I've just sent electrical signals through my body; also I'm still alive WOOT!

Abstract

This project demonstrates the feasibility of sending electrical signals through a human body. A portion of the project involves the research and review of previous work. The result of witch pointed to the primary work done by Thomas Zimmerman[Zimmerman85]. The rest of the project was a review of his prototype. Which was adapted and recreated, based on published electrical characteristics. 

Further Research

Areas of further research includes the capacitance of transmission and reception pads based on location on the body, and the effects of various pad implementation. Frequency response of various pad location and dimensions should also be reviewed. The influence of every day interference should be measured and published. 

The most promising avenue of future work remain in the design and implementation of the sensor equipment as well as the construction of a two way protocol that can take advantage of the body node as a transmission medium. 

Background

All materials demonstrate electrical properties of resistance, inductance and capacitance. Using these properties we can incorporate the human body into electrical circuits to become part of the device we are trying to use.

The ground breaking work was done by Thomas Guthrie Zimmerman. The initial inspiration was in developing a system to measure the location of Yo-Yo Ma’s cello  that did not interfere with his playing. After discovering the extremely  sensitive nature of near fields detector Zimmerman went to design and work on near field communications.  

He currently holds a US patient,on a near field control system for computers and other objects. A German based company IDENT Technology AG is currently in development of near field control devices, such as monitors TVs and remotes. 

There is a lot work in intra(within one body) communications done in Japan. The more recent advance communications article are largely Japanese. The most notable include “Basic study of a transcutaneous information transmission system using intra-body communication”[Okamoto09], and “TCP/IP body area network in intra-body transmission using OFDM-based wide band modulation”[Koshiji09]. Any future work in Body Area network design should reference the last paper to include a discussion on interoperability. Because no established standard has been approved by the IEEE, all communications protocol used by any device remains a proprietary standard.  

Principles

The primary work done by Zimmerman involves the transmission of electrical signal by the induced electric field, see Figure 1.The voltage difference across the transmitter “A”, induces an electrical potential across the transmission pad “C”.  This further induces pico-currents to flow across the body and charge the pad “F”. The sensor will detect the voltage difference between pad “F” and pad “G”. And thus producing a signal output. 

The system is highly dependent of the subject being insulated from ground. The signal attenuates drastically when the subject grounds himself / herself. 

Human body as a node

Early work done by Webster J.G., on Electrical Impedance Tomography[webster89] , published the Resistivity of Mammalian Tissue. Because of this work the known resistivity of human tissue can be approximated as 10Ωmeters. Table I shows a larger list of mammalian Resistivity. 

Because the internal portions of the human body is very conductive, it can be approximated as a node in an idealized circuit diagram. The diagram in figure 2, shows how how the electrical coupling as capacitors. 

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Frequency response

Figure 3 shows the frequency response of a person with business card size transmission and reception pads. The frequency peaks at about 330kHz. The Zimmerman study decided to use that frequency for their prototype. Because my decode mechanism involved 3 frequency dividers and a IR demodulator operating at about 40kHz, I decided to use 300kHz carrier. See Decode section. 

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Requirements

The system must produce a readable binary signal generated and applied on one end of the body and read on the other side of the body.  

Design

Data in is represented by presence of carrier wave. The original design, shown below in Figure 4, expected a differential amplifier to read the difference in voltage difference across the sensor. 

Based on the principle above the initial design included a differential amplifier. This is the same system used to amplify the signal produced by a beating heart. This was done because a working schematic and parts for this type of amplifier was available. However during testing the system could only detect the change in contact status. The output voltage rose when contact was applied but did not send the carrier frequency. 

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Transimpedance Amplification

The output from the right side of the system acts more like pico current source and thus responds more readily to a current amplifier. These Transimpedance amplifiers could can be calibrated for specific frequency response. An application note from National Semiconductor[Pachchigar] was used to design the amplifier. Figure 5 shows the frequency response of the the TIA used by National Semi, and the equations used to size the resistor R_F and C_F.  The capacitor used was a 0.150nF, and the resistor used was a 1kΩ.

  

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Transmission

Figure 6 shows the main transmitter schematics. Figure 7 shows the main transmitter used. The main transmitter was constructed from a 555 timer set to output at 300kHz, potentiometers at both resistor locations R1, and R2 were used to dial in precisely the frequency required. Both C1 and C2 were 0.01μF. 

The final values produced by the transmitter was a square wave with a 9V_PP at 50% duty cycle, at 300kHz. This circuit does not employ a resonance tank circuit, because RadioShack does not carry a proper supply of inductors.  

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Transmission pads

Figure 8 shows the transmission pads used in this project. They were constructed from tinfoil wrapped business cards, the outer layer was wrapped in packing tape to serve as a dielectric. 

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Filter Problems

The main receiver unit went through 4 variations, during testing. Largely due to anomalous signal characteristics during the implantation of the bandpass filter. Signal attenuation after a voltage follower stage other issues lead to the filtering section being removed and a comparator LM337 to be used instead. Voltage spikes and inconsistent signal voltage made this setup nonviable. During the last day of testing, the main problem of the output stage was discovered to be a saturation problem. The output signal from the TIA stood at 4.5 V or more. This lead to a saturation of the voltage follower stage. And adding a non-cascadeable banpass filter design added poles to the first opamp, destroying the output signal. 

A DC blocking circuit, and a cascadable opamp based banpass filter should be employed to reduce these same problems.  

