycling. In this paper, an industrial grade epoxy thermoset was utilized as a model system to demonstrate the thermoset dissolution via solvent assisted transesterification (or alcoholysis) with high efficiency under mild conditions. The anhydride–cured epoxy thermoset was depolymerized by selective ester bond cleavage in 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD)–alcohol solution below 180 °C at ordinary pressure in less than two hours. The epoxy dissolution proceeded in a surface erosion mode via transesterification that was coupled with catalyst–alcohol diffusion. Based on this observation, a surface layer model containing three layers, namely the gel layer, solid swollen layer and pure polymer layer was used to analyze the thermoset dissolution kinetics. The epoxy dissolution kinetics was derived from the surface layer model, which could be used to predict the dissolution rate during the diffusion-rate-controlled dissolution process well. The results show that alcohols with larger diffusivity and better solubility lead to a higher alcohol/catalyst concentration in the gel layer and promote faster erosion and dissolution of epoxy. This is the first work to show that it is possible to depolymerize industrial epoxy using the principle of dynamic bonds with fast dissolution rate at mild temperature under ordinary pressure.

An industrial grade epoxy thermoset was utilized as a model system to demonstrate the thermoset dissolution via solvent assisted transesterification with high efficiency under mild conditions.

Iodine-responsive poly (HEMA-PVP) hydrogel for self-regulating burst-free extended release

Swollen hydrogels with extended iodine release kinetics is highly desirable for burn and scald treatment. In this paper semi-interpenetrating poly (HEMA-PVP) hydrogels were prepared by radical polymerization followed by thermo-treatment to crosslink its PVP component. Incorporation of PVP component endows the hydrogels responsive to loaded iodine undergoing a reversible shrunken/swollen volume transition. This resulted in a self-regulating iodine release model, in which shrunken hydrogel at high iodine loading decreased drug diffusion thereby reducing burst release, and then gradually swollen hydrogel as drug release ensures rapid release of dissociated drug from strong affinity sites on hydrogel backbone, achieving a burst-free extended release. The hydrogels demonstrated 11.5-fold higher iodine loading than pure pHEMA hydrogel and showed a highest 40% volume shrink. Initial burst release of iodine was efficiently decreased from 12,894 μg/day of pure pHEMA hydrogel to 2570 μg/day of pHEMA/PVP hydrogel with 37% PVP content. Iodine-loaded hydrogels showed zero-order release at three time periods of 0-15 h, 15 h-3.5 days and 3.5-23.5 days corresponding to release rate of 2570, 776 and 493 μg/day. The work gained a new insight into swollen hydrogel drug delivery system with burst-free extended drug release kinetics.

Design, engineering and structural integrity of electro-responsive carbon nanotube- based hydrogels for pulsatile drug release

Triggerable drug delivery from polymeric implants offers the possibility to generate remote-controlled drug release profiles that may overcome the deficiencies of conventional administration routes (intravenous injections and oral administration) including the toxicity due to overdose and systemic administration. An electro-responsive delivery system was engineered to deliver drug molecules in a pulsatile manner, controlled by the on/off application of electric voltage. Pristine multi-walled carbon nanotubes (pMWNTs) were incorporated into a polymethacrylic acid (PMAA)-based hydrogel matrix by in situ radical polymerisation. The effect of pMWNTs and cross-linker concentration on the electrical and mechanical properties of the hydrogel hybrids was thoroughly investigated. The incorporation of pMWNTs into the polymeric network improved the electrical properties of the hydrogel hybrids and drug release from the gels was significantly enhanced at high pMWNT concentrations, reaching 70% of the loaded dose after two short electrical stimulations. The presence of pMWNTs within the hydrogel matrix affected however the mechanical properties of the hydrogel by decreasing the pore size and therefore the swelling/de-swelling of the gels. The damage to the hybrid gel surfaces after electrical stimulation and the loss of the pulsatile release profile at high cross-linker concentrations suggested that the mechanism of drug release involved a compressing effect and intensified the stress on the polymeric network as a result of the electrical properties of pMWNTs.

