Biologic Response to Polymer-coated Stents: In Vitro Analysis and Results in an Iliac Artery Sheep Model

In vivo evaluation of safety and performance of a tapered nitinol venous stent with inclined proximal end in an ovine iliac venous model

A tapered stent with inclined proximal end is designed for fitting the iliac anatomically. The aim of the present study was to evaluate the safety and performance of the new stent in ovine left iliac veins. The experiment was performed in 30 adult sheep, and one nitinol-based VENA-BT® iliac venous stent (KYD stent) was implanted into each animal's left common iliac vein. Follow-up in all sheep consisted of angiographic, macroscopic, and microscopic examinations at Day 0 (< 24 h), Day 30, Day 90, Day 180 and Day 360 post-stenting (six animals per each time-point). 30 healthy ~ 50 kg sheep were included in this study and randomly divided into five groups according to the follow-up timepoint. All stents were implanted successfully into the left ovine common iliac vein. No significant migration occurred at follow-up. There is no statistically significant difference between the groups (p > 0.05), indicating no serious lumen loss occurred during the follow-up period. Common iliac venous pressure was further measured and the results further indicated the lumen patency at follow-up. Histological examinations indicated that no vessel injury and wall rupture, stent damage, and luminal thrombus occurred. There was moderate inflammatory cell infiltration around the stent in Day-0 and Day-30 groups with the average inflammation score of 2.278 and 2.167, respectively. The inflammatory reaction was significantly reduced in Day-90, Day-180 and Day-360 groups and the average inflammation scores were 0.9444 (p < 0.001, Day-90 vs Day-0), 1.167 (p < 0.001, Day-180 vs Day-0) and 0.667 (p < 0.001, Day-90 vs Day-0), respectively. The microscopic examinations found that the stents were well covered by endothelial cells in all follow-up time points. The results suggested that the KYD stent is feasible and safe in animal model. Future clinical studies may be required to further evaluate its safety and efficacy.

Clickable and Photo-Erasable Surface Functionalities by Using Vapor-Deposited Polymer Coatings

A prospective design for interface properties is enabled to perform precise functionalization, erasure capability for existing properties, reactivation of surface functionality to a second divergent property. A vapor-deposited, 2-nitro-5-(prop-2-yn-1-yloxy)methylbenzyl carbamate-functionalized poly-para-xylylene coating is synthesized in this study to realize such tasks by offering the accessibility of the azide/alkyne click reaction, an integrated photochemical decomposition/cleavage moiety, and the reactivation sites of amines behind the cleavage that allow the installation of a second surface function. With the benefits from the mild processing conditions used for the coatings and the rapid response of the photochemical reaction, the creation of sophisticated interface properties and localized chemical compositions was elegantly demonstrated with a hybrid functionality including a confined hydrophlic/hydrophobic wetting property and/or a cell adherent/repellent platform on such a coating surface.

Enhanced bone morphogenic property of parylene-C

The ability to induce osteointegration was introduced to a parylene-C surface via the simple and intuitive process of protein adsorption mediated by hydrophobic interactions. In this way, bone morphogenetic protein (BMP)-2, fibronectin, and platelet-rich plasma (PRP) could be immobilized on parylene-C surfaces. This approach alleviates concerns related to the use of potentially harmful substances in parylene-C modification processes. The adsorbed protein molecules were quantitatively characterized with respect to adsorption efficacy and binding affinity, and the important biological activities of the proteins were also examined using both early and late markers of osteogenetic activity, including alkaline phosphatase expression, calcium mineralization and marker gene expression. Additionally, the adsorbed PRP exhibited potential as a substitute for expensive recombinant growth factors by effectively inducing comparable osteogenetic activity. In addition to the excellent biocompatibility of parylene-C and its ability to coat a wide variety of substrate materials, the modification of parylene-C via protein adsorption provides unlimited possibilities for installing specific biological functions, expanding the potential applications of this material to include various biointerface platforms.

Vapor sublimation and deposition to build porous particles and composites

The vapor deposition of polymers on regular stationary substrates is widely known to form uniform thin films. Here we report porous polymer particles with sizes controllable down to the nanometer scale can be produced using a fabrication process based on chemical vapor deposition (CVD) on a dynamic substrate, i.e., sublimating ice particles. The results indicate that the vapor deposition of a polymer is directed by the sublimation process; instead of forming a thin film polymer, the deposited polymers replicated the size and shape of the ice particle. Defined size and porosity of the polymer particles are controllable with respect to varying the processing time. Extendable applications are shown to install multiple functional sites on the particles in one step and to localize metals/oxides forming composite particles. In addition, one fabrication cycle requires approximately 60 min to complete, and potential scaling up the production of the porous particles is manageable.

