A mixture model for water uptake, degradation, erosion and drug release from polydisperse polymeric networks

Structural descriptor and surrogate modeling for design of biodegradable scaffolds

Biodegradable scaffolds are important to regenerative medicine in that they provide an amicable environment for tissue regrowth. However, establishing structure-property (SP) relationships for scaffold design is challenging due to the complexity of the three-dimensional porous scaffold geometry. The complexity requires high-dimensional geometric descriptors. The training of such a SP surrogate model will need a large amount of experimental or simulation data. In this work, a schema of constructing SP relationship surrogates is developed to predict the degraded mechanical properties from the initial scaffold geometry. A new structure descriptor, the extended surfacelet transform (EST), is proposed to capture important details of pores associated with the degradation of scaffolds. The efficiency is further enhanced with principal component analysis to reduce the high-dimensional EST data into a low-dimensional representation. The schema also includes a kinetic Monte Carlo biodegradation model to simulate the biodegradation of polymer scaffolds and to generate the training data for the formation of SP relationships. The schema is demonstrated with the design of polycaprolactone biodegradable scaffolds by connecting the initial scaffold geometry to the degraded compressive modulus.

Chitosan-based sponges containing clotrimazole for the topical management of vulvovaginal candidiasis

Vulvovaginal candidiasis (VVC) persists as a worrying women's healthcare issue, often relying on suboptimal therapeutics. Novel intravaginal dosage forms focusing on improving patient acceptability and featuring improved biopharmaceutical properties could be interesting alternatives to available antifungal products. Different formulations of sponges based on chitosan (Ch), with or without crosslinking and co-formulated with poly(N-vinylcaprolactam) (PNVCL), were produced for the topical administration of clotrimazole (CTZ) and further tested for physicochemical properties, drug release, cytotoxicity and antifungal activity. Results showed that high amounts of CTZ (roughly 30-50 %) could be incorporated into sponges obtained by using a simple freeze-drying methodology. Cross-linking of Ch with ammonia affected the morphology and mechanical features of sponges and shifted the release profile from sustained (around 20 % and 60 % drug released after 4 h and 24 h, respectively) to fast-releasing (over 90 % at 4 h). The combination of PNVCL with non-crosslinked Ch also allowed tuning drug release, namely by increasing the initial amount of CTZ released in simulated vaginal fluid (roughly 40 % after 4 h), as compared to sponges featuring only non-crosslinked Ch. All formulations displayed low toxicity to cell lines derived from the female genital tract, with viability values kept above 70 % after 24 h incubation with sponge extracts. These also allowed maintaining the rapid onset of the antifungal effects of CTZ at minimum inhibitory concentrations ranging from 0.5 to 16 μg/mL for a panel of six different Candida spp. strains. Overall, proposed sponge formulations appear to be promising alternatives for the safe and effective management of VVC.

Topical Formulations Based on Ursolic Acid-Loaded Nanoemulgel with Potential Application in Psoriasis Treatment

Psoriasis is a chronic disorder that causes a rash with itchy, scaly patches. It affects nearly 2-5% of the worldwide population and has a negative effect on patient quality of life. A variety of therapeutic approaches, e.g., glucocorticoid topical therapy, have shown limited efficacy with systemic adverse reactions. Therefore, novel therapeutic agents and physicochemical formulations are in constant need and should be obtained and tested in terms of effectiveness and minimization of side effects. For that reason, the aim of our study was to design and obtain various hybrid systems, nanoemulgel-macroemulsion and nanoemulgel-oleogel (bigel), as vehicles for ursolic acid (UA) and to verify their potential as topical formulations used in psoriasis treatment. Obtained topical formulations were characterized by conducting morphological, rheological, texture, and stability analysis. To determine the safety and effectiveness of the prepared ursolic acid carriers, in vitro studies on human keratinocyte cell-like HaCaT cells were performed with cytotoxicity analysis for individual components and each formulation. Moreover, a kinetic study of ursolic acid release from the obtained systems was conducted. All of the studied UA-loaded systems were well tolerated by keratinocyte cells and had suitable pH values and stability over time. The obtained formulations exhibit an apparent viscosity, ensuring the appropriate time of contact with the skin, ease of spreading, soft consistency, and adherence to the skin, which was confirmed by texture tests. The release of ursolic acid from each of the formulations is followed by a slow, controlled release according to the Korsmeyer-Peppas and Higuchi models. The elaborated systems could be considered suitable vehicles to deliver triterpene to psoriatic skin.

