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Purpose and Features of Therapeutic And Prosthetic Devices


One of the important uses of medical electronics are the diagnostics tools or the tools that aid in surgeries. These instruments have features which are designed to sense, record and display physiological signal. Electric simulators, ventilators, artificial organs (e.g. artificial kidney), prosthetic devices, heart-lung machines, implantable devices(pacemakers), drug delivery pumps (e.g. insulin pumps) etc. are some of the examples of therapeutic and prosthetic devices whose main purpose is to treat human ailments or aid surgeons in the surgeries. Some of these design features of some of the devices are explained in detailed.


1.1​​ Cardiac pacemakers and​​ electric stimulators

 ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​​​ In the field of medical care electric simulators are of common use. In this section two of the most common and important electric stimulator devices used to detect and correct arrhythmias are​​ high-current simulators​​ such as, cardiac defibrillators and​​ the low-current simulators such as the cardiac pacemakers and of the​​ 


Cardiac Pacemakers

The purpose of the cardiac pacemakers is to maintain the​​ natural heartrate.​​ It can work to provide aid to first degree, second degree and​​ third-degree​​ heart block.​​ This electric simulator​​ which produces electric​​ pulses (synchronized with the heart’s pulse rate) in specific period, delivered through the​​ electrodes; which are situated in the inner lining of the heart, hence regulating the heartbeat. Two of the types of these artificial cardiac pacemakers are asynchronous and synchronous.​​ 

The asynchronous pacemaker (running at the uniform rate of 70-90bpm) maintains a constant heart rate, meaning the electric impulses are delivered at a constant rate.​​ Whereas synchronous pacemakers usually provide the heartrate when required by the patient; demand pacemaker provides a heartrate of 60-90bpm, which has a feedback and reset mechanism and is on a standby mode whenever the heart is functioning normally.​​ 

Another type​​ which is called the​​ atrial-synchronous pacemaker uses an atrial electrode to detect atrial contraction, use ventricular electrodes to stimulate the ventricle synchronized with the atrial contraction. Combination​​ of atrial​​ and demand synchronous​​ pacemakers (operate as a demand-pacemaker if no atrial conduction occurs) and a rate responsive pacemaker (programmed​​ to control the pulse generator using various physiological variables measured by implanted sensors).

​​ A primary​​ battery (lithium iodide: 2.8V, open circuit voltage and high source resistance) is used as a source to provide power to the device. The oscillator​​ in the device basically provides​​ the pulse rate produced in the circuit and delivered through​​ the electrodes.​​ 

The oscillators use​​ either a quartz crystal or microprocessors​​ which​​ are​​ the part of the​​ timing​​ circuit (logic circuits) which is basically a free running oscillator running at the rate of 60-150bpm.

​​ The output circuit is the main part which generates actual electric stimulus being applied to the heart through​​ triggering from the timing circuit. Hence providing a constant voltage-amplitude impulse of 5.0-5.5V at 500-600​​ s​​ or providing a constant current of 8 to 10 mA at 1.0-1.2 ms.​​ The packaging which is the most important part; is sealed is electrically lasered or welding.

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Figure 1.1​​ Block diagram of a rate-responsive pacemaker


There are some limitations on the use of synchronous cardiac pacemakers, especially when the patient is near​​ electromagnetic fields​​ of great strength, such as those produced by nearby microwave ovens, powerful radio transmitters, and radar stations. Discuss these limitations and describe the mechanisms involved.


Lead wires and electrodes

To carry the electric impulses​​ from the circuit to the device,​​ lead wires are used due to good electrical conductivity.​​ 

Due to the patient’s movements, the mechanical strength of the lead wires is suitable to withstand the movements or motion when the patients move​​ and provide constant heart beating​​ at the same time (due to the electric conductivity).​​ 

The​​ insulation of these wires​​ is​​ of great advantage,​​ the​​ electrodes present in the insulation shunts current from the areas where it is not needed. To provide such features these wires are helically interwound (hence minimizing stress and ensuring conductivity if one of the wires ruptures) covered with silicon or polyurethane cylinder.

