How to Improve the Design Efficiency of Blood Glucose Meter System

A blood glucose meter is a medical device used to measure the glucose concentration in the blood. Depending on the specific blood glucose concentration, it may also need to provide hypoglycemia medication management functions. The blood glucose meter uses test strips to react with the patient's blood drop. After a chemical reaction, the blood glucose value expressed in mg/dl or mmol/l units can be read from the blood glucose meter. The blood glucose meter should be portable, low power, easy to use and have an enhanced experience. These requirements will directly affect the technology and niche market of the blood glucose meter product.

Sensors and Amperometry

The first step in measuring blood glucose is to convert the blood glucose concentration into a voltage or current signal, which can be achieved by using a special sensor test strip for current analysis. The sensor uses platinum-silver electrodes to form part of the circuit where hydrogen peroxide is electrolyzed. Hydrogen peroxide is a product of glucose oxidation on the glucose oxide membrane. The current flowing through the circuit can be used to measure the concentration of hydrogen peroxide and thus the blood glucose concentration. It should be noted that the expression in the formula (Figure 1) is linear. This is somewhat different from the actual situation because other biochemical substances may also react.

Figure 1: Electrode reactions.

The sensor in the blood glucose meter uses a glucose oxidase electrode, which is stable in a platinum-plated activated carbon electrode. The enzyme electrode can be used for current analysis through electrochemical detection of enzymatic hydrogen peroxide. The sensor is composed of various electrodes: a glucose oxide film layer, a polyurethane film that can be permeated by glucose, oxygen, and hydrogen peroxide.

Amperometry measures the current between a pair of electrodes that drive an electrolytic reaction. Oxygen diffuses through a thin film and a voltage is applied to a platinum electrode, changing O2 to H2. These reacting electrodes are an amperometric sensor using a three-electrode design. This method is useful when using an amperometric sensor because it is reliable to measure voltage and current in the same chemical reaction.

The three-electrode model uses a working electrode (WE), a reference electrode (RE), and a counter electrode (CE). The resulting current must be converted into a voltage for processing by the microcontroller (MCU). This task is performed by a transconductance amplifier. Finally, the MCU detects and processes this signal using an ADC module.

As an example, here is a practical way to explain current analysis. A voltage ranging from -200mV to 8V is applied to the WE and RE electrodes. This voltage defines the voltage at which the sensor can operate at the maximum current. When the current is 18mA, this voltage value is around 4V. After choosing 4V as the operating voltage, we can get a stabilization time of 2 to 4 seconds. This means that this time a reliable measurement can be obtained because the maximum current value is reached.

Design goals

System characteristics. From a system point of view, the blood glucose meter consists of multiple units that interact with each other to provide the necessary functions. The microcontroller is required to be the main coordinator of the system. Depending on the scope of application, internal or external flash memory or SRAM can be used. Memory is important because the measured values ​​must be stored in order to provide basic functions such as data management and measurement value averaging.

Low power consumption. Low power consumption must be paid attention to. Most current blood glucose meter devices are battery powered, so the microcontroller and LCD power consumption should be as low as possible. The blood glucose meter is in a special stop mode that allows tracking time 99% of the time, but can be woken up by an external interrupt. In order to maximize battery life in run mode, blood glucose meter manufacturers pay close attention to MCU operating frequency and wake-up time. A real-time clock function is also required because most blood glucose meters have an alarm system to remind the user when the measurement time is up.

Keep costs low. This cost includes the costs associated with the device (the blood glucose meter) and future expenditures on additional components (test strips). The blood glucose meter itself should be cost-effective because users will continue to purchase new test strips.

Figure 2: Block diagram of the basic architecture of the test strip.

Data Management. Connectivity through USB and wireless is highly desirable as data management is critical for doctors. Analyzing patient data while using a blood glucose meter is critical and must be connected to a computer to display the measurement information graphically.

The interface must be user-friendly for the patient, yet powerful enough to allow the physician to gain as much information as possible through the interface. Wireless connectivity is becoming increasingly important, not only because it facilitates access to information, but also because it enables the connection of other devices that interact with the blood glucose meter measurement, such as an insulin pump, to help the user manage the correct insulin dosage.