Kinetics of re-equilibrium of oppositely charged hydrogel-surfactant system and its application in controlled release

We report a study of re-equilibrium kinetics of an oppositely charged hydrogel-surfactant system (cationically modified hydroxyethyl cellulose (cat-HEC) and sodium dodecyl sulfate (SDS)) and an application of the formulation for delivery of a water-insoluble molecule. Hydrogels have been applied for long-term delivery of water-soluble drugs due to their controlled-release property. However, the release mechanism of drugs solubilized by surfactants has not been clear. In the present study, SDS was used to solubilize a hydrophobic model drug, and thereafter, by electrostatic interaction between cat-HEC and SDS, the solubilized model drug was loaded into two types of cat-HEC hydrogels with different charge density. We found that the charge density of the polymers had a crucial effect on the loading capacity, without affecting the re-equilibrium kinetics. By an elaborate design of the experiments, the release profiles were fitted with one-dimensional Fickian law where we found the diffusivity of the drug to be constant and comparable to free micelles over a wide region of surfactant concentrations. The observed long-term controlled diffusion is discussed from a thermodynamic point of view.

Theory of effective drug release from medical implants based on the Higuchi model and physico-chemical hydrodynamics

Combining the approach of colloid transport with the generalized Higuchi theory of drug release and with the concept of minimum inhibitory concentration (MIC) known in microbiology, the theory of effective drug release from implants has been developed. Effective release of an antibiotic at a concentration above MIC is a necessary condition to achieve protection against infection from implants such as central catheters. The Higuchi theory in its present form is not predictive of the therapeutic effect from medical implants. The theory of effective release presented in this paper specifies two release modes, namely: one with therapeutic usefulness (effective release) and another without therapeutic effect. Therapeutic usefulness may be achieved when the antibiotic concentration, C_ti , on the implant surface kills the organisms of interest and prevents the formation and propagation of biofilm when C_ti exceeds the corresponding MIC of the released antibiotic compound. Currently, neither the Higuchi theory nor any other theory can provide such prediction. The present approach requires quantification of the antibiotic transport from the drug-polymer blend implant surface into the tissue and accounts for its coupling with drug diffusion inside the blend, a task that has not been developed in existing theories. Our solution to this task resulted in the derivation of an equation for the time of duration of effective release, T_e , which depends on MIC, the Higuchi invariant and the characteristics of convective diffusion within the tissue. The latter characteristics include: diffusivity D_ti and diffusion layer thickness δ which is controlled by the velocity of the interstitial fluid in tissue. A smaller D_ti is favorable because transport from the catheter surface is weaker, while a thinner diffusion layer is harmful because this transport is stronger. The influence of the tangential component of interstitial velocity in the tissue is especially harmful because the diffusion within the incision exit site (IES) will be extremely enhanced such that it may decrease C_ti to zero. The incorporation of convective diffusion into the theory of antibacterial protection by means of antibiotic release has revealed that physicochemical mechanisms predict the effectiveness of antibiotic-loaded catheters and defines the conditions necessary to achieve better protection by means of combining the level of catheter loading with antibiotics and the use of wound (IES) dressing.

Molecularly imprinted therapeutic contact lenses

IMPORTANCE OF THE FIELD

There exists a considerable unmet need for more efficacious delivery of ocular therapeutics. Contact lenses have been developed with high loading and controllable sustained release to overcome limited patient compliance and significant ocular transport limitations. This can best be achieved by extending and controlling the residence time of drugs on the eye surface and thereby limiting drug loss by lacrimation, drainage and non-productive absorption.

AREAS COVERED IN THE REVIEW

Within hydrogels, molecular imprinting can be used to create memory for template molecules embedded within a flexible macromolecular network. Control in therapeutic loading and delay of release have been demonstrated with careful attention to the functional monomer/template ratio, the diversity of functional monomers, and the polymer backbone and network structure. Experimental work has also confirmed that macromolecular memory and not structural differences or phenomena are responsible for delayed drug release kinetics compared with non-imprinted systems. A therapeutically relevant amount of drug can be loaded for release to occur over multiple days, which allows the technique to be applied to daily-wear and extended-wear contact lenses.

WHAT THE READER WILL GAIN

The focus of this article is to review the emerging field of molecularly imprinted contact lenses and highlight significant accomplishments, trends, as well as future strategies and directions.

TAKE HOME MESSAGE

In the past 8 years, molecular imprinting has been used to produce therapeutic contact lenses with enhanced loading and delayed release. Progress in the field has mostly included low-molecular-weight therapeutics such as anti-glaucoma, antihistamine, antibiotic and anti-inflammatory therapeutics used to treat anterior eye disorders. Recently, high molecular weight comfort molecules have also been successfully demonstrated. Current methods can produce lenses of suitable thickness, water content, and mechanical and optical properties compared with commercial lenses on the market today.