A reaction-diffusion framework for hydrolytic degradation of amorphous polymers based on a discrete chain scission model

Hydrolytic degradation of polymers involves the scission of long chain molecules, leading to molecular weight reduction and mass loss. The precise degradation response however depends on the scission probability of individual bonds along the polymer backbone. In particular, bonds near the chain ends are considered to be more susceptible to hydrolysis than inner bonds. In this paper, we incorporate a discrete chain scission model that can handle arbitrary bond scission probabilities within a continuum reaction-diffusion framework. Overall hydrolysis kinetics (including autocatalysis) is described independently of the chain scission model. By decoupling the description of the chain scission mechanism from kinetics, our framework enables the identification of the chain scission mechanism from molecular weight reduction and mass loss curves commonly reported in experimental degradation studies. We further propose a reduced continuum model which is better suited for large-scale simulations while retaining the predictive capability of the full discrete-continuum model. The model capability is illustrated in representative case studies based on experimental data from the literature for different materials and geometries. STATEMENT OF SIGNIFICANCE: Many models have been proposed to predict the evolution of molecular weight and mass loss in biodegradable polymers undergoing hydrolytic degradation. However, existing models remain limited in their ability to describe the degradation mechanism, autocatalytic kinetics and short chains diffusion simultaneously. Moreover, existing models often rely on empirical relations and a large number of fitting parameters. Here, we propose a conceptually simple discrete-continuum mathematical framework with a small number of parameters which all have a clear physical meaning. Model calibration against experimental data is simplified, and further provides insights into the degradation mechanisms at play, namely random scission, chain-end scission, or a combination of both. The framework can serve as a basis for future generalisations, including a description of evolving crystallinity, or other degradation mechanisms, such as thermal oxidation or photo-degradation.

Drug Carriers: A Review on the Most Used Mathematical Models for Drug Release

Carriers are protective transporters of drugs to target cells, facilitating therapy under each points of view, such as fast healing, reducing infective phenomena, and curing illnesses while avoiding side effects. Over the last 60 years, several scientists have studied drug carrier properties, trying to adapt them to the release environment. Drug/Carrier interaction phenomena have been deeply studied, and the release kinetics have been modeled according to the occurring phenomena involved in the system. It is not easy to define models’ advantages and disadvantages, since each of them may fit in a specific situation, considering material interactions, diffusion and erosion phenomena, and, no less important, the behavior of receiving medium. This work represents a critical review on main mathematical models concerning their dependency on physical, chemical, empirical, or semi-empirical variables. A quantitative representation of release profiles has been shown for the most representative models. A final critical comment on the applicability of these models has been presented at the end. A mathematical approach to this topic may help students and researchers approach the wide panorama of models that exist in literature and have been optimized over time. This models list could be of practical inspiration for the development of researchers’ own new models or for the application of proper modifications, with the introduction of new variable dependency.

Formulation of Microwave-Assisted Natural-Synthetic Polymer Composite Film and Its Physicochemical Characterization

This study is aimed at microwave-assisted synthesis of sodium carboxymethylcellulose and Eudragit L100 composite film and its physicochemical characterization. The film was developed with varying quantities of each polymer and treated with microwave at a fixed frequency of 2450 MHz with a power of 350 Watts for 60 and 120 s. All formulations were characterized for thickness/weight uniformity, moisture adsorption, erosion and water uptake, tensile strength, and vibrational, thermal, and surface morphological analysis in comparison with untreated film samples. Results indicated that microwave treatment for 60 s significantly improved the tensile strength, reduced the water adsorption, delayed erosion, and reduced the water uptake in comparison with the untreated and 120 s treated film formulations. The vibrational analysis revealed rigidification of hydrophilic domains at OH/NH moiety and fluidization of hydrophobic domains at asymmetric and symmetric CH moieties, which is envisaged to be due to the formation of new linkages between the two polymers. These were later confirmed by thermal analysis where a significant rise in transition temperature, as well as enthalpy of the system, was recorded. The microwave treatment for 60 s is thus advocated to be the best treatment condition for developing sodium carboxymethylcellulose and Eudragit L100 composite polymeric films.

