Biomaterials for MEMS

Application of MEMS in Drug Delivery 151

fibrinogen layer covered the stent surface further inducing platelet activation and

thrombosis.

23,24,25

In 2001, drug-eluting stents (DES) were introduced as a strategy to minimize

restenosis and requirement for reintervention. The currently available polymer-

coated stents contain antiproliferative agents which elute locally at placement

location to prevent neointimal hyperplasia. At two-year follow-up using both an-

giography and ultrasound, the clinical safety of DES was further established with

minimal late lumen loss observed.

26

A recent pooled analysis demonstrated a 74%

reduction in the risk of target lesion revascularization for both sirolimus-eluting

stents (SES) and paclitaxel-eluting stents (PES) compared to BMS.

27

At present,

90% of all stents placed in the United States and Europe are DES. Despite the en-

thusiasm that resulted with the advent of DES, incomplete endothelialization and

stentthrombosis continue to plague these devices. Despite therapeutic regimens

post-placement, late stent thrombosis (LST), defined as occurring > 30 days post-

stent insertion, remains a significant complication in patients with DES. Late stent

thrombosis carries a 45% mortality rate.

28,29

Conor Medsystems (CMS) had proposed a solution to enhance the perfor-

mance of stent technology by developing a stent platform which is capable of

flexible and controllable pharmacokinetics profiles. Micromachining technology

has been used to create strategically placed drug depots within the depth of the

stent wall. The deep reservoirs of CMS stent were designed to provide greater dose

capacity than thin surface coatings (e.g., DES), allowing these stents to potentially

deliver more drug for an extended period of time, if required. Presently, the

technology is fairly new, and independent clinical feedback is further required.

Recently, in a 1,700 patients clinical trial, Krucoff et al reported that there were no

significant benefits to using CMS stents compared to the traditional DES.

30

7.3 THE DYNAMIC BETWEEN BIOCOMPATIBILITY AND

BIOFUNCTIONALITY

7.3.1 Overview

The onset of the inflammatory and wound healing response resulting from the

placement of BioMEMS in situ affects their biofunctionality. Protein adsorption,

cellular adhesion, and fibrous encapsulation are normal inflammatory and wound

healing phenomena which occur following implantation of any material or device.

Consequently, the role of the in vivo environment as it interacts with a

BioMEMS device must be understood and acknowledged. Critical medical device

development limitations imposed by the intended in vivo placement must be recog-

nized and incorporated early into the idea and prototype design and development.

The fundamental hurdle encountered by BioMEMS in reaching a commercial po-

sition today is directly linked to the initial development of these devices, at times

taking place in an alternative reality where biology is inconsequential, because the

role of the biological environment seems to be neglected.

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