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Qiu et al. Vessel Plus 2018;2:12 I http://dx.doi.org/10.20517/2574-1209.2018.13 Page 5 of 15

that the metallic stent had thinner strut with smaller profile but greater radial strength compared to
polymeric stents. Welch et al. studied the effect of thermal annealing on the mechanical behaviour of
[15]
helical coiled PLLA stents. The stents were annealed at a temperature of 70 °C, 80 °C and 90 °C for 25 min
after manufacturing, and then expanded by balloon catheter up to a pressure of 12 atm. Experimental results
showed that the stent had higher stiffness and greater collapse resistance with the increase of annealing
temperature, whereas the elastic recoil had a decrease. Probably, thermal annealing induced some changes in
crystalline structure of PLLA, and thus affected the stress-strain behaviour and stent expansion behaviour.

The experimental work mainly studied the expansion behaviour of polymeric stents, such as stent diameter
change against inflation pressure, recoil and shortening effect, which are greatly affected by stent materials
and designs. Polymeric BRSs, as the latest generation of stent, are mostly made of biodegradable polymer
PLLA. It is well known that PLLA has low mechanical strength and poor elongation compared to metal, which
limits its application for stents. Consequently, studies have also been performed to investigate and improve
the mechanical behaviour of poly-lactic acid (PLA), including blending PLA with ductile biodegradable
polymers as discussed above. In order to further improve the performance of BRSs and compete with metallic
stents, research into processing and characterisation of biodegradable polymers is particularly required.

Degradation behaviour
One of key features for polymeric stents is their degradation behaviour over time. There were some
experimental studies available in terms of characterization of stent degradation in vitro and in vivo. Xu et al.
[16]
evaluated in vitro and in vivo degradation behaviour of biodegradable tubular stents, made of poly-lactic-
co-glycolic acid (PLGA) with five different molar weight ratios of LA/GA (50/50, 60/40, 71/29, 80/20 and
88/12), for application in common bile duct (CBD) repair and reconstruction. For in vitro test, stents were
placed in bile degradation medium (pH = 7.2-7.6). Morphological results showed that the stents slightly
expanded, deformed and then cracked meanwhile their colour changed from initially transparent to
yellow and opaque due to the bile and water absorption. Weight loss, molecular weight change and water
uptake were also observed, indicating the PLGA stents exhibited different degradation rates due to different
composition ratios. For in vivo test, PLGA (71/29) stents were implanted in the rat CBDs. Results showed
that the polymeric stents can provide the same biomedical functions as typical T tubes and completely
disappeared after 5 weeks.

Hadaschik et al. investigated the degradation behaviour of a new biodegradable ureteral stent (Uriprene)
[17]
made of copolymer PLGA (LA/GA = 80/20) using porcine models. Uriprene stents were implanted in 20
pigs while standard biostable stents were implanted in 16 pigs as a control. Compared to control stents,
Uriprene stents caused significantly less ureteral dilatation and urinary tract infections. Their results also
showed that the Uriprene stents began degradation at week 3, and completely degraded at week 10. The novel
stent was proved to be biocompatible in vivo. Yang et al. carried out degradation experiments on a novel
[18]
biodegradable PGLA ureteral stent, with multilayer design immersed by microsphere zein and BaSO , in
4
human urine in vitro. The scanning electron microscope (SEM) results showed that the novel stent started
degradation at week 2 and fully degraded after 4 weeks, and the degradation happened layer by layer from
outer to inner surface. The stent weight and mechanical strength (i.e., tensile strength, elastic recovery and
radial compression load) showed a decrease over the degradation time.

Currently, commercial degradable polymeric stents are made of PLLA, which breaks down to natural
by-products, i.e., water, gases (CO and N ), biomass and inorganic salts. These natural by-products are
2
2
non-toxic, so there are no negative effects on the blood or vessel wall. Degradation behaviour of polymeric
stents was mainly characterized by in vitro and in vivo methods, focusing on variation of physical (e.g.,
mass weight and molecular weight) and mechanical properties (e.g., tensile strength and elastic modulus)
over degradation times. Key parameters used to assess degradation behaviour of biodegradable polymers