Degradation-Dependent Controlled Delivery of Doxorubicin by Glyoxal Cross-Linked Magnetic and Porous Chitosan Microspheres

Glyoxal cross-linked porous magnetic chitosan microspheres, GMS (∼170 μm size), with a tunable degradation profile were synthesized by a water-in-oil emulsion technique to accomplish controlled delivery of doxorubicin (DOX), a chemotherapeutic drug, to ensure prolonged chemotherapeutic effects. The GMS exhibit superparamagnetism with saturation magnetization, M s = 7.2 emu g-1. The in vitro swelling and degradation results demonstrate that a swelling plateau of GMS is reached at 24 h, while degradation can be modulated to begin at 96-120 h by formulating the cross-linked network using glyoxal. MTT assay, live/dead staining, and F-actin staining (actin/DAPI) validated the cytocompatibility of GMS, which further assured good drug loading capacity (35.8%). The release mechanism has two stages, initiated by diffusion-inspired release of DOX through the swollen polymer network (72 h), which is followed by a disintegration-tuned release profile (>96 h) conferring GMS a potential candidate for DOX delivery.

Tuning the selective permeability of polydisperse polymer networks

We study the permeability and selectivity ('permselectivity') of model membranes made of polydisperse polymer networks for molecular penetrant transport, using coarse-grained, implicit-solvent computer simulations. In our work, permeability P is determined on the linear-response level using the solution-diffusion model, P = KDin, i.e., by calculating the equilibrium penetrant partition ratio K and penetrant diffusivity Din inside the membrane. We vary two key parameters, namely the network-network interaction, which controls the degree of swelling and collapse of the network, and the network-penetrant interaction, which tunes the selective penetrant uptake and microscopic energy landscape for diffusive transport. We find that the partitioning K covers four orders of magnitude and is a non-monotonic function of the parameters, well interpreted by a second-order virial expansion of the free energy of transferring one penetrant from a reservoir into the membrane. Moreover, we find that the penetrant diffusivity Din in the polydisperse networks, in contrast to highly ordered membrane structures, exhibits relatively simple exponential decays. We propose a semi-empirical scaling law for the penetrant diffusion that describes the simulation data for a wide range of densities and interaction parameters. The resulting permeability P turns out to follow the qualitative behavior (including maximization and minimization) of partitioning. However, partitioning and diffusion are typically anti-correlated, yielding large quantitative cancellations, controlled and fine-tuned by the network density and interactions, as rationalized by our scaling laws. We finally demonstrate that even small changes of network-penetrant interactions, e.g., by half a kBT, modify the permselectivity by almost one order of magnitude.

Preparation and optimization of monodisperse polymeric microparticles using modified vibrating orifice aerosol generator for controlled delivery of letrozole in breast cancer therapy

Letrozole (LTZ) is effective for the treatment of hormone-receptor-positive breast cancer in postmenopausal women. In this work, and for the first time, using vibrating orifice aerosol generator (VOAG) technology, monodisperse poly-ε-caprolactone (PCL), and poly (D, L-Lactide) (PDLLA) LTZ-loaded microparticles were prepared and found to elicit selective high cytotoxicity against cancerous breast cells with no apparent toxicity on healthy cells in vitro. Plackett-Burman experimental design was utilized to identify the most significant factors affecting particle size distribution to optimize the prepared particles. The generated microparticles were characterized in terms of microscopic morphology, size, zeta potential, drug entrapment efficiency, and release profile over one-month period. Long-term cytotoxicity of the microparticles was also investigated using MCF-7 human breast cancer cell lines in comparison with primary mammary epithelial cells (MEC). The prepared polymeric particles were monodispersed, spherical, and apparently smooth, regardless of the polymer used or the loaded LTZ concentration. Particle size varied from 15.6 to 91.6 µm and from 22.7 to 99.6 µm with size distribution (expressed as span values) ranging from 0.22 to 1.24 and from 0.29 to 1.48 for PCL and PDLLA based microparticles, respectively. Upon optimizing the manufacture parameters, span was reduced to 0.162-0.195. Drug entrapment reached as high as 96.8%, and drug release from PDLLA and PCL followed a biphasic zero-order release using 5 or 30% w/w drug loading in the formulations. Long-term in vitro cytotoxicity studies indicated that microparticles formulations significantly inhibited the growth of MCF-7 cell line over a prolonged period of time but did not have toxic effects on the normal breast epithelial cells.

