In vitro study of drug-eluting stent coatings based on poly(L-lactide) incorporating cyclosporine A - drug release, polymer degradation and mechanical integrity

In vitro drug release profiling of Sirolimus polymeric microparticles coated long-acting stents

In the realm of arterial disease interventions, drug-eluting stents (DES) have become a vital therapeutic choice in preventing atherosclerotic plaque formation and restenosis and facilitating vessel healing. Sirolimus-encapsulated poly Lactic-co-Glycolic acid (PLGA) Microparticles (MPs) were developed using solvent evaporation. MPs were freeze-dried with a cryoprotectant and coated on the stent surface using an efficient and reproducible nitrogen-assisted spray coating technique. The MPs displayed a uniform distribution particle size of 4.38 ± 1.1 μm, span value of 0.88 ± 0.02, coating mass transfer efficiency of 13.45 ± 1.1 % on the stent, and a coating time of ≤ 2 min per stent. Post sterilization, the particle size and morphology of the coated stents remained unchanged. Accelerated in vitro drug release profiles were evaluated under different conditions, indicating significant influences based on dissolution methods ranging from 28.2 %±4.3 %, 42.5 %±5.3 %, 76.6 %±4.7 %, and 84.25 %±3.1 % for dialysis bag (DB), vessel simulating flow-through cell (vFTC), flow-through cell (FTC), and sample and separate (SS) technique respectively for 48 h. The drug release mechanism from the coated stents is governed by the combination of the Korsmeyer Peppas and Higuchi models. The developed dissolution method exhibited discriminative effectiveness when evaluated with critical formulation attributes and process parameter variations. The 48 h accelerated drug release studies correlated well with the 6-month real-time release rate with an R2 value of 0.9142 and Pearson's R2 of 0.9561. Ex-vivo studies demonstrated the permeation of MPs into artery tissues. Stability studies confirmed that MPs coated stents maintained desired properties at 4 °C and 30 °C/65 % RH for 6 months. Overall, these findings contribute to advancing stent technology, suggesting the potential for improvement of arterial interventions and enhanced patient outcomes.

Influence of Ethylene Oxide and Gamma Irradiation Sterilization Processes on the Properties of Poly-L-Lactic-Acid (PLLA) Materials

In order to encourage the substitution of petrochemical polymers in medical technology with sustainable, bio-based materials, there is an urgent need for further investigations, especially data regarding their sterility performance. Within the scope of the investigations, selected material properties of poly-L-lactic-acid (PLLA), a specific type of poly(lactic-acid) (PLA), were analyzed before and after sterilization (using ethylene oxide or gamma irradiation) in order to investigate deviations in its chemical structure, wettability, optical, and mechanical properties. In particular, parameters such as molecular weight, complex viscosity, tensile strength, water contact angle, and color were discussed. Sterilization temperatures close to the glass transition of PLA, high humidity, and interactions with the ethylene oxide molecules have resulted in an increase in crystallinity, a decrease in elongation at break, and in some cases, a variation in wettability. As a consequence of exposure to high-energy radiation, the material's toughness is reduced due to chain scission, which is manifested through a decrease in molecular weight, an increase in crystallinity, and a partial change in surface energy. For the selected PLLA-materials (Luminy® L130, NP HT 202, and NP HT 203), ethylene oxide sterilization resulted in a comparatively minor variation in the characteristics behavior, and was chosen as the preferred method.

Dual Drug Delivery in Cochlear Implants: In Vivo Study of Dexamethasone Combined with Diclofenac or Immunophilin Inhibitor MM284 in Guinea Pigs

Cochlear implants are well established to treat severe hearing impairments. Despite many different approaches to reduce the formation of connective tissue after electrode insertion and to keep electrical impedances low, results are not yet satisfying. Therefore, the aim of the current study was to combine the incorporation of 5% dexamethasone in the silicone body of the electrode array with an additional polymeric coating releasing diclofenac or the immunophilin inhibitor MM284, some anti-inflammatory substances not yet tested in the inner ear. Guinea pigs were implanted for four weeks and hearing thresholds were determined before implantation and after the observation time. Impedances were monitored over time and, finally, connective tissue and the survival of spiral ganglion neurons (SGNs) were quantified. Impedances increased in all groups to a similar extent but this increase was delayed in the groups with an additional release of diclofenac or MM284. Using Poly-L-lactide (PLLA)-coated electrodes, the damage caused during insertion was much higher than without the coating. Only in these groups, connective tissue could extend to the apex of the cochlea. Despite this, numbers of SGNs were only reduced in PLLA and PLLA plus diclofenac groups. Even though the polymeric coating was not flexible enough, MM284 seems to especially have potential for further evaluation in connection with cochlear implantation.