Keyboards and human machine interfaces can be implemented through buttons and segmented LCDs, touch sensor interfaces and graphical LCDs, all of which are also managed by a microcontroller.

System design elements

A fundamental component of the system is the measurement engine, which is a set of analog and digital IP blocks that interact with the sensors and then provide voltage to the microcontroller, which then processes the measurements. Embedding all the measurement engines in the microcontroller has certain advantages. Some 8-bit microcontrollers and 32-bit microcontrollers have on-chip measurement engines to reduce costs and minimize the number of components. The recommended measurement engine consists of the following elements:

Digital-to-Analog Converter (DAC): Provides signal bias. The DAC outputs a specific voltage to bias the sensor (test strip). The key parameter of the DAC is the settling time, which must be less than or equal to 1ms in high power mode and less than or equal to 5μs in low power mode. Uniformity must be guaranteed so that the correct waveform is used to bias the biosensor.

Transconductance amplifier: used to convert the current input into a voltage that the ADC can read and complete signal conditioning. Its key parameter is the bias current, which must be less than 500pA (at room temperature 25°C) in order to measure the tiny changes produced by the biosensor during the chemical reaction.

Operational amplifier: Compare mode set for "out of range" is used to start the measurement algorithm. Compare mode set for "internal range" can easily identify the spike of chemical reaction. A key parameter of general amplifier is bias current, which must be less than or equal to 2μA (at 25℃) to allow correct design of unity gain buffer, low pass filter, gain amplifier, inverter and non-inverting programmable gain amplifier (PGA).

Analog-to-Digital Converter (ADC): The key parameter of the ADC is accuracy, which should be greater than or equal to 13.5 effective number of bits (ENOB) to facilitate the measurement of small signals generated in biosensors. Signal strength and value depend on manufacturer specifications and technology. Measurement technology (user intellectual property) affects the accuracy value.

Additional modules (VREF, Programmable Delay Module, and Date and Time Module): VREF is a trimmable reference voltage that can be used as a reference for analog peripherals; the Programmable Delay Module is glue logic that controls the timing and triggers of the ADC and DAC modules. The Programmable Delay Module is used with the ADC to perform tests at preset time intervals and calculate glucose concentration; the Date and Time Module is used to keep track of time and record when the measurement occurred.

Figure 3: Blood glucose meter system.

Software and USB Connectivity

Software components are also important for blood glucose meter system development. Depending on the software algorithms used, blood glucose meters can be more efficient. Medical devices that comply with medical standards and organizations can be interconnected even if they are manufactured by different vendors.

When addressing USB connectivity issues, an important standard to consider is IEEE 11073. This standard provides the structure of the communication interface, defining not only the commands to access data, but also the structured processing of the data to be sent and defining the communication status. Another important standard is USB itself. The USB organization has defined the Personal Healthcare Device Class (PHDC), which is a standard implementation for medical devices to communicate over USB.

These independent intellectual property blocks provide specific vendors with the tools needed to develop specific implementations of medical USB connectivity. Freescale makes the design of medical devices (such as blood glucose meters) easier by providing these independent building blocks. The system development time can be shortened by having ready-made software developed for the peripherals of a specific microcontroller. These drivers can be used to control LCDs, analog peripherals, and connectivity interfaces. If the drivers are provided when the microcontroller is selected, it is an advantage for medical device developers.

Conclusion

Diabetes is a worldwide health problem that is growing. Fortunately, blood glucose meters facilitate daily care for diabetics by measuring the amount of glucose in the blood and helping patients determine the necessary medications to take. The blood glucose meter test strip reacts with the sensor circuit and blood to generate an electrical current that the meter measures.

Blood glucose meters on the market are differentiated by accuracy, connectivity, LCD display, and data management options. Key features such as low power consumption and medical software support are very important for blood glucose meter design. It is recommended to use a microcontroller that integrates digital and analog functions and reasonably balances cost to achieve small, low-power, and high-performance blood glucose meter devices.