Novel Polymer-Free Everolimus-Eluting Stent Fabricated using Femtosecond Laser Improves Re-endothelialization and Anti-inflammation

The aim of this study was to fabricate a novel polymer-free everolimus-eluting stent with nanostructure using a femtosecond laser (FSL). The stent were coated with everolimus (EVL) using FSL and electrospinning processes. The surface was rendered hydrophobic, which negatively affected both platelet adhesion (82.1%) and smooth muscle cell response. Animal study was performed using a porcine coronary restenosis model. The study groups were divided into 1) bare metal stent (BMS), 2) poly(L-lactide) (PLA)-based EVL drug eluting stent (DES), 3) commercial EVL-eluting DES, and 4) FSL-EVL-DES. After four weeks of stent implantation, various analyses were performed. Quantitative analysis showed that the amount of in-stent restenosis was higher in the BMS group (BMS; 27.8 ± 2.68%, PLA-based DES; 12.2 ± 0.57%, commercial DES; 9.8 ± 0.28%, and FSL-DES; 9.3 ± 0.25%, n = 10, p < 0.05). Specifically, the inflammation score was reduced in the FSL-DES group (1.9 ± 0.39, n = 10, p < 0.05). The increment in re-endothelialization in the FSL-DES group was confirmed by immunofluorescence analysis. Taken together, the novel polymer-free EVL-eluting stent fabricated using FSL can be an innovative DES with reduced risk of ISR, thrombosis, and inflammation.

Engineering approaches in siRNA delivery

siRNAs are very potent drug molecules, able to silence genes involved in pathologies development. siRNAs have virtually an unlimited therapeutic potential, particularly for the treatment of inflammatory diseases. However, their use in clinical practice is limited because of their unfavorable properties to interact and not to degrade in physiological environments. In particular they are large macromolecules, negatively charged, which undergo rapid degradation by plasmatic enzymes, are subject to fast renal clearance/hepatic sequestration, and can hardly cross cellular membranes. These aspects seriously impair siRNAs as therapeutics. As in all the other fields of science, siRNAs management can be advantaged by physical-mathematical descriptions (modeling) in order to clarify the involved phenomena from the preparative step of dosage systems to the description of drug-body interactions, which allows improving the design of delivery systems/processes/therapies. This review analyzes a few mathematical modeling approaches currently adopted to describe the siRNAs delivery, the main procedures in siRNAs vectors' production processes and siRNAs vectors' release from hydrogels, and the modeling of pharmacokinetics of siRNAs vectors. Furthermore, the use of physical models to study the siRNAs vectors' fate in blood stream and in the tissues is presented. The general view depicts a framework maybe not yet usable in therapeutics, but with promising possibilities for forthcoming applications.

Integrated Stent Models Based on Dimension Reduction: Review and Future Perspectives

Stent modeling represents a challenging task from both the theoretical and numerical viewpoints, due to its multi-physics nature and to the complex geometrical configuration of these devices. In this light, dimensional model reduction enables a comprehensive geometrical and physical description of stenting at affordable computational costs. In this work, we aim at reviewing dimensional model reduction of stent mechanics and drug release. Firstly, we address model reduction techniques for the description of stent mechanics, aiming to illustrate how a three-dimensional stent model can be transformed into a collection of interconnected one-dimensional rods, called a "stent net". Secondly, we review available model reduction methods similarly applied to drug release, in which the "stent net" concept is adopted for modeling of drug elution. As a result, drug eluting stents are described as a distribution of concentrated drug release sources located on a graph that fully represents the stent geometry. Lastly, new results about the extension of these model reduction approaches to biodegradable stents are also discussed.

