Dialysis Machines


Dialysis machines perform most of the functions of a human kidney. They remove the unwanted waste products from the blood and are used by patients who have permanent or temporary renal failure.

In operation, a patient's blood is continuously pumped from an artery, a large vein, or a surgically modified vein into the dialysis machine in a way to allow high blood flow rates. Before the blood enters the dialyzer, heparin is added to prevent clotting. A syringe pump is used to deliver the heparin at a precisely controlled rate.

The blood then enters the dialyzer where it passes across a large surface-area, semipermeable membrane with a dialysate solution on the other side. A pressure gradient is maintained across the membrane to ensure the proper flow of compounds out of and into the blood. After cleansing and balancing within the dialyzer, the blood is passed through an air trap to remove any air bubbles before it is returned to the patient. Blood pressure, oxygen saturation, and sometimes hematocrit levels (blood cell concentration) are monitored for proper operation of the machine and to ensure patient safety. For maximum effectiveness, fresh dialysate is continually pumped through the dialyzer during operation.

A dialysis machine consists of multiple pumps, a large number of sensors, and a large number of control valves. Each of these devices requires a relatively high degree of analog signal processing.

Click the "Design Considerations" tab to gain an understanding of the signal chains and circuitry typically used to control these devices within a dialysis machine. Click the "Circuits" or "Block Diagrams" tab to view reference designs and products suggested for use in implementing various design functions.

A dialysis machine is a complex process control machine that pumps blood, injects therapeutic solutions, filters, and then returns the blood to the patient. The machine consists of multiple pumps, ranging from three to 10 depending on capability, multiple sensors that detect and monitor pressures, temperatures, oxygen levels and air bubbles, and a number of mechanical valves. The machines must go through periodic calibrations.

The following pumps are typically found within a dialysis machine:

  • Acid Pump
  • Bicarbonate Pump
  • UF Pump
  • Dialysate Pump
  • Blood Pump
  • Deaeration Pump

The following sensors are typically found within a dialysis machine:

  • Deaeration and Loading Pressure
  • Flow Pressure
  • Dialysate Pressure
  • Arterial Pressure
  • Venous Pressure
  • Inlet Water Pressure
  • Temperature Sensor (Multiple)
  • Conductivity Sensor

The dialysis machine is a precision instrument. Accuracy is extremely important, and pressures, temperatures, and flow rates must all be calibrated on a regular basis to ensure that the machine provides the correct therapy.

The pumps and valves are all controlled by a central controller that must synchronize the pump operation based on inputs from the sensors. From an analog design standpoint, the dialysis machine uses a high percentage of motor control electronics and sensor interface circuits. The user interface typically consists of an LCD screen, and keypad, and audio signaling and alerting that also contain a fair amount of analog components.

Dialysis machines are large machines that primarily operate from line voltage, but require battery operation to ensure a dialysis session can end gracefully in the event of line power loss. Battery systems are of high capacity and are typically housed within the machine. Dialysis machines may draw from 10A to 15A during operation, and multiple voltage rails and point of load regulators are required.

Pumps – Control and Monitoring

A dialysis machine has many pumps. Peristaltic and syringe pump mechanisms are used for the critical blood circuit and therapy injection. Each pump is driven by a motor and the motors are all controlled by the system microcontroller through individual motor control circuits.

For each motor, feedback sensors provide information about rotation count, motor position, and current usage. The type of motor used depends on the application. Stepper motors and BLDC motors are typically used to drive peristaltic and syringe pumps. BLDC motors are typically matched with a gearing system and fitted with angular-position sensors or a rotary encoder to provide rotation feedback to the system controller. Stepper motors can directly drive some pump mechanisms. In addition, the current windings of a BLDC motor are usually monitored to ensure the motor is running efficiently.

Because of the number of motors used in a dialysis machine, the control architecture will incorporate a number of motor-control-specific microcontrollers that take instruction from the main controller. The type of motor-driving circuitry selected depends on the type of motor used and the application. Typically, brushless DC motors are employed because these types of motors provide reliable operation, can be easily controlled, and are low in cost.

Part Selection

BLDC motors are driven by power MOSFETs that are usually driven by half-bridge driver silicon. The driver silicon receives digital timing pulses from a motor control microcontroller. These timing pulses control the speed of the motor. The pump module will usually provide an output signal either digital or analog that is used as a feedback signal into the controller for speed. In addition, most pump motors provide outputs for monitoring the current in the motor windings to ensure the motor is operating smoothly. Refer to the Motor/Pump Driver Signal Chain block diagram for recommended Maxim products for these applications. The products recommended in this diagram will generally work for all the pump control circuits within this machine, regardless of the pump type selected.

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Pressure Sensing

The dialysis machine requires multiple pressure sensors. The pressure differentials that exist within a dialysis machine are moderate, though the pressure values need to be very accurate. Sensor signal chains can be anchored by very accurate 10- and 12-bit analog-to-digital converters (ADCs) in order to achieve the needed resolution. Both the ADCs and any necessary op amps should be as accurate and stable as possible to minimize the amount of calibration time.

Different pressure sensors will have different output levels. Millivolt output sensors will usually require an amplifier. Higher level signals may be fed directly into an ADC, and, depending on needed accuracy, or microcontroller-processing budget, some may be fed directly into a microcontroller.