Combining modeling of drug uptake and release of cyclosporine in contact lenses to determine partition coefficient and diffusivity

Ophthalmic drug delivery via eye drops is inefficient because only about 1-5% of the drug permeates the cornea during the short residence time of a few minutes. Contact lenses are receiving considerable attention for delivering ophthalmic drugs because of higher bioavailability and the possibility of sustained release from hour to days, and possibly longer. The drug release durations from contact lenses are typically measured in vitro and it is challenging to relate the in vitro release to in vivo release, particularly for hydrophobic drugs which may not exhibit sink release in vitro and in vivo. The in vitro release can be fitted to diffusion equation to determine the partition coefficient and diffusivity, which can then be utilized to model in vivo release. The Higuchi equation is frequently used to model the short time release from a contact lens to determine diffusivity with the implicit assumption that the release is under sink conditions and the starting concentration in the lens was uniform. Both conditions may be violated when measuring release of hydrophobic drugs from contact lenses because the diffusivity and partition coefficient, and also the time needed for equilibrium are not known a priori. Here we develop a method to use the data for both loading and release of cyclosporine, which is a common hydrophobic ophthalmic drug, to determine the partition coefficient and diffusivity. The proposed approach does not require sink conditions and also does not require the lens to be fully equilibrated during loading, which may take almost a month for lenses considered here. The model is based on solving the diffusion equation in the gel along with a mass balance in the fluid. The model equations are solved numerically by finite difference. When the value of partition coefficient is high, such as it is for cyclosporine, the dynamic data is only sensitive to a ratio of partition coefficient and diffusivity, and this ratio had to first be determined from the loading data. Then the two unknown parameters were obtained by minimizing the error between the model prediction and experimental data. The method was used to determine D and K for several silicone hydrogel formulations with varying ratio of hydrogel and silicone fractions.

Influence of Drug Incorporation on the Physico-Chemical Properties of Poly(l-Lactide) Implant Coating Matrices-A Systematic Study

Local drug delivery has become indispensable in biomedical engineering with stents being ideal carrier platforms. While local drug release is superior to systemic administration in many fields, the incorporation of drugs into polymers may influence the physico-chemical properties of said matrix. This is of particular relevance as minimally invasive implantation is frequently accompanied by mechanical stresses on the implant and coating. Thus, drug incorporation into polymers may result in a susceptibility to potentially life-threatening implant failure. We investigated spray-coated poly-l-lactide (PLLA)/drug blends using thermal measurements (DSC) and tensile tests to determine the influence of selected drugs, namely sirolimus, paclitaxel, dexamethasone, and cyclosporine A, on the physico-chemical properties of the polymer. For all drugs and PLLA/drug ratios, an increase in tensile strength was observed. As for sirolimus and dexamethasone, PLLA/drug mixed phase systems were identified by shifted drug melting peaks at 200 °C and 240 °C, respectively, whereas paclitaxel and dexamethasone led to cold crystallization. Cyclosporine A did not affect matrix thermal properties. Altogether, our data provide a contribution towards an understanding of the complex interaction between PLLA and different drugs. Our results hold implications regarding the necessity of target-oriented thermal treatment to ensure the shelf life and performance of stent coatings.

Intelligent H2S release coating for regulating vascular remodeling

Coronary atherosclerotic lesions exhibit a low-pH chronic inflammatory response. Due to insufficient drug release control, drug-eluting stent intervention can lead to delayed endothelialization, advanced thrombosis, and unprecise treatment. In this study, hyaluronic acid and chitosan were used to prepare pH-responsive self-assembling films. The hydrogen sulfide (H₂S) releasing aspirin derivative ACS14 was used as drug in the film. The film regulates the release of the drug adjusted to the microenvironment of the lesion, and the drug balances the vascular function by releasing the regulating gas H₂S, which comparably to NO promotes the self-healing capacity of blood vessels. Drug releasing profiles of the films at different pH, and other biological effects on blood vessels were evaluated through blood compatibility, cellular, and implantation experiments. This novel method of self-assembled films which H₂S in an amount, which is adjusted to the condition of the lesion provides a new concept for the treatment of cardiovascular diseases.

