Continuing the series about designing an applied electronic circuits course for our bioengineering majors (with more project ideas and another project idea), I want to talk about another lab-oriented view of what we want the course to teach—what lab skills they should have by the end of the course. Eventually, I’ll get around to textbook-concept view of the course, but I’m trying to stay away from that for a while, since the course we are trying to replace was faulted for being far too theoretical.  I want to start with the practical goals first, and work out form there what are the most important theory topics and what order they should go in.

Again, this is all very preliminary brainstorming—I have to talk with my co-instructor about what lab skills are feasible to teach in the class, which ones he sees as essential, and which are beyond the scope of the class.  Here is a tentative list of technician-level skills that every engineer should have:

• Reading voltage, current, and resistance with a multimeter.
• Using an oscilloscope to view time-varying signals:
  • Matching scope probe to input of scope.
  • Adjusting time-base.
  • Adjusting voltage scale.
  • Using triggering.
  • Reading approximate frequency from display.
  • Measuring time (either pulse width or time between edges on different channels)
• Using a bench power supply.
• Using a signal generator to generate sine waves and square waves.  Hmm, only the salinity conductance meter uses an AC signal so far—I may have to think of some other project-like labs that need the signal generator.  Perhaps we should have them do some capacitance measurements with a bridge circuit before building a capacitance touch sensor.
• Using a microprocessor with A/D conversion to record data from sensors.
• Handling ICs without frying them through static electricity.
• Using a breadboard to prototype circuits.
• Soldering through-hole components to a PC board.  (I think that surface-mount components are beyond the scope of the class, and freeform soldering without a board is too “arty” for an engineering class.)

There are probably a lot more skills to add to this list, which I haven’t thought about yet, and details within these skills that I’ve not thought about.  Luckily, my co-instructor has been teaching beginning students how to use electronic lab equipment for over 15 years, so I’m sure he knows what needs to be covered.

The bigger problem here is motivating students to want to develop these skills quickly—the EE and computer engineering students see the skills as directly related to their chosen profession, but the bioengineers will need to know why anyone would care about resistance, voltage, or current.  Getting a simple biosensor in right from the beginning would probably help.  I wonder if we should start with a thermistor lab for resistance, voltage, and current measurement.  How soon can we cover voltage dividers, so that they can design a resistance-to-voltage converter for interfacing to the ADC on an Arduino? Can we do that in the first lab?

I wrote earlier this week about an applied electronic circuits course for our bioengineering majors  and added some more project ideas in another post.  This weekend, while in southern California for my niece’s wedding, I was given another idea for a project by my sister: a pulse oximeter.

I knew that a pulse oximeter worked by looking at ratios of two different wavelengths of light shining through a finger or earlobe, which have different absorption by oxyhemoglobin and deoxyhemoglobin, but I had never thought about the details.  The first thing that concerned me was how a single ratio of two light intensities could be used—there must be huge variations in how much absorption there is that have nothing to do with oxygenation of hemoglobin.  Rather than puzzle through this on my own, I looked up the Wikipedia article on pulse oximetry.

It turns out that different techniques have been used over the decades.  Some of the early methods used more than two wavelengths of light including some that did not distinguish between the two states of the hemoglobin, in an attempt to quantify the total amount of hemoglobin as well as the ratio of oxy- to deoxy-.  The modern approach, though, is much simpler and more elegant.  The ratio measurements are made repeatedly, and only the fluctuating part of the readings is used.  The arterial blood flow shows a strong pulse signal (at around 0.5–3 Hz), while contaminating signals (hemoglobin in veins, absorption due to tissue other than hemoglobin, fingernail polish, …) are constant. By ignoring the DC signal and using only the fluctuating signal, the oxygen ratios are fairly easily measured.  Note that a pulse is essential to this measurement—continuous flow would not work—hence the name “pulse oximetry”.

There is at least one homebrew pulse oximeter on the web (like this one done by some Polish students and described in Polish, with a Google translation).  As far as I can tell, they used a PWM output from the microprocessor to provide signals alternately to the two LEDs, and to demultiplex the signals from the sensor and do separate analog filtering of the two signals.  It would probably make more sense to reduce the sampling rate to about 100Hz and do all the processing digitally. One thing they did that might be a bit trickier on the Arduino was to adjust the current to the LEDs to keep the DC signal level roughly constant, despite differences in the thickness and opacity of the fingers they were shining the light through.  The Arduino provides PWM outputs, but not real digital-to-analog channels.

They also do not seem to have thought about how to calibrate the device, which could be a problem for our lab also. Since commercial pulse oximeters only cost about $30 these days, we could simply have a couple in the lab for students to compare their results to, making a calibration chart of their reading vs the commercial reading.  (The hard part would be finding variation in oxygenation, since every one in the lab will likely be in the normal range of 95–99% oxygenated.)

The pulse oximeter would be a nice step up from the simple one-channel pulse sensor.  Unfortunately, most of  the design work seems to be on the digital side, rather than the analog side, which may make it a poor fit for the circuits course.

I wrote earlier this week about an applied electronic circuits course for our bioengineering majors that Steve P. and I will be designing over the summer, and how it will be designed backwards from the labs we plan to have them do.  That wasn’t quite an accurate description of our design process though, as the selection of the labs is based on more fundamental curricular goals.  I think that Steve and I have similar goals for the course, based on our discussions of it, but we’ve not tried to write them down formally yet.  I believe that we’ll need to do this for ABET, if the bioengineering program goes for full engineering accreditation.