Maxim's Pressure Sensing Solution provides an excellent background into designing pressure sensor signal chains.

Part Selection

This application needs to be very accurate and stable. The pressures ranges are relatively narrow, so 10- to 12-bit ADCs can be used obtain accurate digital values. The needed sample rate will depend on the number of sensors being monitored. ADCs are available with integrated multiplexers to allow a single ADC to monitor multiple sensors. To see Maxim's most highly recommended ICs for this application, refer to the Pressure Sensor Signal Chain block diagram and click on the ADC, amplifier, and reference blocks.

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Temperature Sensing

The temperature ranges that are seen within a dialysis machine are relatively narrow so lower cost RTDs (resistance temperature detectors) can be used to provide accurate and reliable results over the temperature range. Refer to Maxim's Temperature Sensing Solution for more design information.

Part Selection

The MAX31865 single-chip RTD-to-digital IC is low cost and makes the RTD interface design easy. For more circuit ideas, refer to our Temperature Sensing Solution pages.

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Flow Measurement

Flow rates will be primarily determined by the speed of the pump motors given that, in the absence of any obstructions, the flow rate will be proportional to motor speed. Very precise flows are obtained by the use of special pump types, like the syringe pump, which is geared such that multiple revolutions of a motor shaft result in only very small linear movements of the syringe. Introduction of a physical flow rate sensor could introduce unneeded reliability issues, but devices like ultrasonic flow measures can provide very accurate measurements without introducing an obstruction in the line.

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Air-in-Line Sensing

Air-in-line sensors are ultrasonic sensors that output a voltage that changes when bubbles are detected flowing through the delivery tube. These sensors generally provide higher output levels than a pressure sensor so they can  be attached directly to an ADC input or fronted by a scaling op amp and fed directly into a microcontroller-based ADC. The exact signal chain required will depend on the sensor selected.

Part Selection

Depending on the sensor selection, this signal chain may consist of either a high voltage output (e.g., 0V to 5V) or millivolt output. Either output type might be fed directly into a microcontroller ADC port through a scaling op amp, if the microcontroller has an ADC. Or these sensors can be fed into a single multichannel external ADC that also handles the occlusion sensor. Refer to the Sensors Signal Chain block diagram to view Maxim products recommended for this application.

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The processing architecture will consist of a main processor that controls the system through multiple sensor management microcontrollers as well as the user interface. The main processor may be a high-capacity ARM®-based controller, Intel-based processer, or FPGA-based controller. This processor may require multiple power rails that may require power sequencing. In addition, for safety, the processor may require an external watchdog timer, while the sensor processors may also require external watchdog timers. Depending on the processor selected, an external real-time clock may be required. The system will also generally need system temperature sensors to ensure the system boards are operating within their designated temperature range.

Part Selection

Maxim offers voltage sequencers, watchdog timers, and silicon-based temperature sensors for system monitoring and real-time clocks. Click the "Block Diagram" tab and click on the relevant block of the main diagram to view optimal products for this application.

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User Interface

Setting up and calibrating a dialysis machine can be complicated. To optimize the user interface, most machines have a large color display, touch-based for newer machines, and a keyboard or touch screen for user input. Newer dialysis machines may use standard embedded Windows®- or Linux®-based operating systems running on the main controller. The main controller is a high-performance device that has a large I/O capacity along with extensive firmware memory and program memory. The main controller will directly interface to the user interface screen and keyboard and will also provide any external communications, for example, USB, RS-232 for diagnostics, and possibly Ethernet.

The machine will also have an audio alarm function. Some machines have synthetic voice audio capability and most have audio alarms with multiple sound indicators.

Part Selection

Many LCD displays are selected as a module, so the dialysis machine designer most likely will not be involved in designing the display electronics or the keyboard electronics. The display may, however, require custom power rails, thereby requiring a system engineer to select unique switching regulators for the function. Refer to the User Interface block diagram for products that might apply to your design.

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Power Supply

Dialysis machines are typically line powered and also include a battery power system that can allow a dialysis process to proceed to a state that is safe for the patient in the event of a power failure. The main power supply converts line power to an intermediate voltage level for distribution to the various processing boards. Voltages are converted to the needed level on the individual processing boards using step-down switching regulators and low-voltage linear regulators.

Part Selection

Maxim offers a wide range of switching regulators and LDOs. Refer to the "Block Diagram" tab and click the relevant DC-DC converter blocks to view optimal products for this application.

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Motor/Pump Control Circuits Heater Control Sensor Signal Processing User Interface Communications Audio Signal Chain I/O Expander Watchdog RTC Temp/Fan Control Multivoltage Supervisor LDO Load Switch Battery Charger Step-Up DC-DC Step-Down DC-DC

Pump Motor Signal Chain

Half-Bridge Driver CS Amp ADC

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USB Protection Isolation RS-232 Transceiver

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Sensor, Signal Chains

VREF Amplifier ADC

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Heater Control

Temp Sensor Heater Actuator

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User Interface

LCD Power and Control LCD Backlight Drivers Key Scanner GPIO Indicator Drivers

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Speaker Driver Audio Codec Piezo Driver

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