IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 53, NO. 5, MAY 2006 921 Biotelemetric Passive Sensor Injected Within Tendon for Strain and Elasticity Measurement Sérgio Francisco Pichorim* and Paulo José Abatti Abstract—A passive and injectable (using hypodermic needle) biotelemetric sensor for measurements of tendon length changes has been developed. From these measurements tendon strain and Young’s modulus of elasticity can be derived. The sensor (about 2.1 29 mm) is a LC circuit fixed in tendon by metallic anchors (barbs), where the value of the resonance frequency is modulated by displacement of a mobile ferrite core. The sensor was injected into digital extensor tendon of pig, allowing the determination of its stress-strain curve and, consequently, of Young’s modulus of elasticity of the tendon. Practical results, such as sensitivity of 18.199 kHz/mm (correlation coefficient of 0.9891) for strains up to 5.17%, mechanical hysteresis of 6.5%, and Young’s modulus of 0.9146 GPa for a pig tendon (post mortem), are presented and discussed. Index Terms—Biomedical telemetry, passive and injectable sensor, tendon, Young’s modulus. I. INTRODUCTION T HE mechanical properties of tendons (stress, strain, and Young’s modulus of elasticity), particularly those of pigs, rabbits and horses, have been determined using several methods and sensors [1], [2]. This experimental information can be useful to understand the behavior of the human tendon during long efforts, as sports [1]; to foresee its rupture or damage by mechanical forces or metabolic problems, such as diabetes, tendinitis or arthritis [2]; and, to develop prevention strategies and optimize healing treatment [3]. The mechanical tension or stress suffered by the tendon can be determined by the ratio between the applied force and the tendon cross-sectional area (1) The strain is calculated from displacement divided by the reference length [3] (2) Manuscript received December 20, 2004; revised September 11, 2005. Asterisk indicates corresponding author. *S. Pichorim is with the CPGEI/CEFET-PR-Paraná Federal Center of Tech- nological Education, Av. Sete de Setembro, 3165, CEP, 80230-901 Curitiba, Paraná, Brazil (e-mail: pichorim@cefetpr.br). P. J. Abatti is with the CPGEI/CEFET-PR-Paraná Federal Center of Techno- logical Education, 80230-901 Curitiba, Paraná, Brazil. Digital Object Identifier 10.1109/TBME.2006.872157 Finally, the Young’s modulus of elasticity is the slope of the strain-stress curve [4] (3) For direct measurement of strain or force in vivo, transducers have been attached directly to the tendon that is under evalua- tion. For instance, strain gages are implanted surgically or glued in the calcified part of the tendon and transducers, called belt buckle, are mounted outside of tendon for force or tension mea- surement [3]–[7]. These extratendon transducer may modify the course of the tendon, shorten the tissue, impinge with bone, and, for buckle transducer, after the implantation, a significant level of pain may be provoked, especially if the dimensions of the transducer are too large [6]. In order to minimize these problems, small probes, which can be implanted within tendon, have been developed. These probes employ strain gages trans- ducers, Hall effect transducers, linear variable differential trans- formers (LVDTs) or differential variable reluctance transducers (DVRTs) [6], [7], and must be as small as possible in order not to damage the tissue itself and alter its mechanical behavior [6]. In these approaches, electrical or optical cables trespass the skin of animal or man. These cables may cause biological infections, discomfort, and artifacts related to the movement of the skin [6]. In addition, for electrical cables, there are risks of microshocks. Moreover, these cables may alter the mechanical behavior of tendon. To prevent these problems the communication between the implanted transducer and the instrumentation is made using a biotelemetric link through skin [8]. Among the transcutaneous telemetric systems, those that use units which can be directly injected into the body, using a hy- podermic needle, do not require surgical intervention, reducing the patient recovering period. Devices for animal identifications (called transponders) and microstimulators are examples of in- jectable devices that work with transcutaneous communication by radio frequency (RF) signals [9], [10]. These devices do not have internal batteries, thus energy, and also communica- tion signals, must be transmitted using inductive coupling. All of them are encapsulated with biocompatible material and are composed by an antenna (inductor coil with ferrite core), capac- itor (for tuning), and an electronic integrated circuit [9], [10]. The aim of this paper is to present a passive telemetric sensor for measurement of length changes, and so, tendon strains can be derived. This sensor is composed of a capacitor and an in- ductive transducer, being small enough to be injected through a hypodermic needle within the tendon. The resonance frequency of the injected sensor is modulated by deformation suffered by the biological tissue. A biotelemetric system can read this fre- quency and the strain can be determined. The sensor has been 0018-9294/$20.00 © 2006 IEEE