C 2005) Journal of Medical Systems, Vol. 29, No. 5, October 2005 ( DOI: 10.1007/s10916-005-6107-2
A New Microcontroller-Based Human Brain Hypothermia System ˙ Metin Kapıdere,1 Ras¸it Ahıska,1 and Inan Guler ¨ 1,2
Many studies show that artificial hypothermia of brain in conditions of anesthesia with the rectal temperature lowered down to 33◦ C produces pronounced prophylactic effect protecting the brain from anoxia. Out of the methods employed now in clinical practice for reducing the oxygen consumption by the cerebral tissue, the most efficacious is craniocerebral hypothermia (CCH). It is finding even more extensive application in cardiovascular surgery, neurosurgery, neurorenimatology and many other fields of medical practice. In this study, a microcontroller-based designed human brain hypothermia system (HBHS) is designed and constructed. The system is intended for cooling and heating the brain. HBHS consists of a thermoelectric hypothermic helmet, a control and a power unit. Helmet temperature is controlled by 8-bit PIC16F877 microcontroller which is programmed using MPLAB editor. Temperature is converted to 10-bit digital and is controlled automatically by the preset values which have been already entered in the microcontroller. Calibration is controlled and the working range is tested. Temperature of helmet is controlled between −5 and +46◦ C by microcontroller, with the accuracy of ±0.5◦ C. KEY WORDS: microcontroller; brain hypothermia; thermoelectric cooler.
INTRODUCTION It is observed that in various medical treatments in human body temperature is needed to be changed—it has to be either decreased or increased. When the human body temperature is made higher than the normal level, it is a state of hyperthermia. Laboratory and clinical evidence indicates that if tumor temperature can be maintained at elevated levels for a specific period of time (to achieve a specified thermal dose), the rate of tumor cell killing can be increased. In the hyperthermia, tumors are heated to temperatures above 43◦ C. This technique is used as a radiation therapy in treating cancer.(1–3) 1 Electronics
and Computer Education Department, Faculty of Technical Education Gazi University, 06500 Teknikokullar, Ankara-Turkey. 2 To whom correspondence should be addressed; e-mail:
[email protected]. 501 C 2005 Springer Science+Business Media, Inc. 0148-5598/05/1000-0501/0
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When the temperature is lower than the normal level, it is a state of hypothermia. Lowering the body’s temperature by even 1◦ within a few hours of a stroke can reduce brain damage and the risk of death.(4) Hypothermia, which is core point in this study, is used both for brain and open-heart operations without the application of artificial circulation apparatus. It is seen that brain hypothermia eliminates the brain edema and swelling as well as other pathological consequences of acute hypoxia, treatment of closed and open cranio cerebral injuries.(5,6) This work aims to design and construct a new microcontroller-based human brain hypothermia system (HBHS) for cranio cerebral hypothermia (CCH). This system is significant in the sense that when it is needed it can be used both as hyperthermia as well as hypothermia. Besides the old methods, this new microcontroller-based HBHS is designed to be faster, more reliable, smaller and simple to use. Flexible thermoelectric (TE) modules make the system faster than the traditional cooling methods, while the microcontroller increases the reliability. On the other hand, the less number of components used in the system makes it simpler. HBHS system is made up of a microcontroller-based control card, four different temperature measurement circuits, a electronic control card module, a water circulation system, a switching mode power supply (SMPS), and a helmet. In the control card PIC16F877 microcontroller four analog inputs are used and clock frequency is chosen as 20 MHz. In the system, four thermocouples are used to measure temperature from four different points. As a cooling method for the overheated surfaces of the helmet, a water circulation system is applied. Considering the weight and size of the helmet, a switching mode power supply is preferred. Since this study aims at contributing to brain hypothermia/hyperthermia, a helmet is designed and constructed as a device to adjust the temperature in human brain. To control the current and its direction of the helmet, four MOSFET and a driver is used. DESIGNED SYSTEM Microcontroller-Based Human Brain Hypothermia System (HBHS) Control of human brain temperature has many benefits in the precaution against diseases. Hypothermia (33◦ C) has been shown to be beneficial in preventing further brain damages. Hyperthermia (>37.5◦ C), either prior to or during a stroke, negatively affects the outcomes. Aggressive body and cerebral temperature management during the first 24 h have been shown to be the most effective in preventing detrimental outcomes related to hyperthermia. In that case, the core heat of the brain needs to be cooled. In this study, for cooling or heating the brain, thermoelectric modules are used. Since thermoelectric cooling systems are most often compared with conventional systems, the best way to show the differences in the two refrigeration methods is to describe the systems themselves. A conventional cooling system contains three fundamental parts: an evaporator, a compressor, and a condenser. The evaporator or cold section is the part where the pressurized refrigerant is allowed to expand, boil, and evaporate. During this change of state from liquid to gas, energy (heat) is absorbed. The compressor acts as the refrigerant pump and