The anisotropic mechanical behaviour of electro-spun biodegradable polymer scaffolds: Experimental characterisation and constitutive formulation

Electro-spun biodegradable polymer fibrous structures exhibit anisotropic mechanical properties dependent on the degree of fibre alignment. Degradation and mechanical anisotropy need to be captured in a constitutive formulation when computational modelling is used in the development and design optimisation of such scaffolds. Biodegradable polyester-urethane scaffolds were electro-spun and underwent uniaxial tensile testing in and transverse to the direction of predominant fibre alignment before and after in vitro degradation of up to 28 days. A microstructurally-based transversely isotropic hyperelastic continuum constitutive formulation was developed and its parameters were identified from the experimental stress-strain data of the scaffolds at various stages of degradation. During scaffold degradation, maximum stress and strain in circumferential direction decreased from 1.02 ± 0.23 MPa to 0.38 ± 0.004 MPa and from 46 ± 11 % to 12 ± 2 %, respectively. In longitudinal direction, maximum stress and strain decreased from 0.071 ± 0.016 MPa to 0.010 ± 0.007 MPa and from 69 ± 24 % to 8 ± 2 %, respectively. The constitutive parameters were identified for both directions of the non-degraded and degraded scaffold for strain range varying between 0% and 16% with coefficients of determination r(2)>0.871. The six-parameter constitutive formulation proved versatile enough to capture the varying non-linear transversely isotropic behaviour of the fibrous scaffold throughout various stages of degradation.

Some design considerations for polymer-free drug-eluting stents: a mathematical approach

In this paper we provide the first model of drug elution from polymer-free arterial drug-eluting stents. The generalised model is capable of predicting drug release from a number of polymer-free systems including those that exhibit nanoporous, nanotubular and smooth surfaces. We derive analytical solutions which allow us to easily determine the important parameters that control drug release. Drug release profiles are provided, and we offer design recommendations so that the release profile may be tailored to achieve the desired outcome. The models presented here are not specific to drug-eluting stents and may also be applied to other biomedical implants that use nanoporous surfaces to release a drug.

Application of mathematical modeling in sustained release delivery systems

INTRODUCTION

This review, presenting as starting point the concept of the mathematical modeling, is aimed at the physical and mathematical description of the most important mechanisms regulating drug delivery from matrix systems. The precise knowledge of the delivery mechanisms allows us to set up powerful mathematical models which, in turn, are essential for the design and optimization of appropriate drug delivery systems.

AREAS COVERED

The fundamental mechanisms for drug delivery from matrices are represented by drug diffusion, matrix swelling, matrix erosion, drug dissolution with possible recrystallization (e.g., as in the case of amorphous and nanocrystalline drugs), initial drug distribution inside the matrix, matrix geometry, matrix size distribution (in the case of spherical matrices of different diameter) and osmotic pressure. Depending on matrix characteristics, the above-reported variables may play a different role in drug delivery; thus the mathematical model needs to be built solely on the most relevant mechanisms of the particular matrix considered.

EXPERT OPINION

Despite the somewhat diffident behavior of the industrial world, in the light of the most recent findings, we believe that mathematical modeling may have a tremendous potential impact in the pharmaceutical field. We do believe that mathematical modeling will be more and more important in the future especially in the light of the rapid advent of personalized medicine, a novel therapeutic approach intended to treat each single patient instead of the 'average' patient.

Control of scaffold degradation in tissue engineering: a review

Tissue engineering has shown a great promise as a solution to the high demand for tissue and organ transplantations. Biomaterial scaffolds serve to house and direct cells to grow, exposing them to an adequate perfusion of nutrients, oxygen, metabolic products, and appropriate growth factors to enhance their differentiation and function. The degradation of biomaterial scaffolds is a key factor to successful tissue regeneration. In this article, the existing degradation control approaches in the context of scaffold tissue engineering were reviewed and a new paradigm of thinking called active control of scaffold degradation, proposed elsewhere by us, was also revisited and discussed in light of its benefit and requirement of this new technology.