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PH-responsive self-assembling films were used to intelligently release the drugs at atherosclerotic lesions.

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As a gaseous signaling molecule, H₂S donor ACS14 was loaded into the films used in the field of cardiovascular disease treatment.

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H₂S can help to regulate vascular remodeling and balance the vascular function.

Drug-Eluting Stents and Balloons-Materials, Structure Designs, and Coating Techniques: A Review

Controlled drug delivery is a matter of interest to numerous scientists from various domains, as well as an essential issue for society as a whole. In the treatment of many diseases, it is crucial to control the dosing of a drug for a long time and thus maintain its optimal concentration in the tissue. Heart diseases are particularly important in this aspect. One such disease is an obstructive arterial disease affecting millions of people around the world. In recent years, stents and balloon catheters have reached a significant position in the treatment of this condition. Balloon catheters are also successfully used to manage tear ducts, paranasal sinuses, or salivary glands disorders. Modern technology is continually striving to improve the results of previous generations of stents and balloon catheters by refining their design, structure, and constituent materials. These advances result in the development of both successive models of drug-eluting stents (DES) and drug-eluting balloons (DEB). This paper presents milestones in the development of DES and DEB, which are a significant option in the treatment of coronary artery diseases. This report reviews the works related to achievements in construction designs and materials, as well as preparation technologies, of DES and DEB. Special attention was paid to the polymeric biodegradable materials used in the production of the above-mentioned devices. Information was also collected on the various methods of producing drug release coatings and their effectiveness in releasing the active substance.

Customized Therapeutic Surface Coatings for Dental Implants

Dental implants are frequently used to support fixed or removable dental prostheses to replace missing teeth. The clinical success of titanium dental implants is owed to the exceptional biocompatibility and osseointegration with the bone. Therefore, the enhanced therapeutic effectiveness of dental implants had always been preferred. Several concepts for implant coating and local drug delivery had been developed during the last decades. A drug is generally released by diffusion-controlled, solvent-controlled, and chemical controlled methods. Although a range of surface modifications and coatings (antimicrobial, bioactive, therapeutic drugs) have been explored for dental implants, it is still a long way from designing sophisticated therapeutic implant surfaces to achieve the specific needs of dental patients. The present article reviews various interdisciplinary aspects of surface coatings on dental implants from the perspectives of biomaterials, coatings, drug release, and related therapeutic effects. Additionally, the various types of implant coatings, localized drug release from coatings, and how released agents influence the bone–implant surface interface characteristics are discussed. This paper also highlights several strategies for local drug delivery and their limitations in dental implant coatings as some of these concepts are yet to be applied in clinical settings due to the specific requirements of individual patients.

Influence of Dissolution Vessel Geometry and Dissolution Medium on In Vitro Dissolution Behaviour of Triamterene-Coated Model Stents in Different Test Setups

The aim of this study was to investigate if the geometry of the dissolution vessel, the dissolution medium volume and composition might contribute to the variation in drug release from drug-eluting stents (DES) in different test setups, which has been observed in previous in vitro studies. Therefore, DES containing triamterene as model substance were produced via fluidised-bed technology. Dissolution testing was carried out using different incubation setups, the reciprocating holder (USP Apparatus 7) and two flow-through methods, a method similar to the USP Apparatus 4 (FTC) and the vessel-simulating flow-through cell (vFTC) equipped with a hydrogel as a second compartment simulating the blood vessel wall. The results indicate that dissolution vessel geometry and medium volume had no influence on the release behaviour and only the flow-through cell methods yielded a lower dissolution rate than the incubation setups (80.6 ± 2.0% released in the FTC after 14 days compared to > 90% for all incubation setups). The composition of the hydrogel used in the vFTC also affected the dissolution rate (53.9 ± 4.5% within 14 days with a hydrogel based on phosphate-buffered saline compared to 78.2 ± 1.2% obtained with a hydrogel based on water) possibly due to different solubility of triamterene in the release media as well as interactions between the coating polymer and the release medium. Hence, the introduction of a hydrogel as a second compartment might lead to a more biorelevant test setup.