Swelling, erosion and drug release characteristics of salbutamol sulfate from hydroxypropyl methylcellulose-based matrix tablets

BACKGROUND

Hydrophilic matrix formulations are important and simple technologies that are used to manufacture sustained release dosage forms.

METHOD

Hydroxypropyl methylcellulose-based matrix tablets, with and without additives, were manufactured to investigate the rate of hydration, rate of erosion, and rate and mechanism of drug release. Scanning electron microscopy was used to assess changes in the microstructure of the tablets during drug release testing and whether these changes could be related to the rate of drug release from the formulations.

RESULTS

The results revealed that the rate of hydration and erosion was dependent on the polymer combination(s) used, which in turn affected the rate and mechanism of drug release from these formulations. It was also apparent that changes in the microstructure of matrix tablets could be related to the different rates of drug release that were observed from the test formulations.

CONCLUSION

The use of scanning electron microscopy provides useful information to further understand drug release mechanisms from matrix tablets.

The Long-term Release of Antibiotics From Monolithic Nonporous Polymer Implants for Use as Tympanostomy Tubes

A technology is elaborated for the fabrication of a novel tympanostomy tube (TT) from solidified polymer melts (Elvax and Polyurethane) and antibiotics (Ciprofloxacin and Usnic acid) for insertion into tympanic membrane (ear drum) according to the established surgical procedure. The long-term in vitro release kinetics of the antibiotics into liquid water has been assessed using standard methods. The measured kinetic curves revealed two stages of antibiotic release into the finite space. During the first stage (fast), the fast release rate is almost invariant and is determined by the diffusion through the steady diffusion layer formed due to solution agitation. In this first stage, the influence of the initial internal transport is weak because it takes place at negligibly small distance from interface and accordingly, at negligibly concentration drop. After the antibiotic concentration decreases within the much broader layer of matrix near interface, the internal transport becomes important. This manifests itself as the second stage in measured kinetics of release curves which is characterized by a gradual decrease in rate. The minimum inhibition concentrations of three antibiotics/antimicrobial compounds for four bacterial species were measured. The first stage of fast release from the polymer implant lasts 6 days at a polymer loading by Ciprofloxacin (0.03 g/cm(3)) and this was sufficient for preventing biofilm formation on the surface of the implant material. The measured kinetic curves of drug release showed more rapid decrease in the release rate compared to the Higuchi approximation. Comparison with existing theories, which account for the finite rate of drug dissolution, showed that this may explain the observed deviation from the diffusion-controlled Higuchi model. Large dimensions of drug particles and their aggregation retard the dissolution stage and consequently the release rate. Melt blending was found to cause the drug particle aggregation within polymer matrixes which was confirmed by microscopic reexamination of the polymer implant materials.

Modeling of Drug Release from Bioerodible Polymer Matrices

Models for drug release from bioerodible polymer matrices are proposed in this article. We consider that drug is released continually by diffusion that is influenced by polymer chain degradation, and polymer matrix erosion starts and enhances the drug release at a certain time. The models give excellent reproduction of drug release profiles within the whole release period, and the parameters can be correlated to various factors such as gamma-irradiation dose, copolymer composition, and initial drug loading, this correlation indicates that the new models can be used to predict the effects of various factors on drug release profiles based on limited experimental data.

Sustained-release from Layered Matrix System Comprising Chitosan and Xanthan Gum

Sustained-release tablets of propranolol HCl were prepared by direct compression using chitosan and xanthan gum as matrix materials. The effective prolongation of drug release in acidic environment was achieved for matrix containing chitosan together with xanthan gum which prolonged the drug release more extensive than that containing single polymer. Increasing lactose into matrix could adjust the drug release characteristic by enhancing the drug released. Component containing chitosan and xanthan gum at ratio 1:1 and lactose 75% w/w was selected for preparing the layered matrix by tabletting. Increasing the amount of matrix in barrier or in middle layer resulted in prolongation of drug release. From the investigation of drug release from one planar surface, the lag time for drug release through barrier layer was apparently longer as the amount of barrier was enhanced. Least square fitting the experimental dissolution data to the mathematical expressions (power law, first order, Higuchi's and zero order) was performed to study the drug release mechanism. Layering with polymeric matrix could prolong the drug release and could shift the release pattern approach to zero order. The drug release from chitosan-xanthan gum three-layer tablet was pH dependent due to the difference in charge density in different environmental pH. FT-IR and DSC studies exhibited the charge interaction between of NH3+ of chitosan molecule and COO- of acetate or pyruvate groups of xanthan gum molecule. The SEM images revealed the formation of the loose membranous but porous film that was due to the gel layer formed by the polymer relaxation upon absorption of dissolution medium. The decreased rate of polymer dissolution resulting from the decreased rate of solvent penetration was accompanied by a decrease in drug diffusion due to ionic interaction between chitosan and xanthan gum. This was suggested that the utilization of chitosan and xanthan gum could give rise to layered matrix tablet exhibiting sustained drug release.