Fabrication of Functional Polymer Structures through Bottom-Up Selective Vapor Deposition from Bottom-Up Conductive Templates

An electrically induced bottom-up process was introduced for the fabrication of multifunctional nanostructures of polymers. Without requiring complicated photolithography or printing techniques, the fabrication process first produced a conducting template by colloidal lithography to create an interconnected conduction pathway. By supplying an electrical charge to the conducting network, the conducting areas were enabled with a highly energized surface that generally deactivated the adsorbed reactive species and inhibited the vapor deposition of poly- p-xylylene polymers. However, the template allowed the deposition of ordered poly- p-xylylene nanostructures only on the confined and negative areas of the conducting template, in a relatively large centimeter-scale production. The wide selection of functionality and multifunctional capability of poly- p-xylylenes naturally rendered the synergistic and orthogonal chemical reactivity of the resulting nanostructures. With only a few steps, the construction of a nanometer topology with the functionalization of multiple chemical conducts can be achieved, and the selected deposition process represents a state-of-the-art nanostructure fabrication in a simple and versatile approach from the bottom up.

Development of Antifouling Hyperbranched Polyglycerol Layers on Hydroxyl Poly-p-xylylene Coatings

Antifouling surfaces that are resistant to protein adsorption and cell adhesion are desirable for many biomedical devices, such as diagnostic devices, biosensors, and implants. In this study, we developed an antifouling hyperbranched polyglycerol (hPG) surface on hydroxyl poly-p-xylylene (PPX-OH). PPX-OH was deposited via chemical vapor deposition (CVD), and an hPG film was then developed via the ring-opening reaction of glycidol. The hPG film greatly reduced the adhesion of L929 cells and platelets as well as protein adsorption. The addition of alkenyl groups in the hPG layer allows the conjugation of biomolecules, such as peptides and biotin, and elicits specific biological interactions. Since the CVD deposition of PPX-OH could be applied to most types of materials, our approach makes it possible to decorate an antifouling hPG film on most types of materials. Our method could be applied to biosensors, diagnostics, and biomedical devices in the future.

Non-enhanced magnetic resonance angiography can evaluate restenosis after carotid artery stenting with the Carotid Wallstent

BACKGROUND

Carotid artery stenting (CAS) requires follow-up imaging to assess in-stent restenosis (ISR). This study aimed to determine whether non-enhanced magnetic resonance angiography (NE-MRA) is useful for evaluating ISR.

METHOD

Between 2009 and 2013, we performed 118 consecutive CAS procedures using the Precise stent (n = 78) and the Carotid Wallstent (n = 40). We reviewed 1.5 T NE-MRA and examined visualization of the stent lumen and the degree of ISR if present. Other imaging modalities were used as references.

RESULTS

NE-MRA performed just after CAS was not able to visualize the stent lumen in all patients because of metal artifacts. In the Carotid Wallstent group, follow-up NE-MRA was available in 22 patients. The stent lumen was visible more than three months after CAS in all patients. Among them, >40 % ISR was observed by other modalities in eight lesions. The degree of restenosis measured by NE-MRA (y%) had a linear relationship with that measured by conventional angiography (x%) (y = 0.97x-0.4, r = 0.79, P = 0.021). In one case among 17 without ISR (6 %), NE-MRA showed false ISR. In the Precise stent group, NE-MRA did not visualize the stent lumen in the follow-up period.

CONCLUSIONS

NE-MRA can visualize the stent lumen in the Carotid Wallstent more than three months after CAS, but not in the Precise stent at follow-up. This delayed visualization might depend on endothelialization of the stent lumen. The degree of ISR measured by NE-MRA is comparable to that by conventional angiography. NE-MRA can evaluate ISR after CAS with the Carotid Wallstent (100 % sensitivity and 94 % specificity).