Coating and Pharmacokinetic Evaluation of Air Spray Coated Drug Coated Balloons

Drug coated balloons (DCB) are becoming the standard-care treatment for peripheral arterial disease (PAD). DCB use excipients to transfer and retain anti-proliferative drugs, such as paclitaxel. Excipients thus play a vital role in the design and function of DCB, however methods to coat balloons with excipients and anti-proliferative drugs remain unknown. The goal of this study was to thus develop an approach to coat and evaluate DCB for various excipients. An air sprayer method was developed to deposit paclitaxel and various excipients onto non-coated commercially available angioplasty balloons. The coating of the angioplasty balloons was evaluated for drug deposition and coating efficiency using high performance liquid chromatography tandem mass spectrometry. Drug transfer and retention of the coated angioplasty balloons into arterial segments were evaluated ex vivo using harvested pig arteries in a pulsatile flow bioreactor. The air sprayer method successfully delivered varying excipients including bovine serum albumin (BSA), urea and iohexol. The air spray method was configured to coat four angioplasty balloons simultaneously with paclitaxel and iohexol with an average paclitaxel load of 4.0 ± 0.70 µg/mm². The intra-day (within) and inter-day (between) coating precisions, defined as relative standard deviation (RSD), was 17.2 and 15.5%, respectively. Ex vivo deployment of iohexol-paclitaxel DCB yielded an arterial paclitaxel concentration of 123.4 ± 44.68 ng/mg (n = 3) at 1 h, 126.7 ± 25.27 ng/mg (n = 3) at 1 day, and 12.9 ± 12.88 ng/mg (n = 3) at 7 days. This work provides proof-of-concept of a quick, inexpensive approach to coat commercially available angioplasty balloons with paclitaxel and various excipients.

Stereocomplex Film Using Triblock Copolymers of Polylactide and Poly(ethylene glycol) Retain Paxlitaxel on Substrates by an Aqueous Inkjet System

The stereocomplex formation of poly(L,L-lactide) (PLLA) and poly(D,D-lactide) (PDLA) using an inkjet system was expanded to the amphiphilic copolymers, using poly(ethylene glycol) (PEG) as a hydrophilic polymer. The diblock copolymers, which are composed of PEG and PLLA (MPEG-co-PLLA) and PEG and PDLA (MPEG-co-PDLA), were employed for thin-film preparation using an aqueous inkjet system. The solvent and temperature conditions were optimized for the stereocomplex formation between MPEG-co-PLLA and MPEG-co- PDLA. As a result, the stereocomplex was adequately formed in acetonitrile/water (1:1, v/v) at 40 °C. The aqueous conditions improved the stereocomplex film preparation, which have suffered from clogging when using the organic solvents in previous work. The triblock copolymers, PLLA-co-PEG-co-PLLA and PDLA-co-PEG-co-PDLA, were employed for square patterning with the inkjet system, which produced thin films. The amphiphilic polymer film was able to retain hydrophobic compounds inside. The present result contributed to the rapid film preparation by inkjet, retaining drugs with difficult solubility in water, such as paclitaxel within the films.

Development of a tunable drug delivery system based on chitosan/polyacrylic acid polyelectrolyte multilayers from biodegradable scaffolds

Polyelectrolyte multilayer (PEM) films, established by layer-by-layer deposition, are attractive for drug delivery systems as drug delivery can be systematically controlled. However, one limitation of this coating method is the time-consuming character. The adsorption time of one layer typically takes minutes and commonly, investigated multilayers loaded with drugs contain more than 80 layers. Within our study we systematically evaluate parameters allowing the generation of a PEM-based drug delivery system on poly(L-lactide) (PLLA) with a reduced time consuming construction process and a controlled delivery of physiological amounts of active drugs. Therefore, several PEM based on PLLA with only five tetralayers consisting of polycation/polyanion/drug/polyanion are established and investigated regarding the release profiles of avidin fluorescein isothiocyanate as model substance and vascular endothelial growth factor (VEGF). We demonstrate that the polyelectrolyte nature and molecular weight of polyanions (hyaluronic acid, polyacrylic acid) and polycations (poly-L-lysine, chitosan) strongly influence the drug release profiles. Furthermore, in vitro cell culture studies show that the released VEGF amount and the cover layer from the favorite coating systems can beneficially enhance human umbilical vein endothelial cell (HUVEC) proliferation.

