O2Sat
Medical professionals have known for centuries that the color of a patient’s blood changes with changes in the level of oxygen absorption[i]Oxygen gets into our cells and tissues via the lungs, which breathe in oxygen from the air, and pass the oxygen into the bloodstream through millions of tiny air sacs called alveoli. Hemoglobin in the red blood cells picks up the oxygen and carries it to the rest of the body.. I suppose it has been 200 or 300 years that “doctors” have been using the darkening of the color of blood to diagnose how well a patient’s respiratory system is performing. Prior to the 1980s, it was common for doctors to compare a blood sample to a color chart on the wall for that purpose, but for a precise measurement, a sample would be sent to a lab, where a technician would use a spectrograph or scanning spectral radiometer to measure the color and send back a measurement in a few hours. In those days, the percentage number was called the “O2Sat” number or “Oxygen Absorption Saturation Percentage”.
In 1981, a company called Nellcor[ii]… currently owned by Medtronic after multiple acquisitions, was founded in Hayward, California, to develop technology that could provide the same information faster and less expensively. Work in the area had been done as early as the 1930s, but it was necessary to rely upon clumsy and expensive thin-film filters to obtain the wavelength control required for the measurement. The Nellcor initiative, some half-century later, was sparked in part by Monsanto’s invention of the LED, which was the first truly monochromatic light source with a spectral bandwidth of no more than a couple of nanometers. Having a light source of such wavelength precision eliminated the need for filtering on the measurement end.
Nellcor did not have the light-physics technology to be able to design and build the device, so they went to one of the Siemens medical device companies in Germany for help. From there, they were directed to the III-V Materials Division of Siemens in Regensburg, Germany, a facility where I later visited my friend Hrr. Spath many times. Regensburg was able to contribute the compound semiconductor materials expertise, but they lacked the technology necessary to reduce the Nellcor idea to a manufacturable product.
Meanwhile, Siemens had recently acquired Litronix[iii]… one of several LED companies that had spun out of Monsanto, and hired me as CTO for my experience with chip-level hybrid technology, coupled with my earlier experience at Fairchild with Silicon bipolar and MOSFET development and wafer fab. My experience, coupled with the Litronix LED technology, held the following four essential elements needed for a successful Nellcor product development, and the project eventually ended up in my group.
Red LED
At that time, visible LEDs were either red, orange or yellow, made from GaAsP (Gallium Arsenide Phosphide).[iv]Green came much later as GsAlAs (Gallium Aluminum Arsenide) and blue even later as InGaN (Indium Gallium Nitride). The GaAsP epitaxial layer was grown using a homemade, gas-phase reactor, which Litronix had developed. The substrate was GaAs, which was also grown in the Litronix facility using custom-made float-zone reactors.
Neither the GaAs nor the GaAsP is transparent at the wavelengths needed, so rather than bulk-emitter junctions, the process of planar-diffused junctions yielded extremely inefficient surface emitters, requiring lead-bonding [v]This was long before the perfection of flip-chip, beam-lean, and similar technologies.. Quantum efficiencies (useable photons per spent electron) of the LEDs were in the low, single-digit range, but that was good enough at the time.
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NIR LED
All things organic, such as human tissue, tend to reflect very strongly in the NIR. In the Nellcore product, it was used to normalize the red reflection to compensate mostly for variations in skin tone. The actual wavelength was not particularly important as long as it was constant.
GaAs emitters in the range of 890 nm to 940 nm had become relatively common for use in night-vision cameras and for plastic fiber optic communications, and Litronix was a major player in that market, so that was an easy choice.
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Silicon Phototransistor
Litronix owned controlling interest in a Silicon wafer fab company at the time, and I had considerable experience with bipolar semiconductor devices, so designing and making a custom photodetector was not a problem. The questions were PN junction, PIN junction, or transistor. If transistor, should it be PNP or NPN, open-Base or otherwise, and how large? The tradeoffs are always between active area (sensitivity), junction capacitance (switching time), and cost (die size).
We were able to make up for much of the lack in LED efficiency by using a relatively large NPN with open-Base junction covering almost the entire surface. Junction capacitance was not a concern because the more than a thousand-fold bandwidth margin between switching time and the human pulse rate allowed plenty of room for oversampling.
Hybrid Integration Technology
The first iterations of the finished module were “reflective” configurations, meaning that the two LEDs and the phototransistor were all in the same plane and facing the same direction. It was a single-use device that looked a bit like a bandaid. Litronix was a major supplier of both reflective and slotted beam interrupters, in addition to optical signal couplers, so the required tape-and-reel and lead-frame technology was very familiar.
Later versions used a “transmissive” configuration, where light passed through the patient’s finger or toe to be detected on the other side. Eventually, the whole module became reusable, and requirements for this transducer dropped from millions to thousands.
We executed the development activity and put the finished device into production within a matter of months, and continued to manufacture countless millions of them over the succeeding several years. The product went through several design iterations, and eventually, the patents ran out, allowing others to enter the field.
Today, they call it a pulse-oximeter, and it’s the first thing you see when you walk into a hospital or doctor’s office.
By: Jim
Written: September 2024
Published: September 28, 2024 [vi]This happened a long time ago, so I will add revisions as I slowly remember details.
Revised:
footnotes
| ↑i | Oxygen gets into our cells and tissues via the lungs, which breathe in oxygen from the air, and pass the oxygen into the bloodstream through millions of tiny air sacs called alveoli. Hemoglobin in the red blood cells picks up the oxygen and carries it to the rest of the body. |
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| ↑ii | … currently owned by Medtronic after multiple acquisitions |
| ↑iii | … one of several LED companies that had spun out of Monsanto |
| ↑iv | Green came much later as GsAlAs (Gallium Aluminum Arsenide) and blue even later as InGaN (Indium Gallium Nitride). |
| ↑v | This was long before the perfection of flip-chip, beam-lean, and similar technologies. |
| ↑vi | This happened a long time ago, so I will add revisions as I slowly remember details. |