Automatic Detection of Arrow Annotation Overlays in Biomedical Images

Images in biomedical articles are often referenced for clinical decision support, educational purposes, and medical research. Authors-marked annotations such as text labels and symbols overlaid on these images are used to highlight regions of interest which are then referenced in the caption text or figure citations in the articles. Detecting and recognizing such symbols is valuable for improving biomedical information retrieval. In this research, image processing and computational intelligence methods are integrated for object segmentation and discrimination and applied to the problem of detecting arrows on these images. Evolving Artificial Neural Networks (EANNs) and Evolving Artificial Neural Network Ensembles (EANNEs) computational intelligence-based algorithms are developed to recognize overlays, specifically arrows, in medical images. For these discrimination techniques, EANNs use particle swarm optimization and genetic algorithm for artificial neural network (ANN) training, and EANNEs utilize the number of ANNs generated in an ensemble and negative correlation learning for neural network training based on averaging and Linear Vector Quantization (LVQ) winner-take-all approaches. Experiments performed on medical images from the imageCLEFmed’08 data set, yielded area under the receiver operating characteristic curve and precision/recall results as high as 0.988 and 0.928/0.973, respectively, using the EANNEs method with the winner-take-all approach.

Cellular transduction gradients via vapor-deposited polymer coatings

Spatiotemporal control of gene delivery, particularly signaling gradients, via biomaterials poses significant challenges because of the lack of efficient delivery systems for therapeutic proteins and genes. This challenge was addressed by using chemical vapor deposition (CVD) polymerization in a counterflow set-up to deposit copolymers bearing two reactive chemical gradients. FTIR spectroscopy verified the formation of compositional gradients. Adenovirus expressing a reporter gene was biotinylated and immobilized using the VBABM method (virus-biotin-avidin-biotin-materials). Sandwich ELISA confirmed selective attachment of biotinylated adenovirus onto copolymer gradients. When cultured on the adenovirus gradients, human gingival fibroblasts exhibited asymmetric transduction with full confluency. Importantly, gradient transduction occurred in both lateral directions, thus enabling more advanced delivery studies that involve gradients of multiple therapeutic genes.

Designable biointerfaces using vapor-based reactive polymers

Functionalized poly(p-xylylenes) constitute a versatile class of reactive polymers that can be prepared in a solventless process via chemical vapor deposition (CVD) polymerization. The resulting ultrathin coatings are typically pinhole-free and can be conformally deposited onto a wide range of substrates and materials. More importantly, appropriately selected functional groups can serve as anchoring sites for tailoring biointerface properties via the immobilization of biomolecules. In this article, controlled surface chemistries are outlined that use functionalized poly(p-xylylenes) as reactive coatings, including alkyne-functionalized coatings for Huisgen 1,3-dipolar cycloaddition reactions or aldehyde-functionalized coatings. The reactive coatings technology provides flexible access to a range of different surface chemistries, enabling a broad range of potential applications in microfluidics, medical device coatings, and biotechnology. In this feature article, we will highlight recent progress in vapor-based reactive coatings and will discuss potential benefits and current limitations.

Surface patterning strategies for microfluidic applications based on functionalized poly-p-xylylenes

Microfluidic systems require precise surface modification in order to tailor the interfacial properties. For instance, in lab-on-a-chip research, defined surface chemistry is key to minimizing contamination and to increasing signal-to-noise ratios for bioconjugation schemes. Device efficiency and analytical output can also be maximized with devices that have defined surfaces. Similarly, minimizing biofouling is also crucial to suppress background noise and ensure device functions. Once defined, surface properties have been engineered, microstructuring of surfaces can provide defined microenvironments for cell-based culture systems. In this report, we highlight the use of functionalized poly-p-xylylenes for surface modification with a specific focus on microfluidic systems. Functionalized poly-p-xylylenes constitute a versatile group of reactive coatings that can provide a defined chemical makeup of substrate surfaces irrespective of underlying bulk material properties. Recent advances using reactive coatings for surface modification of microfluidics are introduced, including use as nonfouling coatings, fabrication of patterned surfaces, functionalization of previously assembled devices, as well as device-bonding applications.

Granulation encapsulated stent: a new therapeutic approach for vascular implantation

OBJECTIVES

To determine if stents could be encapsulated with immunocompatible granulation tissue for the treatment of vascular diseases.

METHODS

Bare metal stents were implanted in New Zealand white rabbits so they would be encapsulated with immunocompatible granulation tissue. The granulation encapsulated stents (GES) were then treated with either mitomycin C or saline, and implanted into rabbit iliac arteries for 4 weeks. To test whether the effect of mitomycin C was retained, we co-cultured smooth muscle cells for 3 h with subcutaneous tissue (as control) or with granulation tissue from GES treated with mitomycin C and saline.