I know that the EE Department wants the course to have a wider appeal than just to bioengineering majors taking it because it is (well, will be) required, as fluctuations in the cohort size for engineering majors has been large (biggest in computer science, which has been on a roller-coaster ride nationwide for the past couple of decades). The BME department chair, who is also the bioengineering undergrad director and main adviser, estimates a class size of 30–50 students if the course is offered this coming winter, and expects growth to 50–60 over the next few years.  The labs have room for about 20 students at a time, so this means 2 lab sections this year, growing to 3.  If we had a wider appeal, we might be able to pick up some computer science students, some game design students, or some Digital Arts and New Media (DANM) students, who are currently avoiding EE courses despite some interest in electronics because of the highly theoretical EE 101 course that is prereq to everything.  One problem with this broadening is that most of the potentially larger audience will not have been required to take the physics electricity and magnetism class, which I think is going to be an essential prereq.

Trying to put these two ideas, the need for more explicit goals for choosing labs and a wider potential market for the course, together, I came up with a possible theme for the class to guide lab selection and topics for the course:

Students completing this course will be able to design and build simple biosensors  for measuring chemical, electrical, or physical properties of biological systems (especially humans) and recording the measurements for computer analysis or interaction.

I still have to run this idea by Steve, and we’ll almost certainly need to tweak it a bit before we’re happy with it.  I’m particularly interested in two words in the goal (which I think Steve will agree with): design and build.  I don’t want this class to be a theoretical circuits course, in which students learn to formulate and solve large systems of linear equations, with no connection in their minds to any design problems.  I also want the students to learn to make things that they can take home, not just breadboard prototypes, though I may have to compromise a bit on this one, with most labs being breadboard only, and only one or two getting them to the next stage: a soldered prototype. Designing PC boards is probably beyond the scope of the class.

The labs I talked about in the previous post all fit within this theme:

• Skin conductance meter.
• Electrical field measurements in an electrophoresis gel.  This one is a bit of a stretch for the new theme, as electrophoresis gels are a standard lab tool, but not a biological system.
• Conductivity of saline solutions. Used for measuring ecosystems (like rivers).
• Do-it-yourself EKG
• Optical pulse detector
• Breathalyzer
• Thermistor-based temperature measurement.

Since that post I’ve come up with a couple of other ideas:

• Capacitance touch sensor (nice intro in Microchip application note AN1101).  This one is more ambitious than the EKG even, in that it involves a capacitance-sensitive oscillator design, but the oscillator is low frequency and the frequency sensing can be done by a microprocessor. They could use the same Arduino that they use for recording voltage data in other labs, but we’d have to provide them with a different program, since we are planning on having no programming required in the class or as prereqs to the class.  If the oscillator is a low enough frequency (say around 1kHz), then the program could be almost identical to the interrupt-driven data logger that my son wrote for the homemade super pulley, but interpreting the interrupt times differently.
• Potentiometer-based goniometer for measuring joint angles.  Like the thermistor lab, this one is mainly a voltage-divider lab. The design problems are more mechanical than electrical, but it would be useful for students in understanding noise problems in sensors, since potentiometers are notorious for the noise in the signals.  It would also prepare students well for later work (outside this class) using servos, since most servo motors use a potentiometer-based angle measurements in their feedback loop.  I wonder whether the guts of a servo circuit are within the scope of this class—probably not, as much of it is concerned with handling the power demands and voltage spikes of the inductive load of the motor.

There have been a few ideas that I’ve had to reject as being overly ambitious for a first circuits course or too expensive to implement:  anything involving a microscope or micromanipulator setup is too expensive (eliminating most of the nanopipette work done in Pourmand’s lab), anything requiring weeks of practice to set up is also out (eliminating much of the nanopore work in Akeson’s lab, since forming the bilayers is still an art that few master quickly, also eliminating most work that would require growing neural cell cultures).  The nanopipettes and nanopores also require measuring currents in the picoamp range, which requires very sensitive electronics and extreme attention to noise control (including doing everything in a Faraday cage), which is probably beyond the scope of a first circuits class.  A subsequent bioelectronics class could cover patch-clamp amplifiers and other specialized measuring circuitry.

We probably do want some lab that measures ionic current, though, as that is fundamental to both the nanopipette and nanopore work.  I wonder whether we can do a scale model of the nanopore (with pin-prick size holes in plastic or Teflon), so that the students can measure the currents without the picoamp scale problems.

I suspect that when we try out the labs, we’ll find that only about half of them work well for us, and that several of them rely on the same basic circuit ideas (like voltage dividers to convert resistance into voltage).  In the latter case, we may give students the choice of several different labs to do.  I’ve always liked the approach of giving students a number of different design problems (of roughly comparable difficulty and covering the same fundamental concepts) and letting them choose which ones to work on.  I’ve never implemented this approach in any of my classes, though, as I’ve either been faced with a pre-designed set of lab exercises that I didn’t have the authority or time to redesign or I’ve gone for quarter-long independent projects which are not even attempting equivalence between projects.

Whether we end up rejecting some of these labs or merging some into alternative choices for a single lab, we’re obviously going to need more lab ideas.  Does anyone have any?