VEGF-releasing suture material for enhancement of vascularization: development, in vitro and in vivo study

As it has been demonstrated that bioactive substances can be delivered locally using coated surgical suture materials, the authors developed a vascular endothelial growth factor (VEGF)-releasing suture material that should promote vascularization and potentially wound healing. In this context, the study focused on the characterization of the developed suture material and the verification of its biological activity, as well as establishing a coating process that allows reproducible and stable coating of a commercially available polydioxanone suture material with poly(l-lactide) (PLLA) and 0.1μg and 1.0μg VEGF. The in vitro VEGF release kinetics was studied using a Sandwich ELISA. The biological activity of the released VEGF was investigated in vitro using human umbilical vein endothelial cells. The potential of the VEGF-releasing suture material was also studied in vivo 5days after implantation in the hind limb of Wistar rats, when the histological findings were analyzed. The essential results, enhanced cell viability in vitro as well as significantly increased vascularization in vivo, were achieved using PLLA/1.0μg VEGF-coated suture material. Furthermore, ELISA measurements revealed a high reproducibility of the VEGF release behavior. Based on the results achieved regarding the dose-effect relationship of VEGF, the stability during its processing and the release behavior, it can be predicted that a bioactive suture material would be successful in later in vivo studies. Therefore, this knowledge could be the basis for future studies, where bioactive substances with different modes of action are combined for targeted, overall enhancement of wound healing.

New stent surface materials: the impact of polymer-dependent interactions of human endothelial cells, smooth muscle cells, and platelets

Despite the development of new coronary stent technologies, in-stent restenosis and stent thrombosis are still clinically relevant. Interactions of blood and tissue cells with the implanted material may represent an important cause of these side effects. We hypothesize material-dependent interaction of blood and tissue cells. The aim of this study is accordingly to investigate the impact of vascular endothelial cells, smooth muscle cells and platelets with various biodegradable polymers to identify a stent coating or platform material that demonstrates excellent endothelial-cell-supportive and non-thrombogenic properties. Human umbilical venous endothelial cells, human coronary arterial endothelial cells and human coronary arterial smooth muscle cells were cultivated on the surfaces of two established biostable polymers used for drug-eluting stents, namely poly(ethylene-co-vinylacetate) (PEVA) and poly(butyl methacrylate) (PBMA). We compared these polymers to new biodegradable polyesters poly(l-lactide) (PLLA), poly(3-hydroxybutyrate) (P(3HB)), poly(4-hydroxybutyrate) (P(4HB)) and a polymeric blend of PLLA/P(4HB) in a ratio of 78/22% (w/w). Biocompatibility tests were performed under static and dynamic conditions. Measurement of cell proliferation, viability, glycocalix width, eNOS and PECAM-1 mRNA expression revealed strong material dependency among the six polymer samples investigated. Only the polymeric blend of PLLA/P(4HB) achieved excellent endothelial markers of biocompatibility. Data show that PLLA and P(4HB) tend to a more thrombotic response, whereas the polymer blend is characterized by a lower thrombotic potential. These data demonstrate material-dependent endothelialization, smooth muscle cell growth and thrombogenicity. Although polymers such as PEVA and PBMA are already commonly used for vascular implants, they did not sufficiently meet the criteria for biocompatibility. The investigated biodegradable polymeric blend PLLA/P(4HB) evidently represents a promising material for vascular stents and stent coatings.

Numerical modelling of the physical factors that affect mass transport in the vasculature at early time periods

Coronary artery disease results in blockages or narrowing of the artery lumen. Drug eluting stents were developed to replace bare metal stents in an effort to combat re-blocking of the lumen. A key element in determining the therapeutic success of a drug eluting stent is an in-depth understanding of the physical factors that affect mass transport of the drug into the arterial wall, over early time periods. The numerical models developed within this study focus on assessing the influence of a host of physical factors that either facilitate or impede therapeutic drug delivery into the arterial wall from the unit cell of an idealised stent. This study demonstrates that model reduction strategies to 2D and 1D can still adequately represent a 3D curved arterial wall and strut polymer coating, respectively, using an idealistic stent geometry. It was shown that the level of strut compression can have a significant impact on therapeutic drug delivery in the arterial wall.

