JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 5, OCTOBER 2006 1289Micromachined Antenna Stents and Cuffs forMonitoring Intraluminal Pressure and FlowKenichi Takahata, Member, IEEE, Yogesh B. Gianchandani, Senior Member, IEEE, andKensall D. Wise, Fellow, IEEEAbstract—This paper describes two stainless steel microstruc-tures that are microelectrodischarge machined from 50-m-thickplanar foil for intraluminal measurements of pressure and flow(with potential for applications ranging from blood vessels to bileducts). The first structure is an inductive antenna stent (stentenna)with 20-mm length and 3.5-mm expanded diameter. It is coupledwith capacitive elements to form resonant LC tanks that can be tele-metrically queried. The resulting LC tanks are deployed inside sil-icone mock arteries using standard angioplasty balloons and usedin a passive telemetry scheme to sense changes in pressure and flow.Using water as the test fluid, the resonant peaks shift from about215 to 208 MHz as the flow is increased from 0 to 370 mL/min.The second structure is a ring-shaped intraluminal cuff with two4002750-m2electrodes that are used to provide a direct trans-duction of flow velocity in the presence of a magnetic field. It isfabricated in a manner similar to the stentenna, but with an insu-lating segment. The voltage has a linear dependence on flow rate,changing by 3.1–4.3Vper cm/s of flow (of saline) over a 180 cm/sdynamic range, with a magnetic field of about 0.25 T. [1733]Index Terms—Bloodflow, cardiac, microelectrodischarge ma-chining, sensor network, wireless.I. INTRODUCTIONIN recent years, stents have come to play an essential rolein the treatment of cardiovascular and other diseases. Mostcommonly implanted in the coronary artery, the cardiac stenttypically has mesh-like walls in a tubular shape, and once po-sitioned by a catheter, is expanded radially by the inflation ofan angioplasty balloon. Its primary task is to physically expandand scaffold blood vessels that have been narrowed by plaqueaccumulation. However, renarrowing (restenosis) often occursdue to blood clot (thrombus) formation, excess growth of in-travascular tissues (proliferation), and further plaque deposition[1]. While the availability of drug-eluting stents can suppressthese failures in certain cases, chronic monitoring of pressureor flow is still needed in others. Another category of vascularproblems in which stents are used is aortic aneurysms, whichManuscript received December 11, 2005; revised February 20, 2006. The ex-ploration of microdischarge based process methods was supported in part by aGrant from the National Science Foundation. The design and fabrication of thepressure sensors was supported by the Engineering Research Centers Programof the National Science Foundation under Award Number EEC-9986866. Sub-ject Editor S. Shoji.K. Takahata was with the Department of Electrical Engineering and ComputerScience, University of Michigan, Ann Arbor, MI 48109 USA. He is now withthe University of British Columbia, Vancouver, BC V6T 1Z4, Canada (e-mail:[email protected]).Y. B. Gianchandani and K. D. Wise are with the Department of ElectricalEngineering and Computer Science, University of Michigan, Ann Arbor, MI48109 USA (e-mail: [email protected]; [email protected]).Digital Object Identifier 10.1109/JMEMS.2006.880229Fig. 1. A wireless link to an implanted sensor can be monitored by a portabledevice placed in the residence or on the person of the patient. This can thenprovide a link to a database through the internet or a broader wireless network,permitting physicians to review the sensed parameters.may be thoracic or abdominal. Such aneurysms have been sur-gically treated in the past, but the use of stents for this purposeis increasing [2], [3]. While use of wireless pressure sensorshas been reported in association with these stents [4], the sen-sors are located in the aneurysm, not within the stent or pathof blood flow. For this reason, miniaturization is not essential,and sensors of 1–2 cm length can be accommodated. Stents arealso used in the carotid artery [5], bile duct [6], pancreatic duct[7], tracheobronchial airways [8], and in the esophagus [9] totreat diseases ranging from atherosclerosis to carcinoma. Sincetheir intent is inevitably to facilitate the movement of some kindof fluid, the innate ability to monitor intraluminal pressure orflow can provide advance notice of the need for further tests orintervention.For many applications, the absolute value of pressure andflow is not required; a simple change in the average or instanta-neous signal over time can provide a warning to be heeded bya more direct and precise diagnosis. This eases the burden offunctional performance on the implanted device, and opens upsome options for engineering simpler solutions.The use of wireless telemetry can alleviate a lot of the prac-tical challenges associated with monitoring implanted sensors.For example, pressure can be monitored using a microchip thathas a planar thin film inductor integrated with a micromachinedcapacitive pressure sensor [10]–[14]. This LC tank circuit cou-ples to a separate external transmitting coil via mutual induc-tance, responding to a change in pressure by a shift in the fre-quency at which the external coil shows a characteristic dip inimpedance or phase. If a stent is able to serve as an inductor/an-tenna and is integrated with microsensors, the planar inductorcan be eliminated from the LC tank (saving considerable space),and the stent can become an inherent element in the wireless link(Fig. 1).1057-7157/$20.00 © 2006 IEEEAuthorized licensed use limited to: University of Michigan Library. Downloaded on February 25,2010 at 12:01:09 EST from IEEE Xplore. Restrictions apply.1290 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 5, OCTOBER 2006While most commercially available vascular stents are madeby laser machining of stainless steel tubes, other construc-tions—such as threaded wire structures, or rings made fromshape-memory alloy and sutured to woven polyester—arealso used [3], [15]. It has been recently shown that stents cutfrom planar metal foils by microelectrodischarge machiningoffer appropriate mechanical properties [16]. Thesestents use flexural designs and do not have any bonded orwelded seams. In assembling the device, a deflated angioplastyballoon is threaded alternately above and below a series ofinvolute cross bands that lie between two longitudinal