Zero-order therapeutic release from imprinted hydrogel contact lenses within in vitro physiological ocular tear flow

Zero-order or concentration independent release kinetics are highly desirable from drug delivery devices. In this paper we demonstrate experimentally, for the first time, zero-order release of a small molecular weight therapeutic, ketotifen fumarate (MW=425), from molecularly imprinted hydrogels used as therapeutic contact lenses. We performed dynamic, in vitro drug release studies from imprinted hydrogel contact lenses within a novel microfluidic device that simulates the volumetric flow rates, tear volume and tear composition of the eye. Imprinted gels with multiple functional monomers and complexation points to the drug demonstrated a significantly delayed release of drug compared to less functionalized systems. There were no statistical differences in experimentally determined equilibrium swollen polymer volume fractions, which correlate with molecular weight between crosslinks and mesh size of the gel. Under infinite sink conditions, imprinted contact lenses demonstrated Fickian (concentration dependent) release kinetics with diffusion coefficients ranging from 4.04 x 10(-9) to 5.57 x 10(-10) cm(2)/s. The highest functionalized gel exhibited a diffusion coefficient averaging ten times smaller than less functionalized gels and released drug for over 5 days with 3 distinct rates of release. Under physiological volumetric flow rates, the release rate was constant for a duration of 3.5 days delivering a therapeutically relevant dosage and was fit to a power law model indicating zero-order release characteristics with n=0.981+/-0.006 (r(2)=0.997). This work demonstrates the potential of micro/nanofluidic devices to determine physiological release rates and stresses the importance of matching local conditions to adequately characterize drug delivery devices. It also demonstrates the enormous potential for molecular imprinting to further tailor therapeutic release kinetics via the imprinting process.

Characterizing multilaminated hydrogels with spatially varying network structure and solute loading using confocal laser scanning microscopy

Multilaminated controlled release devices were formed through photopolymerization techniques to produce hydrogels with spatially varying solute loadings and network structures composed of poly(hydroxyl ethyl methacrylate) (PHEMA) and poly(ethylene glycol) (PEG). Using low molecular weight fluorescent dyes as model drugs, the distribution profiles were characterized non-invasively in pseudo-real-time with confocal laser scanning microscopy (CLSM) during release studies. For comparison, theoretical modeling based on Fickian diffusion theory was performed in conjunction with experimental work to identify any deviations from expected behavior and to guide in the development of future devices. In multilaminates composed of only PHEMA, the evolution of dye distribution during release and cumulative release profiles agreed well with theoretically predicted data, indicating continuity of diffusion and insignificant interfacial hindrance between layers. However, in devices composed of alternating layers of PHEMA and PEG, differences from predicted behavior were experimentally observed in both concentration profiles and release rates, suggesting interfacial obstruction of diffusion, possibly due to the formation of interpenetrating networks. Finally, the simultaneous release of two dyes at different rates from a PEG/PHEMA multilaminate was monitored to demonstrate the usefulness of CLSM in understanding the complex temporal changes in solute distributions in gel devices.

Modeling of degradation and drug release from a biodegradable stent coating

Biodegradable polymeric coatings on cardiovascular stents can be used for local delivery of therapeutic agents to diseased coronary arteries after stenting procedures. This can minimize the occurrence of clinically adverse events such as restenosis after stent implantation. A validated mathematical model can be a very important tool in the design and development of such coatings for drug delivery. The model should incorporate the important physicochemical processes responsible for the polymer degradation and drug release. Such a model can be used to study the effect of different coating parameters and configurations on the degradation and the release of the drug from the coating. In this paper, a simultaneous transport-reaction model predicting the degradation and release of the drug Everolimus from a polylactic acid (PLA) based stent coating is presented. The model has been validated using in vitro testing data and was further used to evaluate the influence of various parameters such as partitioning coefficient of water, autocatalytic effect of the lactic acid and structural change of the matrix, on the PLA degradation and drug release. The model can be used as a tool for predicting drug delivery from other coating configurations designed using the same polymer-drug combination. In addition, this modeling methodology has broader applications and can be used to develop mathematical models for predicting the degradation and drug release kinetics for other polymeric drug delivery systems.