The electrodes are designed to withstand stress​​ against the tensile forces. It is introduced​​ to the body​​ through a​​ vein in the shoulder or the neck. This device​​ could be either unipolar (one electrode in the heart) or bipolar (two electrodes in the heart). The materials are almost the same as lead wires although platinum alloys and stainless steel could also be used.​​ 


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Figure​​ 1.2​​ (a)​​ Bipolar intraluminal electrode.​​ (b)​​ Intramyocardial electrode.​​ 



The package of the pacemaker must be biocompatible with the body and it must be protective of the corrosive​​ environment​​ (both damaging device and may cause risk to the body). It must be designed to be compact, so it should have minimal volume and mass. Usually they are designed titanium or stainless steel.​​ Or of the alloys of titanium or stainless steel, so it doesn’t corrode in the corrosive environment that is provided by the body; these alloys are also friendly to the environment of the human body hence no harm is done internally.​​ 

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Figure 1.3​​ A transcutaneous RF powered electronic​​ (passive​​ simulator, amplitude applied to the electrodes is dependent on the coupling coefficients of the internal and external coils)



  • Failure to pace the appropriate cardiac chamber:

  • Output failure

  • Capture failure

  • Problems with detecting intracardiac signals

  • Under sensing

  • Oversensing (sensing signals that it should not normal encounter)

  • Pseudo malfunction

  • Crosstalk with resultant safety pacing (inappropriate inhibition in the second chamber)


Bladder Stimulators

It is used to provide aid to the patients with urinary incontinence (due to​​ weak sphincter muscle surrounding the urethra​​ or increased pressure within the bladder due to coughing or laughing​​ or neurologically​​ excited excessive contraction of the detrusor muscle of the bladder) or another neurological​​ dysfunction.​​ To aid the patients, electric stimuli is needed​​ in the muscles or in the nerves of the nervous system of the human body.​​ 

These stimulations are provided through the electrodes in or near the muscles or nerves involved in the sphincter control of the​​ urethra; which is introduced​​ through anal or vaginal insertion. The pulse duration is 0.5-5 ms for muscle electrodes or 100-400 ms for neural electrodes, it has the pulse repetition rate of 20-100 pulses/second.​​ The circuit provides​​ an​​ average current of about 1 mA,​​ transcutaneous RF powered electric stimulator is used​​ or to provide continuous stimulations.​​ 


Muscle Stimulator

It is used to provide physical therapy to the muscle contractibility which could be used to provide​​ exercise,​​ or the patient temporarily paralyzed from muscle atrophy (disuse may cause muscle loss). It provides functional electric stimulation to the muscles’ damaged nervous system. It has multiple electrodes with different stimulation sequence from a controller produce movement (usually controlled by the patient).​​ 

These are also used for the patients with drop foot prothesis(strokes)​​ consider figure 1.3​​ who are unable to lift the ball of the foot hence susceptible to tripping).

Stimulator​​ provides constant-current charge transfer per pulse. It is constant for varying load impedance with pulse current of 2 -20 mA with pulse duration of 1 ms and has peak voltage of 3-30 V at the electrodes depending on the load.​​ 


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Figure 1.4​​ A simulator system for use on a strokes patients suffering from gait problems(associated with dropfoot).


Cochlear Prosthesis​​ 

A stimulator for the patients with hearing impairment from dysfunction of the sound-transducing an​​ apparatus of the middle and inner ear. There is​​ an​​ externally designed​​ unit and implanted unit. Electrodes are inserted into the scala tympani, facing the basilar membrane; the thin wires cast in an elastomer (silicone) or an array of thin-film deposited​​ on the​​ electrodes on a flexible polymer substrate;​​ resulting in encoding of speech into stimulus patterns meanwhile​​ providing​​ limited recognition of words. Consider figure 1.4 to determine the working of the cochlear prosthesis.​​ 


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Figure 1.5​​ block diagram of cochlear prosthesis​​ 


Visual Prosthesis

To aid certain types of blindness this device provides electrical stimulation to​​ the occipital​​ cortex and the optic nerve which senses the​​ light,​​ it is made of​​ networks of miniature array or​​ of stimulating electrodes.​​ Another type of visual prosthesis is under development that directly simulates the retina and the ganglion cells in it due to this approach a direct image is show in the retina; to make use of this the optic nerve must intact for patient to see the image. In this device minute and​​ high-density​​ electrodes are fabricated using silicon which​​ is designed with microelectronics and is implanted in the eyes using surgery. This is design is one of the examples of​​ neuro-prosthetics.


1)Severe headaches​​ 

 ​​ ​​ ​​ ​​ ​​ ​​​​ 2)Damage of some nerves


Pain Suppression and Transcutaneous Nerve Stimulation

The amount of paresthesia and anesthesia usually depends from one patient to another and in some cases the effectiveness of pain suppression decreases over a continuous period. According to the gate-theory of pain which basically says that simulation of certain neurons is inhibited by the transmission of pain information, which basically leads to these pain suppresser devices.​​ It’s basically​​ a battery-powered or transcutaneous RF-powered stimulator​​ which​​ has a gate-control theory of pain​​ experienced​​ (stimulation of certain neurons can have an effect inhibiting​​ the transmission of pain information through peripheral nerves to the spinal cord).

