250 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 60, NO. 1, JANUARY 2013 A Minimally Invasive Implantable Wireless Pressure Sensor for Continuous IOP Monitoring Girish Chitnis*, Teimour Maleki, Member, IEEE, Brian Samuels, Louis B. Cantor, and Babak Ziaie, Senior Member, IEEE Abstract—This paper presents a minimally invasive implantable pressure sensing transponder for continuous wireless monitoring of intraocular pressure (IOP). The transponder is designed to make the implantation surgery simple while still measuring the true IOP through direct hydraulic contact with the intraocular space. Fur- thermore, when IOP monitoring is complete, the design allows physicians to easily retrieve the transponder. The device consists of three main components: 1) a hypodermic needle (30 gauge) that penetrates the sclera through pars plana and establishes direct ac- cess to the vitreous space of the eye; 2) a micromachined capacitive pressure sensor connected to the needle back-end; and 3) a flex- ible polyimide coil connected to the capacitor forming a parallel LC circuit whose resonant frequency is a function of IOP. Most parts of the sensor sit externally on the sclera and only the needle penetrates inside the vitreous space. In vitro tests show a sensitiv- ity of 15 kHz/mmHg with approximately 1-mmHg resolution. One month in vivo implants in rabbits confirm biocompatibility and functionality of the device. Index Terms—Glaucoma, implantable microdevice, intraocular pressure (IOP), wireless sensing. I. INTRODUCTION G LAUCOMA, the second leading cause of blindness, is most accurately defined as a collection of diseases that have in common, damage to the optic nerve and loss of visual field with increased intraocular pressure (IOP) being the pri- mary risk factor [1]. According to National Institutes of Health (NIH) approximately 120 000 Americans are blind from glau- coma which accounts for 9–12% of all cases of blindness in the U.S. Worldwide 79.6 million people are expected to suffer from glaucoma by 2020 increasing from 60.5 million in 2010 [2]. Although there are treatments available, there is a need to de- velop improved diagnostic and therapeutic techniques to fight this disease. Increased IOP is one of the primary factors used Manuscript received December 3, 2011; revised March 26, 2012; accepted May 29, 2012. Date of publication June 19, 2012; date of current version Decem- ber 14, 2012. This work was supported by the AMIPurdue. Asterisk indicates corresponding author. *G. Chitnis is with the School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 USA (e-mail: gchitnis@purdue.edu). T. Maleki and B. Ziaie are with the School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906 USA (e-mail: tmalekij@purdue.edu; bziaie@purdue.edu). B. Samuels and L. B. Cantor are with the Department of Ophthalmology, Indiana University School of Medicine, Indianapolis, IN 46202 USA (e-mail: bcsamuel@iupui.edu; lcantor@iupui.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBME.2012.2205248 to diagnose glaucoma and is also a clinically significant risk factor for its progression. Goldmann tonometry performed dur- ing the office visit is considered to be the gold standard for the measurement of IOP [3]. However, given that IOP fluctuates over time, a single office visit gives only a snapshot of what the true IOP is between measurements, which is often weeks or months depending on the patient. Recently, a variety of stud- ies, such as Advanced Glaucoma Intervention Study (AGIS), have identified IOP fluctuation as an independent risk factor for progression of glaucoma [4]–[7] but current methods of IOP measurement do a poor job at allowing physicians to examine these fluctuations. A more continuous pressure measurement during the patients’ routine daily activity can provide a valuable insight into the true IOP throughout the day. Furthermore, the ability to monitor IOP on a more frequent basis will also be use- ful to clinicians in assessing the effectiveness of modifications in a patient’s treatment regimen. Wireless measurement of IOP has been pursued for several decades following the pioneering work of Collins in 1967 using a passive LC transponder [8]. Much effort in this area has been focused on passive (LC) and active (pressure sensor plus inter- face and RF electronics powered through inductive methods) transponders implanted in the eye or embedded into contact lens [9]–[16]. These efforts suffer from several shortcomings which have so far prevented their widespread application. The transponders implanted inside the eye are naturally too inva- sive for a widespread application across a majority of glaucoma population; meanwhile, IOP measurement across the cornea by embedding a pressure sensor into contact lens is not accurate enough due to its reliance on a good long-term physical contact between the sensor and the cornea. In addition, the contact-lens method relies on an accurate knowledge of the mechanical prop- erties of cornea which is different in each individual depending on age, sex, and other conditions [17], [18]. The most recent academic effort in wireless measurement of IOP is from a group at the University of Michigan [15]. Using optical powering with on-board solar cells and rechargeable batteries, they were able to shrink the system size to 1 mm 3 . Although, technologically a great feat, the system requires elaborate on-board electron- ics, relies on custom-made thin film batteries (not commercially available), and requires an invasive surgery similar to that of cataract. The solution to successful development of a clinically accept- able IOP measuring microdevice relies on careful consideration of engineering design and medical/surgical tradeoffs. Consider- ing the problems associated with contact-lens approaches which rely on transcorneal measurements, we believe that one has to 0018-9294/$31.00 © 2012 IEEE