Design and evaluation of 1- and 3-layer matrices of verapamil hydrochloride for sustaining its release

The present study was performed to design oral controlled delivery systems for the water-soluble drug, verapamil hydrochloride, using natural and semisynthetic polymers as carriers in the forms of 1- and 3-layer matrix tablets. Verapamil hydrochloride 1-layer matrix tablets containing hydroxypropylmethylcellulose, tragacanth, and acacia either alone or mixed were prepared by direct compression technique. 3-layer matrix tablets were prepared by compressing the polymers as release retardant layers on both sides of the core containing the drug. The prepared tablets were subjected to in vitro drug release studies. Tragacanth when used as the carrier in the formulation of 1- and 3-layer matrices produced satisfactory release prolongation either alone or in combination with the other 2 polymers. On the other hand, acacia did not show enough prolonging efficiency in 1- and 3-layer matrix tablets. The results also showed that the location of the polymers in the 3-layer tablets has a pronounced effect on the drug release. Kinetic analysis of drug release from matrices exhibiting sustained release indicated that release was predominantly attributable to the contribution made by Fickian diffusion, while the erosion/relaxation mechanisms had a minor role in the release.

Controlled release from triple layer, donut-shaped tablets with enteric polymers

The purpose of this research was to evaluate triple layer, donut-shaped tablets (TLDSTs) for extended release dosage forms. TLDSTs were prepared by layering 3 powders sequentially after pressing them with a punch. The core tablet consisted of enteric polymers, mainly hydroxypropyl methylcellulose acetate succinate, and the bottom and top layers were made of a water-insoluble polymer, ethyl cellulose. Drug release kinetics were dependent on the pH of the dissolution medium and the drug properties, such as solubility, salt forms of weak acid and weak base drugs, and drug loading. At a 10% drug loading level, all drugs, regardless of their type or solubility, yielded the same release profiles within an acceptable level of experimental error. As drug loading increased from 10% to 30%, the drug release rate of neutral drugs increased for all except sulfathiazole, which retained the same kinetics as at 10% loading. HCl salts of weak base drugs had much slower release rates than did those of neutral drugs (eg, theophylline) as drug loading increased. The release of labetalol HCl retarded as drug loading increased from 10% to 30%. On the other hand, Na salts of weak acid drugs had much higher release rates than did those of neutral drugs (eg, theophylline). Drug release kinetics were governed by the ionization/erosion process with slight drug diffusion, observing no perfect straight line. A mathematical expression for drug release kinetics (erosion-controlled system) of TLDSTs is presented. In summary, a TLDST is a good design to obtain zero-order or nearly zero-order release kinetics for a wide range of drug solubilities.

Novel carboxymethylcellulose-based microporous hydrogels suitable for drug delivery

Several materials capable of acting as structures for controlled release were analysed for the fabrication of matrices. Among those used, hydrophilic polysaccharides appeared to be the most suitable materials. Carboxymethylcellulose (a semi-synthetic polysaccharide) was chemically cross-linked with a 60% and 90% cross-linking degree in order to obtain hydrogels and utilised as matrix for the realisation of controlled drug release systems. The morphology of the gels was changed in order to obtain a microporous structure with different porosity (14, 30 and 40 microm). The obtained porous matrices were characterised in terms of pore density, dimension and swelling behaviour. The influence of both the pore dimension and technique of loading on the release kinetics was analysed. By increasing the pore dimension the release of ibuprofen-lysin was slower. Inducing the microporous structure after the loading of the hydrogel with the drug resulted in a slower release.