​​ The​​ produces​​ from monopolar rectangular to biphasic spike pulses. Simulations provide up to 60 V and 50​​ V, 2-200 pulses/s, 20 -400 ms pulse​​ width, 2 per second burst rate.​​ There’s a​​ wide variety​​ of skin surface electrodes​​ (slices​​ of silicone elastomer made conductive​​ by loading with material made of carbon).​​ 


1.2​​ Defibrillators and Cardioverters

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Cardiac fibrillation

It occurs when the heart quivers instead of pumping this is due to the asynchronous​​ or irregular​​ contractions of myocardial cells​​ in the heart, decreases​​ the cardiac output to zero which can​​ brain damage and death within 5​​ minutes​​ to avoid the circumstances,​​ defibrillation is needed to provide electric shock using internal or external electrodes​​ to​​ avoid this​​ situation​​ as it causes loss of consciousness. This is a type of cardiac arrhythmia. Two main sources of treatment are the CPR or the cardiac defibrillation devices. The design of each devices is explained below


Capacitive-Discharge DC Defibrillators

This device used the short high-amplitude defibrillation pulse using the capacitive discharge circuit.​​ The clinician charges the​​ capacitor​​ through the power supplied​​ (usually under 10 seconds), in the circuit a​​ step-up​​ transformer is used to charge a capacitor. The capacitors are​​ 

discharged through the electrodes and the patient’s torso basically providing​​ a load of 50 ohms to the circuit. The process could be repeated if necessary. This​​ device depends on two aspects which is the type of capacitor and the voltage used (that’s provided through a step-up transformer in the power supply).​​ 


Defibrillators electrodes​​ 

Defibrillator Electrodes needs to be fully in​​ contact to minimize heat dissipation at the​​ end of the​​ electrode-skin contact it is basically a​​ switch activated through switch (basically a relay like​​ mechanism)​​ which is safe to use though needs a good insulation to protect the operator it also could be a spoon-shaped internal electrode.​​ 

The electrode electrodes with about 100 mm in diameter, requires electrolyte gel for good contact. Or disposable electrodes Large is already manufactured gelled in the​​ ECG electrode with a 50-cm2 pre-gelled sponge material backed by foil and surrounded by foam with pressure-sensitive adhesive. The metal foil faces the​​ conductive adhesive polymer, entire surface has a high sensitivity to pressure.​​ 


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Figure 1.6.​​ Electrodes in the defibrillation (a) The spoon shaped internal electrode that is applied directly to the heart (b) a paddle-type electrode that is applied against the anterior chest wall.​​ 



Automatic external defibrillator:

an automatic external defibrillator that can be used by an operator with little training. The electrodes are placed on the body, which ensures that the operator is not exposed to high voltages.



When an electric shock is needed to provide the patient through the chest​​ using​​ a dc defibrillator may cause the risk of producing ventricular fibrillation hence in the cardioverters​​ circuit​​ it​​ produces a rectangular-Wave instead for the patients of atrial tachycardia.​​ This device​​ is basically a combination of a cardiac defibrillator and a cardiac motor.​​ The​​ power​​ is​​ controlled using SCR, which is basically amplified and shown in a cardio scope. It has less peak​​  ​​ ​​ ​​​​ , no inductor and with smaller size of electrolytic capacitors. This device has no relay used in it.​​ On switching off the switch the defibrillator discharges in​​ 



  • Due to the surgery swelling, bruising at the area where ICD (implantable cardioverter) is placed

  • Blood vessel heart or nerve damage

  • More prone to heart failure as it doesn’t pump enough to meet the body’s requirement.


Implantable Automatic Defibrillators​​ 

The Internal electrodes basically requires less energy (5 -30 J) for defibrillation and achieves earlier defibrillation. It basically provides electrical and mechanical signals for patients of tachyarrhythmias. The power supply and the energy storing elements basically provide the electrical and mechanical signals in this defibrillator.​​ 


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Figure 1.7.​​ Block diagram of a cardioverter: in this case defibrillation pulse must be synchronized with the R wave of the ECG​​ that is applied to the patient.​​ 



1.3 Mechanical Cardiovascular Orthotic and Prosthetic Devices

These devices are usually mechanical in nature but has electronic instrumentation. Some of the examples are monitoring of hemodynamics, ECG (aortic and central-venous pressure waveforms), controlling the saturation of O2 and PO2 or controlling the malfunctioning of device itself (gas leakage and blood loss etc.).​​ 