RESULTS

Vessels with GES treated with mitomycin C (MS) and washed with saline had significantly less neointimal area (NA) after 4 weeks (0.27 (SD 0.03) mm(2) than vessels containing bare metal stents (B) (1.15 (SD 0.10) mm(2), n = 5, p<0.05) or GES treated with saline (S) (4.78 (SD 0.72) mm(2), n = 5, p<0.05). The average vessel injury score was not significantly different among these three groups (S: 1.98 (SD 0.51), MS: 1.46 (SD 0.18) and B: 1.51 (SD 0.32)). GES treated with saline had significantly less NA than the other two groups and also blocked blood flow in the contralateral iliac artery in the abdominal aortic bifurcation immediately after implantation and 4 weeks later. Histology also showed neointimal overgrowth in the vessel wall over the contralateral iliac artery.

CONCLUSIONS

GES treated with mitomycin C can significantly inhibit neointimal formation in rabbit arteries due to the formation of granulation tissue. GES treated with saline demonstrated significantly increased NA and resisted normal rabbit artery pressures.

In Vivo Evaluation of Patency and In-Stent Stenoses After Implantation of Nitinol Stents in Iliac Arteries Using MR Angiography

OBJECTIVE

Our study was a prospective in vivo study to evaluate whether MR angiography is suitable for assessing stent patency and grading in-stent stenoses and to examine whether the accuracy of MR angiography changes with time after stent implantation.

SUBJECTS AND METHODS

In a prospective study, 34 iliac stenoses in 27 patients were treated by implantation of 35 nitinol stents. MR angiography was performed immediately after stent placement for 32 stents, and both digital subtraction angiography (DSA) and MR angiography were repeated at the 6-month follow-up for 23 stents. Three blinded observers assessed stent patency and the degree of in-stent stenoses on MR angiography and DSA (the standard of reference) images. The difference between the observers' grading of stenoses on DSA and on MR angiography was determined. Statistical analysis was performed using the Student's t test for paired samples.

RESULTS

Stent patency was assessed correctly for all stents and both sets of MR angiography images. Evaluation of DSA 1 images (obtained at end of implantation procedure) revealed that 96.9% of in-stent stenoses were less than 50%. On DSA 2 images (obtained at follow-up), 95.7% of in-stent stenoses were graded as less than 50%. The difference between grading of stenoses on DSA and MR angiography images was 15.0% +/- 16.0% (minimum, 0.0%; maximum, 63.3%) for DSA 1 versus MR angiography 1 (statistically significant, p = 0.037) and 9.8% +/- 13.5% (minimum, 0.0%; maximum, 63.3%) for MR angiography 2 versus DSA 2 (not statistically significant, p = 0.355).

CONCLUSION

Patency was correctly assessed for all stents on MR angiography. The quality of MR angiography regarding characterization of in-stent stenoses improved with time after stent placement. However, discrepancies of more than 60% between grading of lumen narrowing on DSA and MR angiography images occurred even at the 6-month follow-up. Thus, MR angiography is not yet a reliable technique for characterization of in-stent stenoses.

In Vivo Evaluation of the Carotid Wallstent on Three-dimensional Contrast Material–enhanced MR Angiography: Influence of Artifacts on the Visibility of Stent Lumina

PURPOSE

Contrast material-enhanced magnetic resonance (MR) angiography is increasingly used in postinterventional imaging after implantation of endovascular stents. The main limitations are stent-related artifacts compromising the visibility of the stent lumen. The aim of this in vivo study is the evaluation of contrast-enhanced MR angiography imaging characteristics of the carotid Wallstent.

MATERIALS AND METHODS

The carotid arteries of 29 patients were examined with contrast-enhanced MR angiography 3-6 days and/or 7-23 months after implantation of a carotid Wallstent into the internal carotid artery. Images were evaluated with regard to the diameter and signal intensity (SI) of the visible stent lumen. Digital subtraction angiography (DSA) was used as the standard of reference.

RESULTS

Stent-related artifacts on contrast-enhanced MR angiography caused an artificial lumen narrowing and a reduction of the SI within the stent. Artifacts were pronounced on imaging 3-6 days after stent implantation, but 68% of stents imaged 7-23 months after stent implantation presented with a significantly decreased artificial signal reduction and an improved visibility of the stent lumen.

CONCLUSIONS

The results of this study indicate that a reliable evaluation of the stent lumen is limited as a result of an artificial decrease of the SI inside the stent. However, in follow-up examinations 7-23 months after stent implantation, visibility of the stent lumen was improved and diagnostic reliability of contrast-enhanced MR angiography was markedly increased. A probable explanation for this phenomenon might be the formation of a neointimal layer covering the stent struts and thereby reducing stent-related artifacts.