Biomedical Engineering Reference
In-Depth Information
24
Biomaterials in Urology - Beyond Drug
Eluting and Degradable - A Rational
Approach to Ureteral Stent Design
Dirk Lange, Chelsea N. Elwood and Ben H. Chew
The Stone Centre at Vancouver General Hospital, University of British Columbia
Canada
1. Introduction
Ureteral stents are commonly used in urology to provide urinary drainage of the upper
tracts, particularly following treatment of urolithiasis. Stents are commonly plagued with
infections and encrustation, particularly in stone-forming patients (Denstedt and Cadieux
2009). This involves a multistep process outlined in Figure 1. The first step is formation of a
conditioning film comprised of urinary proteins, ions, and crystals that are deposited at the
stent surface (Tieszer, Reid et al. 1998). The conditioning film becomes an attractive surface
for bacteria to adhere to and forms a biofilm which can lead to a urinary tract infection or
encrustation (Wollin, Tieszer et al. 1998; Choong and Whitfield 2000; Choong, Wood et al.
2001; Shaw, Choong et al. 2005). Bacteria have been demonstrated to adhere to the stent
surface in up to 90% of indwelling stents, which in 27% of cases leads to a positive urine
culture (Reid, Denstedt et al. 1992).
Ideally, ureteral stent biomaterials would be able to limit or completely prevent the
processes shown in Figure 1. Various attempts have been made to reduce the deposition of
crystals, bacteria, and protein on stent surfaces including using low surface energy
biomaterials (Tieszer, Reid et al. 1998), heparin coating (Cauda, Cauda et al. 2008),
antimicrobial eluting biomaterials (Cadieux, Chew et al. 2006; Chew, Cadieux et al. 2006;
Wignall, Goneau et al. 2008; Cadieux, Chew et al. 2009), diamond-like carbon coatings
(Laube, Kleinen et al. 2007), polyethylene glycol and marine mussel adhesive proteins
(Pechey, Elwood et al. 2009) to name just a few. Most have limited effectiveness and some
have even shown increased bacterial adhesion compared to controls in the case of heparin
coating (Lange, Elwood et al. 2009). While drug-eluting technology of biomaterials is both
readily available and used clinically in other fields, its role in urology has been limited.
Triclosan was used recently in ureteral stents. It held promise in
in vitro
(Chew, Cadieux et
al. 2006) and animal infection models (Cadieux, Chew et al. 2006) but it fared poorly in those
patients requiring chronic ureteral stents (Cadieux, Chew et al. 2009). The current
methodology of technological implementation surrounding ureteral stent coating and
design have come from trial and error or are borrowed technologies from coronary and
vascular stenting. The time has come for the urological world to apply the same types of
scientific discovery and development to problems specific to urinary devices rather than just
applying the end product from other areas of medicine. What works in vascular stenting
may not be directly applicable to the urinary environment.
