Mechatronic for Health Sciences MEC 141 Lecture 3

Mechatronic for Health Sciences [MEC 141] Lecture No. 3

• Lecture is on the design of biomedical instrumentation
• A "medical device" is any instrument, machine, appliance, implant, software, material or similar item intended for medical use
• Medical device purposes include: investigating, replacing, modifying or supporting anatomy/physiology; supporting or sustaining life; or controlling conception/disinfecting medical devices
• Generalized medical instrumentation differs from conventional instruments as the signal source is living tissue or energy applied to living tissue.
• Design of instrument must match measurement needs (environmental conditions, safety, reliability); and instrument performance (speed, power, resolution, range)
• A generalized medical instrumentation system includes an energy source, sensors, transducers, signal conditioning, a control system, alarms, a display, data storage and output.

Biomedical Instrumentation Design

• Biomedical instrumentation systems contain three fundamental components: a sensor, a signal processor, and a storage device
• A biomedical sensor in the context of instrumentation is the interface between the electronic instrument and the biological system
• Sensor characteristics include:
  • Range (difference between maximum and minimum sensed parameter value) exemplified by a thermocouple's 25-225°C range
  • Resolution (smallest change sensor can differentiate, or least count of sensor)
  • Sensitivity (ratio of output change to a unit change of input), which is closely related to sensor resolution
  • Error (difference between measured result and true value)
  • Accuracy (difference between measured and true value, thus showing closeness to true value)
  • Response time (time sensor takes to respond completely to input change)

Standard Signal Types

• Most modern equipment uses standard signal ranges for consistency and calibration
• Electrical signal range: 4 to 20 mA
• Pneumatic signal range: 0.2 to 1.0 bar (or 3 to 15 psi)
• Standard range ensures equipment is readily calibrated
• Minimum signal represented by 4 mA/0.2 bar and maximum by 20 mA/1.0 bar

Hydraulic Signal Transmission System

• Hydraulic systems consist of storage tanks, filters, hydraulic pumps, pressure regulators, control valves, hydraulic cylinders, pistons, and leak-proof fluid pipelines
• Output shaft with piston transfers motion/force, while other components control the system

Pneumatic Signal Transmission System

• Involves an intake compressor, cooler, separator, receiver, actuator, control valve, secondary air treatment, pressure switch, and motor control centre

Comparison of Electrical, Hydraulic & Pneumatic Systems

• Presents a comparison table for different systems in terms of energy source, storage, distribution, energy cost, rotary actuators, linear actuators, controllable force, points to note (including danger, leakage, and noise issues)

Classification of Biomedical Sensors

• Biomedical sensors are classified by how they are used relative to the biological system
  • Noninvasive sensors do not contact the biological system (e.g., skin surface thermometers, biopotential electrodes, strain gauges)
  • Indwelling sensors are minimally invasive, placed into natural body cavities (e.g., oral/rectal thermometers, intrauterine/stomach pH sensors)
  • Invasive sensors require surgical placement and tissue damage
• Sensors are also classified by the quantities they measure
  • Physical sensors measure physical quantities like displacement, pressure, flow
  • Chemical sensors determine concentrations of chemical substances within the host (often regarding biochemical materials)
  • Bio-analytical or biosensors measure internal quantities like enzymes

Physical Sensors

• Include temperature, strain, force, pressure, displacement, position, velocity, acceleration, optical radiation, sound, flow rate, viscosity, and electromagnetic fields
• Temperature sensors:
  • The mercury thermometer produces a non-electronic output. Other examples include thermocouples, thermistors, and resistance thermometers.
• Thermocouples use the Seebeck effect to create voltage differences across dissimilar conductor junctions dependent on temperature difference.
• Resistance thermometers rely on metal wire resistance increasing with temperature. Scattering events from electrons reduce mobility and increase resistance inversely
• Thermistors are resistive semiconductor elements with a negative temperature coefficient, relying on increasing electron numbers due to increasing temperature. Consequently, thermistor resistance decreases with increasing temperature.

Displacement and Force Sensors

• Force can be sensed through displacement, like a spring stretching due to mass acceleration.
• Force is also associated with membrane displacement from pressure differences

Chemical Sensors

• Used in biomedical settings to measure chemical concentrations or activity in biological specimens.

Bio-Analytical Sensors

• A special class of sensors that use biological molecules (such as enzyme-substrate or antigen-antibody reactions) to target specific molecules in biological samples
• Highly specific for biological molecules and can provide a high degree of sensitivity and selectivity
• Basic structure of a bioanalytical sensor includes a recognition component (proteins), physical or chemical transducer (detectors), and a biological sensing area. Sensors could be electrical (electrodes, capacitance measurements) or optical (photodetectors) based.

Example of Bio-Analytical Sensor (Glucose Sensor)

• The first part of glucose sensor contains glucose oxidase, which oxidizes glucose into glucuronic acid and hydrogen peroxide.
• This process consumes oxygen. Thus, a placed oxygen sensor can measure the amount of glucose oxidized by measuring hydrogen peroxide produced or oxygen consumed,

Applications of Biomedical Sensors

• Biomedical research (quality control of products)
• Patient care (monitoring blood pressure, oxygen saturation, body temperature, and ECG)
• Specimen analysis (home blood glucose analyzers, blood analyzers)

Signal Conditioning

• This is a crucial element of biomedical instrument design and processing of a signal generated from the interaction between the biological system and the sensor system
• Pre-processing: amplifier gain dependent on next stage needs; usually, converting millivolt sensor output to digital; use of specialized digital circuits/microcomputers for logic and arithmetic
• Logic and Arithmetic Control: Performing calculations – including filtering- on raw amplified data from patients to measure parameters
• Post-processing: Final processing that matches output/adjusts scale of time, frequency, and level; functions which include averaging repetitive signals, reducing noise and converting signal from time to frequency domain

Many Sensors Have

• Primary sensing elements (diaphragms converting pressure to displacement), variable conversion elements (strain gauges converting displacement to electrical signals), and the ability to adjust sensitivity by altering the primary sensing component
• Variable conversion elements often need external power.

This lecture focuses on the design and function of biomedical instrumentation, highlighting the importance of medical devices in healthcare. It includes their purposes, performance requirements, and the components of a generalized medical instrumentation system. Understanding these concepts is essential for students in the field of health sciences and engineering.