**3. Current stent biomaterials**

460 Biomaterials – Physics and Chemistry

Fig. 1. The processes following stent insertion that lead to bacterial adhesion, infection, and encrustation. Following stent insertion, urinary components deposit on the surface, forming anchor points for both bacteria as well as ions/minerals. Subsequent bacterial colonization and proliferation leads to the formation of a resistant biofilm that leads to subsequent infection. In addition the interaction of ions/minerals with conditioning film components and bacterial induced crystallization will facilitate encrustation of the device. Inhibition of the common step of conditioning film deposition is a potential step in preventing patient

Stents have been associated with increased morbidity causing infection,(Riedl, Plas et al. 1999) encrustation,(Paick, Park et al. 2003) hematuria,(Damiano, Oliva et al. 2002; Joshi, Newns et al. 2003; Joshi, Chitale et al. 2005) and discomfort (Joshi, Stainthorpe et al. 2001; Joshi, Okeke et al. 2002). The Ureteral Stent Symptom Questionnaire (USSQ) looks at different facets of life including sexual function which has been shown to be negatively affected by the presence of a stent (Sighinolfi, Micali et al. 2007). In fact, ureteroscopes, intracorporeal lithotriptors, and ureteroscopic techniques have improved to the point that the major morbidity of ureteroscopy has become the stent left *in situ* postoperatively*.*  Studies evaluating differences in postoperative complications and stone-free rates in stented compared to non-stented patients have shown that stents are not a routine requirement following uncomplicated ureteroscopy (Hosking, McColm et al. 1999; Borboroglu, Amling et al. 2001; Denstedt, Wollin et al. 2001). The uncommon, but most severe problem arising from stents is the "forgotten stent" that is left in place for several months to years. These stents become encrusted and create difficulty for both patient and urologist, especially since their

symptoms associated with indwelling ureteral stents.

**2. Problems arising from stents** 

The synthetic polymer, polyethylene, was previously used in stent construction, but was abandoned due to its stiffness, brittleness, and tendency to fragment. Blends of polyethylene and other polymers, such as polyurethane, have been shown to resist encrustation (Gorman, Tunney et al. 1998; Gomha, Sheir et al. 2004). Silicone is currently the most biocompatible stent material as it is the most resistant to biofilm formation, infection and encrustation (Watterson, Cadieux et al. 2003), and is one of the most lubricious materials available (Jones, Garvin et al. 2004); however, its softness and elasticity make it difficult to handle, particularly through tortuous or tight ureters. In addition, the low tensile strength of silicone makes it susceptible to extrinsic compression. The development of new stent materials aimed to meld the flexible and elastic properties of silicone with the rigidity of polyethylene which resulted in the development of polyurethane, the most common class of polymer currently used in stents. Polyurethane, however, is a stiff material that causes patient discomfort and significant ureteral ulceration and erosion have been reported in an animal model (Marx, Bettmann et al. 1988). New proprietary materials and combinations are softer, more comfortable, and easier to maneuver within the urinary tract. Examples of commonly used materials in stents include Percuflex® (Boston Scientific Corporation, Natick, MA), Silitek® (Surgitek, Medical Engineering Corporation, Racine, WI, USA), C-Flex® (Consolidated Polymer Technologies, Clearwater, FL, USA), Tecoflex® (Thermedics, Wilmington, MA, USA), and ethylene-vinyl-acetate (from the polyefin family of which polyethylene is a member). They have been designed to provide rigidity to facilitate handling by the surgeon and to provide adequate drainage while being soft enough to limit patient discomfort.
