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

Abstract 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.

1 Figure S1. Scanning electron microscope image of an integrated stent device. An example of sensor integration on the tab-like platform of an inductive stent using conductive epoxy adhesive, showing a thick adhesive layer with partly unfilled spacing between them. and after (right) graft bypass surgery. The test used a carotid artery that had ~2 mm outer diameter with normal blood flow. After temporarily stopping blood flow through the exposed artery with vascular clamps and tying up the artery using a silk ligature, two incisions were made proximal and distal to the ligature, at which the ends of the vascular graft that contained stent devices were sutured. The clamps were then removed to detour blood flow through the graft as a bypass. b) Exposure of left carotid artery. c) Suturing of one end of device-deployed graft at incision on carotid artery. In our accelerated aging tests in 1× phosphate buffered saline at a temperature of 67 C (8× compared with the case of normal body temperature at ~37 C), the laser-microwelded sample (316L stainless steel) showed no failure over a consecutive period of three months, while epoxy jointed sample (same steel) exhibited cracks after 42 days, verifying significant merit of laser microwelding in achieving robust, safe packaging and long-term reliability for smart stent technology.

Wireless sensing principle.
The wireless RF sensing system consists of two main parts, the passive LC tank (smart stent, the focus of this study) and the readout unit (commercial spectrum-impedance analyzer in this study).
The resonance of a LC tank occurs when the capacitive (pressure sensor) and inductive (stent) reactances become equal to each other. We use two frequency-domain telemetry methods to interrogate the tank's resonant frequency that reflects the amount of pressure locally applied to the capacitive sensor integrated on the stent. One method probes the impedance phase of a loop 7 antenna placed over the inductive stent through inductive coupling made between them. As the analyzer sweeps frequency in the antenna's impedance phase, a dip appears in the spectrum, with an approximate phase amount of tan -1 (k 2 Q) (where k is the coupling coefficient between the loop antenna and the inductive stent, and Q is the Q factor of the stent device), when the sweeping frequency matches the resonant frequency of the tank (given the principle, this is often called "phase-dip" method). Thus, a pressure change applied to the on-stent sensor leads to a shift in the phase-dip frequency, which can be used to back-calculate the applied pressure change with a known sensitivity of the sensor. A high Q factor of the stent device as well as a high coupling coefficient are two important factors that enhance the dip signal over a baseline noise and thus improve the sensing distance and resolution. The other method is based on the use of reflection coefficientthe intensity ratio of the reflected electromagnetic wave to the incident one. This method is useful for extending the working range between the external antenna and a readout unit with a coaxial extension cable, which can be essential for in vivo tests and real use in clinics.
The wireless sensing ranges with the above two sensing methods are up to ~3 cm and ~1 cm when stent devices are present in air and blood, respectively, using the set-up based on commercial spectrum-impedance analyzer as described in the current paper. The smaller distance in blood is mainly attributed to increased RF damping caused by the conductive liquid ambient.
The wireless distance requirement varies depending on the application (e.g., from ~2.5 cm for carotid artery to ~5 cm for coronary artery). There are different routes to extending the distance.
One path is to improve the Q factor of the device further. Another path is to use a different telemetry scheme. The time-domain transient resonance (often called "ring-down") method is a promising alternative. This method transmits RF bursts with varying frequencies to a LC-tank sensor and analyzes the reflected transient wave to determine the resonant frequency of the tank. [S1] Our preliminary study showed 2-3× distance enhancement using smart stents, [S2] and further development is ongoing.