In vitro study of dual drug-eluting stents with locally focused sirolimus and atorvastatin release

Within the context of novel stent designs we developed a dual drug-eluting stent (DDES) with an abluminally focussed release of the potent anti-proliferative drug sirolimus and a luminally focussed release of atorvastatin with stabilizing effect on atherosclerotic deposits and stimulating impact on endothelial function, both from biodegradable poly(L-lactide)-based stent coatings. With this concept we aim at simultaneous inhibition of in-stent restenosis as a result of disproportionally increased smooth muscle cell proliferation and migration as well as thrombosis due to failed or incomplete endothelialisation. The especially adapted spray-coating processes allowed the formation of smooth form-fit polymer coatings at the abluminal and luminal side with 70% respectively 90% of the drug/polymer solution being deposited at the intended stent surface. The impacts of tempering, sterilization, and layer composition on drug release are thoroughly discussed making use of a semi-empirical model. While tempering at 80 °C seems to be necessary for the achievement of adequate and sustained drug release, the coating sequence for DDES should be rather abluminal-luminal than luminal-abluminal, as reduction of the amount of sirolimus eluted luminally could then potentially minimize the provocation of endothelial dysfunction. In vitro proliferation and viability assays with smooth muscle and endothelial cells underline the high potential of the developed DDES.

Novel poly(L-lactide-co-ε-caprolactone) matrices obtained with the use of Zr[Acac]₄ as nontoxic initiator for long-term release of immunosuppressive drugs

Slowly degradable copolymers of L-lactide and ε-caprolactone can provide long-term delivery and may be interesting as alternative release systems of cyclosporine A (CyA) and rapamycin (sirolimus), in which available dosage forms cause a lot of side effects. The aim of this study was to obtain slowly degradable matrices containing immunosuppressive drug from PLACL initiated by nontoxic Zr[Acac]₄. Three kinds of poly(L-lactide-co-ε-caprolactone) (PLACL) matrices with different copolymer chain microstructure were used to compare the release process of cyclosporine A and rapamycine. The influence of copolymer chain microstructure on drug release rate and profile was also analyzed. The determined parameters could be used to tailor drug release by synthesis of demanded polymeric drug carrier. The studied copolymers were characterized at the beginning and during the degradation process of the polymeric matrices by NMR spectroscopy, GPC (gel permeation chromatography), and DSC (differential scanning calorimetry). Different drug release profiles have been observed from each kind of copolymer. The correlation between drug release process and changes of copolymer microstructure during degradation process was noticed. It was determined that different copolymer composition (e.g., lower amount of caprolactone units) does not have to influence the drug release, but even small changes in copolymer randomness affect this process.

Improving smooth muscle cell exposure to drugs from drug-eluting stents at early time points: a variable compression approach

The emergence of drug-eluting stents (DES) as a viable replacement for bare metal stenting has led to a significant decrease in the incidence of clinical restenosis. This is due to the transport of anti-restenotic drugs from within the polymer coating of a DES into the artery wall which arrests the cell cycle before restenosis can occur. The efficacy of DES is still under close scrutiny in the medical field as many issues regarding the effectiveness of DES drug transport in vivo still exist. One such issue, that has received less attention, is the limiting effect that stent strut compression has on the transport of drug species in the artery wall. Once the artery wall is compressed, the stents ability to transfer drug species into the arterial wall can be reduced. This leads to a reduction in the spatial therapeutic transfer of drug species to binding sites within the arterial wall. This paper investigates the concept of idealised variable compression as a means of demonstrating how such a stent design approach could improve the spatial delivery of drug species in the arterial wall. The study focused on assessing how the trends in concentration levels changed as a result of artery wall compression. Five idealised stent designs were created with a combination of thick struts that provide the necessary compression to restore luminal patency and thin uncompressive struts that improve the transport of drugs therein. By conducting numerical simulations of diffusive mass transport, this study found that the use of uncompressive struts results in a more uniform spatial distribution of drug species in the arterial wall.