Effect of microenvironment pH of swellable and erodable buffered matrices on the release characteristics of diclofenac sodium

The aim of this work is to design pH-dependent swellable and erodable-buffered matrices and to study the effect of the microenvironment pH on the release pattern of diclofenac sodium. Buffered matrix tablets containing diclofenac sodium, physically mixed with hydrophilic polymer (hydroxypropyl methylcellulose [HPMC]) and pH-dependent solubility polymer (Eudragit L100-55) were prepared with different microenvironment pHs. The release of diclofenac sodium from the buffer matrices was studied in phosphate buffer solutions of pH 5.9 and 7.4. The swelling and erosion matrices containing only HPMC and Eudragit L100-55 were studied in phosphate buffer solution of pH similar to the microenvironment pHs of the matrices. Drug release from matrices was found to be linear as a function of time. Amount of drug released was found to be higher in the medium of pH 7.4 than that of pH 5.9. The rate of drug release increased with the increase of the microenvironment pH of the matrices as determined from the slope. The pattern of drug release did not change with the change of microenvironment pH. The swelling and erosion occurred simultaneously from matrices made up of HPMC and Eudragit L100-55. Both extent of swelling and erosion increased with increase of the medium pH. It was concluded from this study that changing the pH within the matrix influenced the rate of release of the drug without affecting the release pattern.

Minimization of initial burst in poly(vinyl alcohol) hydrogels by surface extraction and surface-preferential crosslinking

Surface extraction and surface-preferential crosslinking were investigated as effective methods to reduce the burst effect for proxyphylline release from poly(vinyl alcohol) hydrogels. Both these techniques involved changing the surface characteristics to reduce drug diffusion during the early stages of release, with the goal of subtracting the burst effect from the release profile without altering the long-term release rate. The extraction process was carried out on both relaxed and dry gels. Proxyphylline was extracted from both freshly made and dried hydrogel samples, with the extraction from dried samples providing better control of the burst effect with smaller amounts of drug removed from the gels. The success of extracting from the dried samples was attributed to the lack of drug diffusivity and redistribution after extraction when the majority of the device remained dry. Surface-preferential crosslinking, by dipping preformed proxyphylline-loaded samples in a concentrated crosslinking solution, effectively diminished the burst effect by slowing macromolecular relaxation near the surface. Notably, this technique maintained the same long-term drug release rate as the untreated gels and less than 0.2% of the loaded proxyphylline was removed during the crosslinking step.

Implantable, polymeric systems for modulated drug delivery

The ability to deliver therapeutic agents to a patient in a pulsatile or staggered release profile has been a major goal in drug delivery research over the last two decades. This review will cover methods that have been developed to control drug delivery profiles with implantable polymeric systems. Externally and internally controlled systems will be discussed, spanning a range of technologies that include pre-programmed systems, as well as systems that are sensitive to modulated enzymatic or hydrolytic degradation, pH, magnetic fields, ultrasound, electric fields, temperature, light and mechanical stimulation. Implantable systems have the potential to improve the quality of life for patients undergoing therapy with a variable dosing regime by eliminating the need for multiple intravenous injections. Ideally, these systems would also result in increased patient compliance with a given therapy due to the relative ease of self-dosing.

On the optimization of drug release from multi-laminated polymer matrix devices

This work presents a systematic optimization framework to achieve desired release rates in drug delivery devices using multi-laminated layers. A simple mathematical model is used to describe the transient mass transfer between successive layers, laminated together to form matrices with different initial concentrations, drug diffusivities and thickness. First, an efficient analytical-based optimization approach is investigated to define the optimal nonuniform initial drug distribution for constant diffusivity profile. The results obtained are in a good agreement with relevant work from the literature resorting to advanced optimal control techniques. Then, a formal dynamic optimization approach is employed, to systematically explore the synergistic benefits when all the available controllable parameters are simultaneously optimized, in order to achieve a drug release profile as close to a desired profile as possible for the entire period of operation. The optimization results lead to significantly improved constant release profiles.

Photopolymerized, multilaminated matrix devices with optimized nonuniform initial concentration profiles to control drug release

This paper describes a novel approach to obtain desired release profiles from diffusion-controlled matrix devices by employing nonuniform initial concentration profiles theoretically and experimentally. Theoretically, a model was developed to examine the effect of nonuniform initial concentration profiles on matrix release behavior, and an optimization technique was investigated to determine suitable nonuniform initial concentration profiles which provide desired release patterns. Experimentally, release rates of an organic dye from photopolymerized matrix devices were measured to test the application of these mathematical techniques and the efficacy of photolaminated matrices in approximating the optimized release behavior. All system parameters were measured by independent experiments, and the experimental release data agree very well with the computed results.