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Signal received

Figure 9 shows the TIA circuit and the unused comparator. In the breadboard. The output signal from the TIA is shown in figure 10. This is under ideal conditions, with pads highly coupled to the body. The output is 603mV_PP this is after the first gain stage and the resulting signal is -11db signal.  

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Decode section

Figure 11 shows the decode section. The Decode section was tested individually and was proven to work. it included 3 D-flip-flops arranged in a frequency divider combination. The output was then sent to two LEDs green and IR. The Ir LED is used to transmit to a IR sensor that can output to an LED if it receives a 38kHz IR signal. This stage was never used due to filtering problems described above.   

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Zimmerman’s Approach

Figure 12 show’s the transmitter receiver device used in the original study. This device integrates at each rising and falling edge from the transmitter. The integrator is reset. An inverted sample could be read, this option was used to maintain a the proper synchronization with the transmitter section. Zimmerman claimed it was important for the reading phase of the ADC.

 

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Conclusion 

The Zimmerman approach, requires a precise timing between the sending and receiving station clocks. An asynchronous design would be remove these issues. An IR demodulator used in TVs and other IR controlled systems has very similar problems, of attenuation, and noise. Most Demodulators are housed in a 3 pin  package that can received a modulated signal usually between 36kHz to 41kHz. The filtering technique of these devices are outlined in figure 13.

This approach could yield a asynchronous 1st level communications protocol. Building multiple variations of these devices on the same board with a banpass filter at different frequencies could allow for two way communication channels. The next step would be to develop a human Spice model. And then to simulate this receiver device.  

 Woo, E. J., P. Hua, J. G. Webster, W. J. Tompkins, and R. Pallás-Areny. "Skin Impedance Measurements Using Simple and Compound Electrodes." Medical & Biological Engineering & Computing 30.1 (1992): 97-102. Print.

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Zimmerman, Thomas G. "Personal Area Network Near-Field Intra-body Communication." Thesis. Massachusetts Institute of Technology, 1995. Print.

Eiji Okamoto, Eiji, Yusuke Sato, and Kazuyuki Seino. Basic Study of a Transcutaneous Information Transmission System Using Intra-body Communication. SpringerLink. Journal of Artificial Origins, 7 Dec. 2009. Web. 10 Dec. 2010. <http://www.springerlink.com/content/k2t35687v352r538/>.

Koshiji, Fukuro, Shudo Takenaka, and Ken Sasaki. "TCP/IP Body Area Network in Intra-body Transmission Using OFDM-based Wideband Modulation." BodyNets '09 Proceedings of the Fourth International Conference on Body Area Networks 4 (2009). Print.

Okamoto, Eiji, Yusuke Sato, and Kazuyuki Seino. "Basic Study of a Transcutaneous Information Transmission System Using Intra-body Communication." Journal of Artificial Organs (2009). SpringerLink. Department of Human Science and Informatics, School of Bioscience and Engineering, Tokai University,, 7 Dec. 2009. Web. 10 Dec. 2010. <http://www.springerlink.com/content/k2t35687v352r538//fulltext.html#Fig2>.

http://www.ident-technology.com/"Gesture Control." IDENT Technology, Next Generation Mobile User Interfaces & Intelligent Sensing. Web. 10 Dec. 2010. <http://www.ident-technology.com/technology/gesture-control>.

Pachchigar, Maithil. "Design Considerations for a Transimpedance Amplifi Er."Http://www.national.com/. National Semiconductor. Web. 8 Dec. 2010. <http://www.national.com/nationaledge/files/national_AN-1803.pdf>.

Presented

 Intro and demonstration of interbody and intrabody signaling using simple electronics. The next step is to build a better filtering design and then implenent some kind of communications protocol. There is interestng work in Japan over the implementation TCP/IP Body Area done by Koshiji, Fukuro, Shudo Takenaka, and Ken Sasaki. I'm going to start reading that paper now. 

Project overview: Inter-body Signaling

The goal of the project is to send a machine readable signal between two points of a living person. The goal at the end of the quarter is to send a binary message across a person, and to do this in the most efficient manner currently known. This project may evolve into a senior project, where Body Area Network devices are constructed and programed. Possible applications involves contact sharing between phones via (human) handshake, wireless earphones, touch-to-pay blocks in stores, or instant data access to all physically reachable objects.

This project involves three primary stages: broad research, design outline, engineering and execution. In the broad research phase, research papers, patents, and references are collected. At each stage papers are skimmed for further citations, and more materials to collect. The papers are then organized, and skimmed for useful information, and choice papers are perused. The information learned at this stage is used to produce the design constraints. Using the the design constrains the top level schematic (below) is refined and expanded into well defined components and subsystems. Once the subsystems are defined, component engineering can take place.

The broad research phase shouldnʼt take more than a couple of days. The reading and the design outline could take a week or more depending on homework load from the other classes.To speed up prototyping as many readily working subsystems should be used when possible. This will bring down, prototype engineering to a few critical subsystems.

What is the size limiting factor of an inter-body signaling apparatus? Could it be built into a cell phone? Does direct and close contact matter? Does this create an unacceptable amount of noise to the other devices? What modulation is best? What data rates can this support? What is the the overall error rate in this device?

I expect to find one question that is more pressing and answerable than the rest. I expect a limiting minimum size for the capacitors and inductors for the bandpass filters. I expect that the system produces an intense amount of noise making the ground and power rails susceptible to latch up. I expect odd behaviors from capacitive touch screens. I expect the project to be completed in under $300.