Impact of polymer structure and composition on fully resorbable endovascular scaffold performance

Fully erodible endovascular scaffolds are being increasingly considered for the treatment of obstructive arterial disease owing to their potential to mitigate long-term risks associated with permanent alternatives. While complete scaffold erosion facilitates vessel healing, generation and release of material degradation by-products from candidate materials such as poly-L-lactide (PLLA) may elicit local inflammatory responses that limit implant efficacy. We developed a computational framework to quantify how the compositional and structural parameters of PLLA-based fully erodible endovascular scaffolds affect degradation kinetics, erosion kinetics and the transient accumulation of material by-products within the arterial wall. Parametric studies reveal that, while some material properties have similar effects on these critical processes, others induce qualitatively opposing responses. For example, scaffold degradation is only mildly responsive to changes in either PLLA polydispersity or the initial degree of crystallinity, while the erosion kinetics is comparatively sensitive to crystallinity. Moreover, lactide doping can effectively tune both scaffold degradation and erosion, but a concomitant increase in local by-product accumulation raises concerns about implant safety. Optimized erodible endovascular scaffolds must precisely balance therapeutic function and biological response over the implant lifetime, where compositional and structural parameters will have differential effects on implant performance.

Multiscale analysis of water uptake and erosion in biodegradable polyarylates

The role of hydration in degradation and erosion of materials, especially biomaterials used in scaffolds and implants, was investigated by studying the distribution of water at length scales from 0.1 nm to 0.1 mm using Raman spectroscopy, small-angle neutron scattering (SANS), Raman confocal imaging, and scanning electron microscopy (SEM). The measurements were demonstrated using L-tyrosine derived polyarylates. Bound- and free- water were characterized using their respective signatures in the Raman spectra. In the presence of deuterium oxide (D(2)O), H-D exchange occurred at the amide carbonyl but was not detected at the ester carbonyl. Water appeared to be present in the polymer even in regions where there was little evidence for N-H to N-D exchange. SANS showed that water is not uniformly dispersed in the polymer matrix. The distribution of water can be described as mass fractals in polymers with low water content (~5 wt%), and surface fractals in polymers with larger water content (15 to 60 wt%). These fluctuations in the density of water distribution are presumed to be the precursors of the ~ 20 μm water pockets seen by Raman confocal imaging, and also give rise to 10-50 μm porous network seen in SEM. The surfaces of these polymers appeared to resist erosion while the core of the films continued to erode to form a porous structure. This could be due to differences in either the density of the polymer or the solvent environment in the bulk vs. the surface, or a combination of these two factors. There was no correlation between the rate of degradation and the amount of water uptake in these polymers, and this suggests that it is the bound-water and not the total amount of water that contributes to hydrolytic degradation.

Assessment of material by-product fate from bioresorbable vascular scaffolds

Fully bioresorbable vascular scaffolds (BVS) are attractive platforms for the treatment of ischemic artery disease owing to their intrinsic ability to uncage the treated vessel after the initial scaffolding phase, thereby allowing for the physiological conditioning that is essential to cellular function and vessel healing. Although scaffold erosion confers distinct advantages over permanent endovascular devices, high transient by-product concentrations within the arterial wall could induce inflammatory and immune responses. To better understand these risks, we developed in this study an integrated computational model that characterizes the bulk degradation and by-product fate for a representative BVS composed of poly(L-lactide) (PLLA). Parametric studies were conducted to evaluate the relative impact of PLLA degradation rate, arterial remodeling, and metabolic activity on the local lactic acid (LA) concentration within arterial tissue. The model predicts that both tissue remodeling and PLLA degradation kinetics jointly modulate LA fate and suggests that a synchrony of these processes could minimize transient concentrations within local tissue. Furthermore, simulations indicate that LA metabolism is a relatively poor tissue clearance mechanism compared to convective and diffusive transport processes. Mechanistic understanding of factors governing by-product fate may provide further insights on clinical outcome and facilitate development of future generation scaffolds.