Development andIn VitroCharacterization of Hyaluronic Acid-Based Coatings for Implant-Associated Local Drug Delivery Systems

The development of drug-eluting coatings based on hyaluronic acid (HA) is especially promising for implant-associated local drug delivery (LDD) systems, whose implantation provokes high insertion forces, as, for instance, cochlear implants or drug-coated balloons (DCB). The lubricious character of HA can then reduce the coefficient of friction and serve as drug reservoir simultaneously. In this context, we investigated several plasma- and wet-chemical methods for the deposition of HA-based coatings with LDD function on polyamide 12 as a model implant surface, conventionally used for DCB. In contrast to aminosilane, epoxy silane surface layers allowed the covalent attachment of a smooth and uniform HA base layer, which provided good adherence of further HA layers deposited by manual dip coating at a subsequent processing stage. The applied HA-crosslinking procedure during dip coating influences the transfer and release of paclitaxel, which could be reproducibly incorporated via infiltration. While crosslinking with N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride provided HA coatings on DCB, which allowed for an efficient paclitaxel transfer upon expansion in a vessel model, crosslinking with glutardialdehyde resulted in a slower drug release being more appropriate for implants with longer residence time in the body. The developed HA coating is hence well suited for spontaneous and sustained LDD.

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-L-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.

Development of a specially tailored local drug delivery system for the prevention of fibrosis after insertion of cochlear implants into the inner ear

A cochlear implant (CI)-associated local drug delivery system based on dexamethasone (DMS) was developed with the purpose to inhibit the growth of fibrotic tissue which influences the signal transmission from the CI to the neurons of the inner ear. For the realization of a targeted DMS delivery the following concepts were combined: modification of the silicone-based electrode carrier by incorporation of DMS and a DMS-containing polymeric coating chemically attached on the surface of the electrode carrier. It was demonstrated that the coated CI showed a high coating stability in a simulated implantation procedure. The in vitro drug release studies in a quasi-stationary model revealed a faster DMS release in the initial phase originating from the DMS-containing coatings and then a lower and sustained DMS release originating from the DMS-loaded silicone carrier. The performed in vitro biocompatibility study confirmed that the released DMS was non-toxic for cultured spiral ganglion cells.

Enhanced visualization of biodegradable polymeric vascular scaffolds by incorporation of gold, silver and magnetite nanoparticles

Due to improved tissue regeneration and the enabling of post-operative minimally invasive interventions in the same vessel segment, biodegradable polymeric scaffolds represent a competitive approach to permanent metallic stents in vascular applications. Despite these advantages some challenges, such as the improvement of the scaffold mechanics and enhancement of scaffold visibility during the implantation procedure, are persisting. Therefore, the scope of our studies was to investigate the potential of gold, silver and magnetite nanoparticles incorporated in a polymeric blend of poly(L-lactide)/poly(4-hydroxybutyrate) for image enhancement in X-ray, magnetic resonance or near-infrared imaging. Their impact on mechanical properties of such modified scaffold materials was also evaluated.

In vitro and in vivo characterisation of biodegradable polymer-based drug-eluting stent

AIMS

The objective of this study was to investigate the structural integrity and early vascular response of a polylactic acid-coated (i.e., biodegradable polymer) coronary drug-eluting stent (DES) (BioMatrix™; Biosensors International, Singapore) to three currently marketed FDA/CE- mark approved non-erodible polymer-coated DES in a porcine model.

METHODS AND RESULTS

BioMatrix™, XIENCE V™ (Abbott Vascular, Santa Clara, CA, USA), TAXUS® Liberté™ (Boston Scientific, Natick, MA, USA), and Cypher SELECT™ (Cordis, Johnson & Johnson, Miami, FL, USA) stents were implanted in pig coronaries for seven days. Polymer integrity was assessed by scanning electron microscopy (SEM) following tissue digestion. In vitro expansion of the BioMatrix™ was also performed. SEM analysis of in vivo stents demonstrated polymer defects on the abluminal surface of all DES including polymer cracking (BioMatrix™), bridging (TAXUS Liberté™), round-small defects (Cypher SELECT™), and flaking (XIENCE V™). Histologically, the myocardium revealed no evidence of acute myocardial infarction or microscopic scarring, moreover all intramyocardial vessels were found to be patent with no evidence of emboli. In vitro results demonstrated greater BioMatrix™ polymer cracking and lifting.

