Biomedical Engineering Reference
In-Depth Information
and no animal had a distal obstructed ureter Due to retained stent pieces. Properties such as
stent softness from these biodegradable stents may improve patient comfort. Clinical studies
will be necessary to determine if biodegradable stents are more comfortable.
7. Stent coatings
One of the most common stent coatings is hydrogel, which consists of hydrophilic polymers
that absorb water and increase lubricity and elasticity.(Marmieri, Pettenati et al. 1996; John,
Rajpurkar et al. 2007) These properties facilitate stent placement, making the device rigid
and easily maneuverable in its dry state, but once exposed to urine, the hydrogel begins to
absorb and trap water in its polyanionic structure, causing it to soften and theoretically
increase patient comfort. Data on encrustation and infection are less convincing, as hydrogel
has been shown to both reduce
(Gorman, Tunney et al. 1998)
and increase encrustation and
biofilm formation (Desgrandchamps, Moulinier et al. 1997). Hydrogels have been used in an
attempt to soak and retain antibiotics but an
in vitro
study did not show increased efficacy of
bacterial killing compared to non-antibiotic soaked hydrogel coated stents (John, Rajpurkar
et al. 2007).
Glycosaminoglycan (GAG), a normal constituent of urine, is a natural inhibitor of
crystallization. Other novel stent coatings include pentosan polysulfate (Zupkas, Parsons et
al. 2000) (a member of the Glycosaminoglycan family a normal constituent of urine and a
natural inhibitor of crystallization), phosphorylcholine (Stickler, Evans et al. 2002) (a
constituent of human erythrocytes that mimics a natural lipid membrane), and polyvinyl
pyrrolidone (Tunney and Gorman 2002) (a hydrophilic coating, similar to hydrogel, that
absorbs water).
Attempts to reduce encrustation have included other stent coatings, such as the bacterial
enzyme, oxalate decarboxylase, which has been shown to decrease encrustation in silicone
discs placed in rabbit bladders.(Watterson, Cadieux et al. 2003) A novel coating of mPEG-
DOPA
3
, a natural constituent produced by mussels that produces strong adhesive
properties, also has the ability to avoid biofouling in the environment. The polyethyelene
(PEG) component provides the antifouling property while the DOPA
3
provides the
adherence that PEG lacks on its own. Adherence of these combined compounds on silicone
disks has resulted in a strong ability to resist bacterial adherence and growth in vitro.(Ko
2007) Further development of this type of coating was studied
in vivo
using a rabbit
E. coli
cystitis model (Pechey, Elwood et al. 2009). This study showed that the anti-adhesive
coating was successful at inhibiting bacterial adhesion and decreased the incidence of
infection, however it was unable to prevent non-bacterial mediated encrustation.
Plasma deposited diamond like carbon coatings have been used to coat stents in an attempt
to prevent encrustation (Laube, Kleinen et al. 2007).
In vitro
experiments have shown a 30%
decrease in encrustation of these stents in artificial urine compared to the non-coated
controls. Ongoing clinical trials appear to indicate a further enhancement of these results
in
vivo
, however a mechanism for this needs to be elucidated. Encrustation of ureteral stents
remains one of the most common problems associated with ureteral stenting and more
research will need to be done for an optimal stent design which resists the deposition of
bacteria, minerals and proteins.
In vascular medicine, the anticoagulant heparin has been shown to inhibit bacterial
attachment to venuous catheters (Ruggieri, Hanno et al. 1987; Appelgren, Ransjo et al. 1996),
which has been attributed to its highly negative charge. Similarly, effects of heparin have