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recompresses the gas to a liquid. The condenser expels the heat absorbed at the evaporator and the heat produced during compression into the environment or ambient. Among the heat control systems, the most reliable ones are the systems which use microcontrollers. In the HBHS, Arithmetic Logic Unit (ALU) in microcontroller plays an important role in controlling the heat. It compares helmet temperature by means of the set values. In this system, the desired temperature values are entered via keyboard. These values are stored to EEPROM memory addresses. The temperature of the brain is set using the keyboard on the front panel of HBHS. The user can choose the thermocouple for the using modes. Helmet temperature data coming from the temperature sensors are sent to the control card via I/O ports. The maximum temperature value that can be set on the system is 46◦ C and minimum value is −5◦ C. Otherwise this may cause mortal damage to the patient. The temperature can be controlled more precisely by changing the program on the card. When the system temperature reaches the set temperature value, duty value of pulse-width modulation is stabilized by PIC16F877 microcontroller. Meanwhile, the controller checks the helmet temperature, in case it drops from the desired value and it increases duty value of PWM. The function of the control card is to achieve the above processes. All these steps in the HBHS are done by microcontroller itself and the program used for this system is MPASM software.(10) A block diagram for HBHS is shown in Fig 1. Human Brain Hypothermia System (HBHS) Control Card Components Microcontroller-Based Control Card The microcontroller is the most important component of the control card. In this study, PIC16F877 microcontroller is used to measure and control the helmet temperature. Four analog temperature values are converted to 10-bit digital outputs
Fig. 1. A block diagram of human brain hypothermia system (HBHS).
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Fig. 2. Control card circuit diagram.
in the microcontroller. Set temperature values compared with the measurements and required duty ratios of PWM are sent from PWM outputs by microcontroller. All the set temperatures are recorded to the internal EEPROM memory in it. PIC microcontrollers have the proper hardware, memory and software for the control of a stand alone machine or system.(11,12) Microcontrollers require some simple peripheral circuits. For example, power supply is used to supply required energy for both microcontroller peripheral circuits. Microcontroller has an internal oscillator which produces clock signals. This kind of microcontroller requires only external passive timing elements. A 20 MHz crystal and two capacitors were chosen for the required processing speed. Microcontrollers require one or more power supply voltages. These voltage values should be kept in the 5% margin of operation limits. The microcontroller power supply design for positive voltages is dramatically simplified after starting to use 7805 voltage regulators. Some microcontrollers can work directly with the batteries without a voltage regulator. These kind of microcontrollers usually requires low feeding current and high tolerances. Control circuit diagram is shown in Fig. 2. LCD module is used to display thermocouple measurement and adjusted temperatures. 8-bits parallel data was connected to LCD module by using PORTB of the microcontrollers. The module has 4 lines, 20 characters and backline light. The keyboard used in the system is a classical 4 × 4 keyboard and it works as the
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Fig. 3. Full bridge power MOSFET driver and connection to the helmet.
controller of the system by the given instructions. Temperature can be adjusted easily with the keyboard. All the controls of the HBHS are done by this keyboard. PIC16F877 microcontroller has 2-canal of PWM. They are connected to IR2109 which is a high voltage, high speed power MOSFET and IGBT drivers with dependent high and low side referenced output channels. The SMPS is provided 12 V and 40 A power for helmet by the MOSFET. PWM averages the amount of energy provided to the module and reduces the extreme temperature excursions that are experienced with an on/off system. Full bridge power MOSFET driver and connection to the helmet is shown in Fig. 3. Thermocouples and Error Compensation Circuit There is a variety of temperature sensors in the market all of which meet specific application needs. The most common sensors used to solve these application problems include the thermocouple, resistive temperature detector (RTD), thermistor, and silicon-based sensors. For measuring the temperature in the brain a little dimension sensor was needed. Thermocouples have little dimension and widespread applications. These are constructed of two dissimilar metals such as Copper and Constantan (Type T) or Chromel and Alumel (Type K). The two dissimilar metals are bonded together on one end of both wires with a weld bead. This bead is exposed to the thermal environment of interest. If there is a temperature difference between the bead and the other end of the thermocouple wires, a voltage will appear between the two wires at the end where the wires are not soldered together. This voltage is commonly called the thermocouple’s electromotive force (EMF) voltage. This EMF voltage changes with temperature without any current or voltage excitation. An absolute temperature reference is required in most of the thermocouple applications. This is used to remove the EMF error voltage. Therefore, compensation circuit is used at temperature measurement of HBHS. Thermocouple temperature measurement error compensation circuit is shown in Fig. 4. Temperature values are converted to voltage values by thermocouples. Low voltage values are amplified
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Fig. 4. Thermocouple temperature measurement error compensation circuit.