CONCLUSIONS

These results illustrate the presence of polymer defects in all DES (TAXUS Liberté™, Cypher SELECT™, XIENCE V™, BioMatrix™) implanted seven-days in pigs, with absence of myocardial damage in this small number of samples. Polymer coating irregularity was greater in BioMatrix™ stent expanded in vitro as compared to in vivo, suggesting simulated benchtop deployment induces greater damage to the biodegradable polymer coating than in vivo deployment in healthy porcine coronary arteries.

In-vitro dissolution methods for controlled release parenterals and their applicability to drug-eluting stent testing

Objectives

Dissolution testing is a powerful tool for the characterization of dosage form performance in vitro under standardized conditions. In spite of the increasing number of parenterally administered medicinal products, currently there are no compendial dissolution test methods designed especially for these types of dosage forms. In addition to classical drug delivery systems, drug/device combination products, such as drug-eluting stents, are being used increasingly.

Key findings

This review describes the current methods that are used most often for in-vitro dissolution testing of parenteral dosage forms, i.e. the ‘sample and separate’ methods, the ‘dialysis’ methods, and the ‘flow-through’ methods, with a special emphasis on whether these methods can be used for drug-eluting stent testing. In the light of current regulatory requirements and with the exploding costs of preclinical and clinical development, test systems that include biorelevant parameters and are predictive of in-vivo performance are increasingly important. Published attempts to take biorelevant conditions into consideration in the design of dissolution test apparatus developed for parenteral dosage forms, including a method that was designed to emulate the embedding and flow-conditions at the site of stent implantation, have been outlined in this review.

Summary

In spite of the large quantity of highly potent controlled release parenteral products marketed today, there is still a lack of suitable methods for in vitro dissolution testing for these dosage forms especially with regard to biorelevant testing conditions. For dosage forms implanted into tissues it seems of major importance to reproduce the transport forces which are predominant in vivo (diffusive versus convective) in the in-vitro experimental setup.

Current requirements for polymeric biomaterials in otolaryngology

In recent years otolaryngology was strongly influenced by newly developed implants which are based on both, innovative biomaterials and novel implant technologies. Since the biomaterials are integrated into biological systems they have to fulfill all technical requirements and accommodate biological interactions. Technical functionality relating to implant specific mechanical properties, a sufficiently high stability in terms of physiological conditions, and good biocompatibility are the demands with regard to suitability of biomaterials. The goal in applying biomaterials for implants is to maintain biofunctionality over extended periods of time. These general demands to biomaterials are equally valid for use in otolaryngology. Different classes of materials can be utilized as biomaterials. Metals belong to the oldest biomaterials. In addition, alloys, ceramics, inorganic glasses and composites have been tested successfully. Furthermore, natural and synthetic polymers are widely used materials, which will be in the focus of the current article with regard to their properties and usage as cochlear implants, osteosynthesis implants, stents, and matrices for tissue engineering. Due to their application as permanent or temporary implants materials are differentiated into biostable and biodegradable polymers. The here identified general and up to date requirements for biomaterials and the illustrated applications in otolaryngology emphasize ongoing research efforts in this area and at the same time demonstrate the high significance of interdisciplinary cooperation between natural sciences, engineering, and medical sciences.

Examination of drug release and distribution from drug-eluting stents with a vessel-simulating flow-through cell

The recently introduced vessel-simulating flow-through cell offers new possibilities to examine the release from drug-eluting stents in vitro. In comparison with standard dissolution methods, the additional compartment allows for the examination of distribution processes and creates dissolution conditions which simulate the physiological situation at the site of implantation. It was shown previously that these conditions have a distinct influence on the release rate from the stent coating. In this work, different preparation techniques were developed to examine the spatial distribution within the compartment simulating the vessel wall. These methods allowed for the examination of diffusion depth and the distribution resulting in the innermost layer of the compartment simulating the vessel wall. Furthermore, the in vitro release and distribution examined experimentally were modelled mathematically using finite element (FE) methods to gain further insight into the release and distribution behaviour. The FE modelling employing the experimentally determined diffusion coefficients yielded a good general description of the experimental data. The results of the modelling also provided important indications that inhomogeneous coating layer thicknesses around the strut may result from the coating process which influence release and distribution behaviour. Taken together, the vessel-simulating flow-through cell in combination with FE modelling represents a unique method to analyse drug release and distribution from drug-eluting stents in vitro with particular opportunities regarding the examination of spatial distributions within the vessel-simulating compartment.