Effect of hydrophobic and hydrophilic additives on sol-gel transition and release behavior of timolol maleate from polycaprolactone-based hydrogel

The objective of this work was to delineate the effect of hydrophilic and hydrophobic polymeric additives on sol-gel transition and release profile of timolol maleate (TM) from poly (ethylene glycol)-poly (ε-caprolactone)- poly (ethylene glycol) (PEG-PCL-PEG)-based thermosensitive hydrogel. Polycaprolactone (hydrophobic additive) and polyvinyl alcohol (PVA) (hydrophilic additive) reduced critical gel concentration of PEG-PCL-PEG triblock polymer. The effect of PCL on sol-gel transition was more pronounced than PVA. However, with PCL no statistically significant difference in release profile was observed. The effect of PVA on release profile was more pronounced, which reduced the cumulative percentage release of TM from 86.4±0.8% to 73.7±1.8% over 316 h. Moreover, cytotoxicity of the hydrogel was also investigated utilizing rabbit primary corneal epithelial culture cells. No significant cytotoxicity of hydrogel alone or in presence of additives was observed. So, polymeric additive strategy serves as a valuable tool for optimizing TM release kinetics from PEG-PCL-PEG hydrogel matrix.

Mathematical modeling of degradation for bulk-erosive polymers: applications in tissue engineering scaffolds and drug delivery systems

The degradation of polymeric biomaterials, which are widely exploited in tissue engineering and drug delivery systems, has drawn significant attention in recent years. This paper aims to develop a mathematical model that combines stochastic hydrolysis and mass transport to simulate the polymeric degradation and erosion process. The hydrolysis reaction is modeled in a discrete fashion by a fundamental stochastic process and an additional autocatalytic effect induced by the local carboxylic acid concentration in terms of the continuous diffusion equation. Illustrative examples of microparticles and tissue scaffolds demonstrate the applicability of the model. It is found that diffusive transport plays a critical role in determining the degradation pathway, whilst autocatalysis makes the degradation size dependent. The modeling results show good agreement with experimental data in the literature, in which the hydrolysis rate, polymer architecture and matrix size actually work together to determine the characteristics of the degradation and erosion processes of bulk-erosive polymer devices. The proposed degradation model exhibits great potential for the design optimization of drug carriers and tissue scaffolds.

Modeling of Deformation-Accelerated Breakdown of Polylactic Acid Biodegradable Stents

The use of biodegradable polymers in biomedical applications has been successful in nonload bearing applications, such as biodegradable implants for local drug delivery, and in simple load bearing situations such as surgical sutures and orthopedic fixation screws. The desire to incorporate these materials in more complex load bearing situations, such as tissue engineering scaffolds and endovascular or urethral stents, is strong, but the lack of constitutive models describing the evolution of biodegradable polymers over the course of degradation has severely hampered the rational design process for these more complex biodegradable medical applications. With the objective of predicting biodegradable stent behavior, we incorporated constitutive models of biodegradable polymeric materials in a computational setting and the mechanical response of three different stent designs were analyzed as degradation progressed. A thermodynamically consistent constitutive model for materials undergoing deformation-induced degradation was applied to a commonly employed biodegradable polymer system, poly(L-lactic acid), and its specific form was determined by corroboration against experimental data. Depreciation of mechanical properties due to degradation confers time-dependent characteristics to the response of the biodegradable material: the deformation imparted by a constant load increases over time, i.e. the body creeps, and the stress necessary to keep a fixed deformation decreases, i.e. the body relaxes. Biodegradable stents, when subjected to constant pressure in its exterior, deflect inwards and ultimately fail as the structure loses its mechanical integrity. The complex geometry of endovascular stents and their physiological loading conditions lead to inhomogeneous deformations, and consequently, inhomogeneous degradation ensues. Degradation is mostly confined to the bends of the stent rings and junction points, which are the locations that carry most of the deformation, whereas mostly undeformed connector bars remain less degraded. If failure occurs, it will occur most likely at those sensitive locations and large, nondegraded pieces can provoke severe embolic problems. Highly nonuniform degradation indicates that some stent designs are at higher risk for complications. Deformation patterns of stents made of a material that loses its integrity are different than those of permanent stents. Blind adaptation of permanent stent design concepts is ill-suited for biodegradable stent design. The time-dependent aspect of the implant not only must be taken into account but should also be used to interact with the body’s reaction and to enhance healing.