by three OP-07 operational amplifiers. It converts −5◦ C to 0 V and +46.15◦ C to 5.115 volts. Temperature increment of 1◦ C is converted into 0.050 volts by the circuit. A thermocouple temperature measurement circuit is shown in Fig. 5. HBHS has four channel temperature measurement circuits. The helmet has 120 item flexible thermoelectric modules. They are connected to serial shape. They need to work in 12 V and 40 A power supply. When one side of the modules is cooled, the other side is heated. Heated surface of the modules need
Fig. 5. A thermocouple temperature measurement circuit.
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Fig. 6. Water circulation system construction.
to be cooled by fun or water circulation system. This system is used to more cooling of the modules. It is shown in Fig. 6.
SOFTWARE OF THE HUMAN BRAIN HYPOTHERMIA SYSTEM PIC microcontrollers have been widely used in electronic industry. PIC is programmed by the MPASM language. There are only 35 instructions in this instruction set. Because of minimum instructions, this language can be learned easily. Because of the flexibility of the software, the program code designed for PIC16F877 can be used for the other members of PIC microcontrollers.(8,10) The software can be made suitable for the components without fully modifying. Besides that, larger programs can be executed with the help of external memory because these components have 8 K capacity of ROMs. But this needs extra cost. In other words, this should be used only for the situations that do not need larger software. The Flow Chart of Program The microcontroller PIC16F877 that controls HBHS to cool and heat is programmed by the MPASM programming language. The execution of the program is
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as follows: at first it is wrote to LCD module. If data is input, it is checked whether the “#” key is pressed. If the key is pressed then at first the sets are started. The user is set to the working mode of helmet. This is cooler or heater of human brain. Then user selects number of reference thermocouples for using temperature control and set temperature value and polarity. If pressed to “#” key on the keyboard, duty cycle value of PWM calculates in microcontroller and sends PWM signal to driver. On the other hand microcontroller writes all of thermocouples measurement temperatures on LCD module. Helmet temperature measurement and change of the duty cycle of PWM is real time. Duty cycle value can increase or decrease by microcontroller. It has 10-bit duty value which is important for stability of temperature. If ∗ user press “ ” key on the keyboard, program returns to the Start line. It is internal
Fig. 7. The flow chart of the program.
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Fig. 8. Human brain hypothermia system (HBHS) construction.
reset of program and stops cooling or heating of the helmet. Temperature up and down limits are −5 and +46.15◦ C on control parameter. The flow chart of the program is shown in Fig. 7. The flow chart is shown in this figure in a general manner with the subroutines. From the PORTB pin the keyboard values are entered. PORTA is used in the input of analog value of temperature and PORTD reads the keyboard.
Table I. Temperature versus voltage changes Time (minutes)
Th (◦ C)
Tc (◦ C)
VDC (V)
0.01 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.51 5.00 5.50 6.00 6.50 7.01
18.38 27.67 26.55 26.38 26.46 26.88 26.96 27.58 27.96 24.50 19.99 20.16 19.56 19.15 19.02
17.19 2.63 −3.25 −5.61 −6.37 −6.65 −6.87 −6.74 −6.55 −2.64 6.60 12.20 14.82 16.15 16.85
9.70 9.82 9.83 9.84 9.85 9.85 9.85 9.85 9.85 1.44 0.71 0.38 0.22 0.02 0.01
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Fig. 9. For 30 A cold and heat surface temperature and voltage of the helmet graphics.
