Blood glucose monitors are used to measure the amount of glucose in a patient's blood, especially in patients with diabetes symptoms or a history of hyperglycemia or hypoglycemia. In general, blood glucose monitors help diabetics control the right insulin dose. The emergence of home blood glucose meters (non-clinical devices) has greatly improved the quality of life of patients. However, each time you use this monitor to measure, you not only need to collect finger blood, causing pain and inconvenience, but also use new test paper, which increases the cost of using the device.
To determine the most appropriate insulin dose requires regular or continuous monitoring of blood glucose, but current blood glucose meters cannot meet this requirement. Continuous monitors do exist, but need to be implanted under the skin, cause trauma after implantation, and be replaced weekly. There is also a non-invasive blood glucose monitor. This article describes an architecture that uses near-infrared (NIR) spectroscopy to determine blood glucose levels based on the transmission spectrum of the earlobe. Due to the use of various human parameters such as tissue thickness and oxygen saturation and calibration systems based on linear regression analysis, an accurate real-time architecture is recommended. An example of the full analog, digital, and mixed-signal functions of the Cypress programmable system-on-chip PSoC-5LP controller is also presented.
High blood sugar and low blood sugar
Hyperglycemia and hypoglycemia refer to physical conditions in which blood glucose levels are higher or lower than normal. Diabetes is a condition in which the body's pancreas ceases to secrete insulin that controls blood sugar levels. The cause of diabetes is not yet fully understood, but it is widely believed that diabetes may be caused by genetic factors and excessive sugar intake in the daily diet. Once diagnosed with diabetes, blood glucose levels are constantly monitored to allow timely intake of medicinal insulin. Patients with hyperglycemia will show persistent high blood sugar levels and need continuous blood glucose monitoring [1]. Since current measuring devices monitor blood glucose levels by invasive methods, it is necessary to frequently extract blood samples from patients, sometimes leading to other complications such as bleeding, blood loss and allergies. Non-invasive technology can solve the problem of blood collection. This article will explore and implement a non-invasive blood glucose monitoring program.
Because of its high sensitivity, high selectivity, low cost, and ease of portability, NIR spectroscopy has chosen this technology. At the same time, we chose a wavelength of 1550nm, which means that the signal-to-noise ratio (SNR) of the glucose signal is higher at this wavelength.
The near-infrared transmission spectroscopy technique was used to measure blood glucose on both sides of the earlobe, and a light source and a photodetector were placed on both sides of the earlobe. The amount of near-infrared light passing through the earlobe depends on the amount of blood sugar in the area. The earlobe was chosen for measurement because the earlobe position has no bone tissue and is relatively thin. At the same time, near-infrared (NIR) light is required to illuminate one side of the earlobe, while a receiver placed on the other side is used to receive the attenuated light, and then the attenuated optical signal is sampled and processed.
Two Thorlabs LEDs (LED 1550E) were chosen as the light source. Since conventional silicon photodiodes have limited spectral bandwidth and cannot be used to receive near-infrared light, other types of photodiodes must be used. In this case we chose a high sensitivity Marktech indium gallium arsenide photodiode with a wavelength of 1550 nm. Connect the RC low-pass filter to the output of the photodiode to reduce high frequency noise. The cost of a light emitter and receiver having a wavelength of 1550 nm is relatively low compared to other wavelengths having the same or higher glucose responsiveness.
In addition to the amount of glucose in the blood, the transmittance of near-infrared light depends on the amount of blood in the light path. That is, at the same glucose level, a larger amount of blood leads to a lower light transmittance, and vice versa. Therefore, it is necessary to adjust the value of glucose according to the amount of blood in the earlobe at the time of measurement. The amount of blood can be estimated by the oxygen content of the blood. The blood oxygen content can be measured using a pulse oximeter. The pulse oximeter uses red and infrared light to distinguish hemoglobin and oxidized hemoglobin in the blood, and based on this, obtains oxygen saturation.
Another physical parameter that affects glucose measurement is the thickness of the earlobe tissue. This problem occurs when multiple people use a device, because the thickness of the earlobe may vary from person to person in this case. The thickness of the tissue determines the path length of the near-infrared light. The longer the path, the lower the light transmittance. The thickness of the earlobe tissue can be measured by green light with a higher skin attenuation rate.
Indium gallium arsenide photodiodes used to sense near-infrared spectral signals can also be used to sense other wavelengths (such as green, red, and infrared) because the spectral response of such diodes covers the wavelengths used above.
All of these variables are amplified, sampled, and processed in PSoC5LP and then transmitted via Bluetooth to an Android app. Figure 1 is a block diagram of the entire system flow.
Figure 1. System structure diagram
Induction and pretreatment
The indium gallium arsenide photodiode signal is sent to an amplifier to amplify the weak near infrared spectral signal. The attenuation of the red, infrared, and green signals does not affect, so no amplification is required. We can use an internal programmable gain amplifier (PGA) to amplify near-infrared spectral signals. A few millivolts of voltage change was recorded from the glucose change and amplified using a 1.024V reference voltage and a programmable amplifier with a gain of 50. The sensed signal is sampled using a single Δ∑ analog-to-digital converter along with an analog multiplexer. The near-infrared and green-light signals are sampled with an 18-bit resolution, and the red and infrared signals are sampled with a 16-bit resolution to increase the sampling rate and avoid signal aliasing caused by heart rate changes (see Figure 2).
Figure 2. External components and schematics of PsoC
Pulse width modulation (PWM) can be used to control the transmit power of the LEDs. Since five LEDs (2 near-infrared, 1 infrared, 1 red, and 1 green) are used, five 8-bit PWM modules are required with different duty cycles. The transmission wavelength of the near-infrared LED varies with the average value of the DC voltage. The near-infrared LED operates at three different duty cycles to float the wavelength of the light at 1550 nm. This is done to reduce the noise between the original glucose values.
Changes in the amount of blood in the earlobe caused by heart rate will become the main source of noise if not properly handled. To eliminate the effects of heart rate changes, the attenuation signal should be sampled within 100 milliseconds after the red, infrared, and near-infrared LEDs are turned on. A total of 20 samples were collected for each LED output, and a total of 120 samples were collected (60 for the near-infrared wavelength and 20 for the infrared, red, and green wavelengths). Ambient light sources also generate a lot of noise and are collected by optical sensors. To eliminate this noise, several samples should be stored before turning on the LED. The ambient light measurement is then subtracted from the actual signal. All samples are stored with 32-bit integer variables to handle multiplication and addition overflow issues.
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