Stent silicone

In parallel to catheter-based delivery, stent-based approaches, such as passive stent coatings (diamond-like carbon, phosphorylcholine, and silicon carbide coatings) and immobilized drug coatings (heparin-coated stents), were evaluated for their ability to inhibit restenosis. Although animal studies demonstrated some promise, none of these technologies were clinically successful for restenosis prevention. The failure of these surface modification technologies further added to the need for the development of DES based on the principles of sustained CDD,... 

C. Harder, A. Rzany, and M. Schaldach, Coating of vascular stents with antithrom-bogenic amorphous silicon carbide, Prog. Biomed. Res., 1, 71 (1999). 

Silicon carbide was an early approach to the high levels of restenosis caused by BMS. The idea was that interaction between fibrinogen and the stent surface would lead to fibrin deposition, leading to platelet adhesion and thrombus formation thus adding a semiconductor may decrease these electron interactions. [23] Unfortunately the clinical studies of stents coated by amorphous, phosphorous-doped, hydrogen rich silicon carbide showed no difference in restenosis rate when compared to the typical BMS [24]. 

Heublein, B., Pethig, K., Ozbek, C., Elsayed, M., Bolz, A., Schaldach, M. Silicon carbide coating-a new hybrid design of coronary stents. Prog. Biomed. Res. 1, 33-39 (1998)... 

Ozbek, C., Heisel, A., Gross, B., Bay, W., and Schieffer, H., Coronary implantation of silicone-carbide-coated Palmaz-Schatz stents in patients with high risk of stent thrombosis without oral anticoagulation, Cathet. Car-diovasc. Diagn., 1997 41(l) 71-78. 

Parylene is used as a coating on electronics ranging from advanced military and aerospace electronics to general-purpose industrial products, medical devices ranging from silicone tubes to advanced coronary stents, synthetic rubber products ranging from medical grade silicone rubber to ethylene propylene diene monomer rubber (EPDM). 

Dumon (Dumon 1990) introduced another silicone tube stent, which is available in different lengths and diameters and comes with external protrusions for better fixation. The Dumon stent (Novatech, Aub-agne, France) has become the most commonly used silicone stent. This stent requires a rigid bronchoscope and a special introducer system for placement. Because so much clinical experience (Cavaliere et al. 1996 Dumon et al. 1996) has been gained with this stent it has become the gold standard for most bronchoscopists. Therefore, newer stent designs must be measured in comparison to this stent. 

Recent developments of plastic stents have aimed to improve the resistance of plastic stents to external compression forces. Therefore, metal has been incorporated into the plastic material of the stent. One of the latest developments is the dynamic bifurcation stent made of silicone (Freitag et al. 1994). This Dynamic stent (Riisch, Kernen, Germany) is reinforced with horseshoe-shaped steel struts. A posteriorly located flexible membrane allows dynamic compression of the stent during coughing, whereas the steel struts prevent airway compression from external forces. Theoretically, this stent mimics the mechanical dynamics of the normal trachea. The distal end is a Y shape which rides on the carina to prevent distal migration. 

Finally, a silicone stent with self-expanding forces has been created. The Polyflex stent (Riisch) is made of a polyester wire with a thin layer of silicone. The polyester wires are woven like metalfic mesh-work stents in order to create self-expanding forces of this synthetic polyester stent (Wassermann et al. 1997 Bolliger et al. 1999). 

Wallstents are available with silicone and polyurethane covering. Stents can be ordered with complete covering, and with uncovered proximal and distal ends for better mucosal fixation. 

The series from Brescia, Italy reported by Cavaliere et al. comprises 306 patients and placement of 393 silicone stents (Cavaliere et al. 1996). The vast majority of patients were treated with 363 Dumon stents in 27 cases a Y-shaped Hood stent was placed, and in another four cases a Y-shaped Dynamic stent. Stents were inserted into the trachea in 38.7%, right main stem bronchus and bronchus intermedius in 19.8%, left main stem bronchus in 15.5%, trachea and right or left main stem bronchus in 13.5%, and in other positions in 12.5%. Stent insertion resulted in improvement in pulmonary function tests and quality of life in all but six patients (98% success rate). The median survival of patients was 108 days. All patients had malignant tracheobronchial tumors. 

