Research Article: Enabling Angioplasty‐Ready “Smart” Stents to Detect In‐Stent Restenosis and Occlusion

Date Published: February 16, 2018

Publisher: John Wiley and Sons Inc.

Author(s): Xing Chen, Babak Assadsangabi, York Hsiang, Kenichi Takahata.


Despite the multitude of stents implanted annually worldwide, the most common complication called in‐stent restenosis still poses a significant risk to patients. Here, a “smart” stent equipped with microscale sensors and wireless interface is developed to enable continuous monitoring of restenosis through the implanted stent. This electrically active stent functions as a radiofrequency wireless pressure transducer to track local hemodynamic changes upon a renarrowing condition. The smart stent is devised and constructed to fulfill both engineering and clinical requirements while proving its compatibility with the standard angioplasty procedure. Prototypes pass testing through assembly on balloon catheters withstanding crimping forces of >100 N and balloon expansion pressure up to 16 atm, and show wireless sensing with a resolution of 12.4 mmHg. In a swine model, this device demonstrates wireless detection of blood clot formation, as well as real‐time tracking of local blood pressure change over a range of 108 mmHg that well covers the range involved in human. The demonstrated results are expected to greatly advance smart stent technology toward its clinical practice.

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Cardiovascular disease (CVD) is the number one leading cause of mortality worldwide. For example, in the United States, more than two thousand patients die from CVD each day.1 The main pathology of CVD is atherosclerosis, the hardening of blood vessels caused by plaque buildup on the arterial wall, which progressively narrows arteries to obstruct blood flow and thus interrupt normal oxygen and nutrient supply.2 If severe, this leads to heart attack and stroke. One of the most common treatments is stenting. Stents are metallic tubular implants that are permanently placed in narrowed arteries to physically prop open and scaffold the vessels to restore blood flow.3 Backed by its clinical efficacy, millions of stents are implanted every year. The presence of a metallic stent within an artery, however, can cause inflammation, leading to excess growth of arterial tissue that may cause renarrowing within the stent. This complication is known as in‐stent restenosis.4 The likelihood of in‐stent restenosis can reach as high as 50% among stented patients.5 Drug‐eluting stents, or stents coated with medications to be released slowly to suppress cellular proliferation, are currently used to prevent restenosis at an early stage after stenting. Although beneficial within the first few years post implantation, covered stents may pose an increased risk of late thrombosis and resultant heart attacks over time.6 The two primary methods used to identify in‐stent restenosis in arteries, duplex ultrasound and angiography,7, 8 are usually not done unless patients present with chest pain or other symptoms.9

In summary, we have demonstrated advanced smart stent technology using a MEMS‐sensor‐integrated antenna stent with enhanced electromechanical performances. The custom‐designed implantable capacitive pressure sensor chips based on medical‐grade stainless steel were laser‐microwelded on 20 and 30 mm length inductive antenna stents made of the same alloy to form passive resonant LC tanks that served as stent‐based wireless pressure sensors. The gold and Parylene C coatings applied to the devices ensured both biocompatibility and X‐ray opacity of them. Moreover, the adopted microwelding integration method provided clear merits in increasing both mechanical robustness and electrical performance of the device. The microfabricated prototypes were robust enough to achieve compatibility with the standard catheter assembly and PCI procedures. The combination of thick gold coating and microwelding integration was highly effective to raise the device’s Q factor, which in turn led to significant resolution enhancement in wireless pressure sensing. The prototypes deployed in the vascular grafts were tested to demonstrate wireless monitoring of in‐stent blood pressure using a swine model. These results from both the bench tests and the animal study advance smart stent technology further, toward its clinical evaluation and application for long‐term monitoring of implanted stents. The technologies of robust electromechanical integration and biocompatible packaging developed for the current device can benefit the development of other types of smart medical implants.

Fabrication of Inductive Antenna Stent: Similar to commercial stent production, the antenna stents were laser‐micromachined from medical‐grade 316L stainless‐steel tubing (with an inner diameter of 1.8 mm and a wall thickness of 100 µm) after which electropolishing was performed to smoothen the machined surfaces. Two tab‐like structures (0.6 × 2.0 mm2) were fabricated at both ends of the stent to be used as the platforms for sensor integration. The tubing has to be annealed (either before or after laser patterning) so that the stent can be balloon expandable. Postannealing stainless‐steel of fabricated stents exhibited a yield strength of ≈310 MPa and an elongation of >45%, suitable levels that enabled mechanical functionality of the stent.32 After thorough cleaning in sonicated ethanol, the stents were first pretreated through a strike process to deposit thin gold film that serves as an adhesion layer on the stainless‐steel stents, and then subjected to electroplating of 24K gold, using commercially available strike/plating solutions (TriVal‐24K Acid Gold Strike and 24K Bright Gold Plating Solution, respectively, Gold Plating Services, USA). Gold electroplating was performed for a 15–20 µm thickness (with a current of 10 mA for 1 h) to fully exploit the skin effect (considering that the skin depth in gold is up to ≈14 µm for the resonant frequency spectrum (30–100 MHz) adopted for the developed device). The DC resistance of the antenna stents was measured to decrease by 15× (from 22.5 Ω to 1.5 Ω on average) after gold electroplating. Moreover, gold‐covered stents were not only biocompatible and anticorrosive but also had high X‐ray opacity. The fabricated 20 and 30 mm stents (with helical turns of 15 and 23, respectively) were measured to have inductances of 180 and 268 nH (at 10 MHz), respectively.

The authors declare no conflict of interest.




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