RESULTS AND DISCUSSION Shown in Fig. 8, a microcontroller-based human brain hypothermia system (HBHS) is designed and then its temperature is calibrated. The validity of calibration is verified by numerous temperature measurements. The maximum heating temperature of the HBHS is at 46.15◦ C. In order to make exact calibration three different thermometers are used. These thermometers are, respectively, classical thermometer, digital thermometers whose temperature sensors are thermocouple, and integrated circuit. In helmet the controlling of the temperature is an extremely important issue. If the temperature is less or more than the normal value, the helmet will affect the brain and skin. In microcontroller-based HBHS, temperature and voltage values are measured. The measured temperature data is shown in the Table I. Hot and cold surface temperature of the helmet are Th and Tc , respectively. The measured helmet voltage is Vdc . Using a 30 A current, the measured cold and hot surface temperatures and voltages of the helmet graphics are shown in Fig. 9. Table II. Temperature versus current changes Temperature (◦ C) Time (minutes) 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 3.67
5A
10 A
15 A
20 A
25 A
30 A
16.99 17.09 17.05 17.23 17.20 12.06 10.22 7.67 5.60 3.79 9.94 7.43 3.62 0.70 −1.56 9.12 6.26 2.05 −1.19 −3.82 8.86 5.87 1.50 −1.95 −4.78 8.68 5.76 1.15 −1.75 −4.91 8.70 5.71 1.16 −2.09 −5.03 8.63 5.67 1.09 −2.13 −5.28 8.62 5.62 1.04 −2.18 −5.32
17.19 2.44 −3.32 −5.62 −6.39 −6.66 −6.87 −6.75 −6.74
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Fig. 10. Cold surface temperature of the helmet and current graphics.
Cold surface temperature and current measurements of the helmet are shown in the Table II. When current is increased across the helmet, temperature is decrease. Cold surface temperature measurements of the helmet and current graphics are shown in Fig. 10. Furthermore, these are dependent on the ambient values like environment temperatures. The errors could be decreased to the minimum values with the microcontroller-based design. In addition, the usage of temperature sensor with linear structure, and the issues like the tolerances of components and the exact timing are considered. Unnecessary deficiencies occurred due to the magnetic field produced by pump and fun in the control card. These deficiencies are removed with the help of shielding and grounding. The buttons in the keyboard are isolated in order not to be influenced by both magnetic field and network noises. Power of 220 V is used to passive filter and fuses. In this designed system less amount of circuit component is used and all of them are placed on one card. ACKNOWLEDGMENT This Project is supported by Scientific and Research Projects of Gazi University, Project No: 07/2004-04. REFERENCES 1. VanBaren, P., and Ebbini Emad, S., Multipoint temperature control during hyperthermia treatments: Theory and simulation. IEEE Trans. Biomed. Eng. 42(8):818–827, 1995. 2. Mackerle, J., Finite element analyses and simulations in biomedicine: A bibliography (1985–1999). Simul. Biomed. 813–816, 2000. 3. Kyle Potocki, J., and Tharp, H. S., Reduced-order modeling for hypothermia control. IEEE Trans. Biomed. Eng. 39(12):1265–1273, 1992. 4. DeBow, S., and Colbourne, F., Brain temperature measurement and regulation in awake and freely moving rodents, Methods 30:167–171, 2003.
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5. Inamasu, J., Nakamura, Y., and Ichikizaki, K., Induced hypothermia in experimental traumatic spinal cord injury: An update. J. Neurol. Sci. 209:55–60, 2003. 6. Ao, H., Moon, J. K., Tanimoto, H., Sakanashi, Y., and Terasaki, H., Jugular vein temperature reflects brain temperature during hypothermia. Resuscitation 45:111–118, 2000. 7. Chung, M., Miskovsky, N. M., Cutler, P. H., Kumar, N., and Patel, V., Theoretical analysis of a field emission enhanced semiconductor thermoelectric cooler. Solid-State Electron. 47:1745–1751, 2003. 8. PIC16F87X, Microcontroller Data Book, Microchip, DS30292C, pp. 57–63, 2001. 9. International Rectifier (IR), Data Sheet No, PD60163-r, www.irf.com 10. MPASM Assembler User’s Guide, Microchip, pp. 3–9, 1996. ˙ Control of dental prosthesis system with microcontroller, ¨ ur, ¨ S., and Guler, ¨ 11. Kapıdere, M., Muld I., J. Med. Syst. 24(2):119–129, 2000. ˙ Four Temperature Sensor and Microcontroller Based ¨ 12. Kapıdere, M., Ahıska, R., and Guler, I., Thermohypotherm System, 3rd International Advanced Technologies Symposium, August, 18–20, Ankara, Turkey, pp. 90–97, 2003.