The group of Colt et al. (1992) were the first to report double stents for carcinoma of the esophagus invading the tracheobronchial tree. Ten patients with severe dyspnea at rest due to airway obstruction ( =5) or esophagorespiratory fistulas (n=5) had tracheal (n=5) or bronchial (n=5) stents placed additionally to the esophageal stent. The tracheobronchial stents were Dumon silicone stents. The fistulas were sealed in all cases. The mean survival time was 121 days (range, 12-350 days). 

Interaction of the metalhc stent and the tracheobronchial wall is expected unlike plastic tube stents. This leads to specific problems. Removal of a metalhc stent, which is incorporated into the mucosa several weeks after deployment is extremely difficult and sometimes requires laser destruction of the stent struts in order to remove the stent piece by piece . Similarly, repositioning of an embedded metal stent is more difficult than relocation of a silicone stent. Covered metal stents exert less problems regarding removal and repositioning than uncovered mesh stents, where the open mesh design can lead to complete inoculation of the small stent wires into the mucosa. 

Bolliger CT, Heitz M, Hauser R, Probst R, Perruchoud AP (1996) An airway Wallstent for the treatment of tracheobronchial malignancies. Thorax 51 1127-1129 Bolliger CT, Wyser C, Wu X et al. (1999) Evaluation of a new self-expandable silicone stent in an experimental tracheal stenosis. Chest 115 496-501... 

Wassermann K, Ifoch A, Miiller-Ehmsen J et al. (1997) Clinical and laboratory evaluation of a new thin-walled selfexpanding tracheobronchial silicone stent progress and pitfalls. J Thorac Cardiovasc Surg 114 527-534... 

Westaby S, Jackson JW, Pearson FG (1982) A bifurcated silicone rubber stent for relief of tracheobronchial obstruction. J Thorac Cardio Surg 38 200-205... 

Using medical devices such as catheters, angioplasty balloons, pacemaker leads, and cardiovascular stents usually involves insertion of such devices into urinal tracts or blood vessels. During insertion, high surface lubricity of Ihe devices helps to facilitate the insertion process and reduce insertion-associated tissue damage, which benefits both the patient and the surgeon. Early approaches to decrease insertion friction involve using lubricants such as olive oil and silicon oil, or low friction materials like polyethylene. Due to the superior mechanical properties, polyurethane is extensively used in catheters/baUoons and it is desirable to improve the lubricity of polyurethane materials. 

The various tracheal substitutes and techniques of reconstmction were analyzed by GriUo [12], who classified them in five categories foreign materials (silicone tubes [103,104], coated stents, metallic [104,105], and other solid prostheses [106]), nonvi-able tissues, autogenous tissues, TE, and tracheal transplantation. 

Tubular PGA fiber scaffolds, seeded with chondrocytes and implanted into sternohyoid muscle for 4 weeks, have been used, with a silicon tube as stent (removed after 2 months). Six of 10 animals, implanted with vascularization, survived more than 6 months, whereas all the animals in the control group (without vascularization) died within 2 months after reconstruction, due to mucus impaction. Six months after implantation, vascularized constructs retained structures and features of cartUage-like tissue and developed a continuous cihated columnar epithehum layer, suggesting the importance of the prevascularization for the development of a suitable airway graft [126]. A copolymer of L-lactide and e-caprolactone sponge tube reinforced by PGA was implanted into sheep. A silicone stent (7 cm in length) was placed perioperatively to prevent graft collapse. After 9 months, only stent had positive outcomes, even if a complete and spontaneous reconnection of the native trachea was not observed [127]. 

Concerning the therapeutic relevance of diagnosing TBM/EDAC, a short-term follow-up study of patients with severe TBM/EDAC undergoing central airway stabilization with silicone stents concluded that the intervention markedly improved dyspnea, health-related quality of life, and functional status in these patients (Ernst et al. 2007). 

Ernst A, Majid A, Feller-Kopman D etal. (2007) Airway stabilization with silicone stents for treating adult tracheobronchomalacia a prospective observational study. Chest 132 609-616... 

Nondegradable silicone stents are also commercially available. They can be removed more easily than metallic ones, but the large thickness of the tube can cause obstruction. Saito et al. [22] compared the biocompatibility and suitability of a tubular bioabsorbable knitted stent made of PLLA and a conventional silicone stent in normal rabbit airways. Three out of eight rabbits in the silicone stent group died within 4 weeks of implantation as a result of airway obstruction by secretions inside the stent lumen, whereas none of the rabbits in the PLLA stent group died from airway complications. 

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