Device PrincipleThe device is based on the concept that the voltage across our ultrasonic transducer will change when the acoustical impedance of the tissue beneath it changes. The device will be capable of detecting the border between healthy and necrotic tissue.

Divider Chain CircuitThe divider chain to drive the transducer needed to meet a number of requirements:• Output a constant number of pulses• Adjust to the resonant frequency • Have a modifiable duty cycle (<1%, in order to reduce power transmission)

This cascading chain of AND gates and decadecounters was developed to meet exactly these requirements in a robust circuit of digital logic.

A pressure ulcer, more commonly known as a bed sore, is an area of the skin that breaks down when continuous force is applied to an area of the body. The constant pressure against the skin reduces blood supply to that area and can cause major damage if left untreated.

Currently, the only method of detection of these ulcers is the Blanch Response Test that most nurses and doctors use to test their patients. This rudimentary test does not work well for people with darker pigmented skin, and the ulcer frequently progresses to become more severe.

In the chosen approach, ultrasonic transducers are used to detect pressure ulcers by measuring a voltage difference between the impedances of normal and damaged skin. The proposed prototype of the project will be accurate and consistent, as well as portable and easy to use. Most importantly, the prototype would work just as well for people with darker complexions as those with lighter skin color.

The construction of this device will take several years based on the need to follow the very strict FDA regulation for the creation of medical devices. This year was an exploratory year for the project. The goal was to prove the theories behind the initial device design while laying the foundation for subsequent phases of the project. While eventually the project will feature a Force Sensor to detect pressure being exerted by the device and a compact data processing unit, we have begun by creating the driving circuit for the project, characterizing the transducers and proving their worth in detecting changes of impedance.

Driving an Ultrasonic Transducer to Detect

Pressure UlcersAlex Wagner, CTE ‘07

AdvisorsProf. Salvador Castro, Dr. Jorge Santiago, Prof. Phil Farnum, Dr. Philip Bloomfield, Dr .Chulhyun Ahn, Siddarth Deliwala and

Robyn Evelyn

Special ThanksDr. Richard Salcido and the Department of

Physical Medicine and Rehabilitation of The University of Pennsylvania

Demo: April 19, 2007 at _______________

Transducer ConstructionThe transducer was constructed using a thin disk of Lead Zirconate Titanate (PZT), copper tubing for the casing, an epoxy to hold the PZT crystal in place, and silver paint to bond the wires to the PZT and casing. Since the transducers are designed to detect changes in acoustical impedance, they need to resonate at ultrasonic frequencies.

Transducer TestingEach transducer needed to be characterized to determine its resonant frequencies. This testing was done on a high frequency impedance analyzer at Drexel University. This also allows us to pair the transducers up so that those which most closely resemble each other will be working together. We were also able to prove to ourselves that the resistance of the transducer will be substantially altered when the density of the material that the transducer is facing is altered. This theory is the major foundation of the project.

The resonance curve for an unloaded transducer. Note the resonant impedance at 22 Ω

Testing Facilities at Drexel University

The resonance curve for a transducer loaded with impedance matching alumina.

Note the lowered resonant impedance of 16 Ω

Impedance and Phase Measurements for an Ultrasonic PZT Transducer Loaded by Alumina

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Impedance and Phase Measurements for an Ultrasonic PZT Transducer

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Image by Akshay Chugh