Cardiac-Assist Devices

The idea is based on designing a pump that would aid the failing heart after acute traumatic insults (such as myocardial infarction or cardiac surgery). ​​ One of the devices an aortic-balloon system (a sausage shaped balloon) is inserted in the heart through femoral artery; it’s operation is controlled with ECG or pressure signal. After the surgery, the balloon is caused to collapse, and the blood easily replaces the volume that was consumed by the balloon. After the closing of the aortic valve closes and the balloon is filled with CO2 gas (due to solubility of the gas in the blood it gets dissolved) hence forcing blood to the body. This sort of device is usually referred as VAD (ventricular assist device).​​ 

Pump Oxygenators

During the cardiac surgery (open heart surgery) it is often needed to stop it from pumping when certain procedures are performed, the pump oxygenators replaces the function of the heart and lung​​ and provides the functionality of the heart and lungs to the body by providing optimum oxygen throughout.​​ The pump causes arterial blood pressure that’s needed; with oxygenators that provide oxygen and remove carbon dioxide​​ 

This device is connected between the right atrium and femoral artery specifically between the superior and inferior venae cava or femoral artery to femoral vein bypass to keep all the cannula away from the heart.​​ The valves and the tubing are used to direct the flow of the blood throughout the body (to the bladder etc.). Usually the mechanism is such that a thin film (to increase surface area to volume) is brought into contact with 100% Oxygen by rotating disks. This has less damaging effect on the body overall.​​ 


  • Chagas heart disease

  • Precordial problems with constriction

  • Air embolism

  • Intravascular coagulation (thrombosis is occurred due to the mismanagement of blood pressure)


There are various types of pumps which is used in this procedure such as roller pumps and multiple-finger pumps are often chosen (these pumps do not meet the blood). The tubing of the pumps is disposable which is pinched through fingers or propagation rollers is used to contain the blood or pulsatile pump with chambers which reciprocates the​​ motion of the piston and valves.​​ 


This is basically a film type mechanism (a disc of large surface area film of blood is drawn into contact with 100% pure oxygen gas, through rotation of that disc). Another mechanism involves a permeable tubing (permeable to the gas) which lets blood be in direct contact with the gas (hence less chance of the formation of emboli).​​ 

For​​ infants​​ with lung diseases it acts as an extra corporeal membrane oxygenator ECMO.

Artificial heart

It is entirely an artificial heart that is implanted in the human body in the thoracic cavity. The device is connected externally through electric or pneumatic connections. It basically acts temporarily until a donor is available. The patient could survive approximately 620 days after the surgery.​​ 



  • Blood clots

  • Infections

  • Pumping action may not be right

  • Power may fail

  • Parts may stop working well


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​​ Figure​​ 1.8.​​ ​​ Connection of a pump oxygenator to bypass the heart (A disk type oxygenator is used with the roller pump; the blood is drawn through a cannula from the vein in the right atrium and oxygenated blood is returned through another cannula from the femoral artery).​​ 


1.4 Hemodialysis​​ 

It basically is an artificially kidney that replaces the circulatory system of the uremic patients’ body, solely to remove the metabolic wastes that is in the blood.​​ 

The mechanism of hemodialysis initiates with an exchanger chamber that contains the patient’s blood, another compartment that contains the dialysate and a semipermeable compartment membrane between them. Then follows the second stage, the dialysis delivery process. There are three basic exchangers.

​​ Since the functionality of kidney to remove waste materials is gone, dialysis is used. It poses life threatening situation due to the electronic instrumentation, the patient must be protected of any sort of leakage or clothing in the blood which is life threatening. Even a small bubble would​​ 

Through a small incision a probe is inserted under X-ray; shock waves are delivered mechanically that break up the stones. The pieces are then removed through another tube or through urinary tract.​​ 


  • Can lead to arrhythmia​​ 

  • Trapped air in the dialysis tube is risky

  • Inadequate filtering of waste products

Coil Dialyzer

Known for being the most commonly used dialyzer. It’s basically a tube made of a material that is semi-permeable membrane wounded into a coil. The dialysate is made to circulate in each turn of the coil, the mechanism is workable only when the coil is​​ large (to have more surface area) and high resistance to blood pressure, to let it flow needs constant pumping which also increase the rate of ultrafiltration in the membrane. To constantly remove waste through osmosis, the dialysate also needs to be moved through a pump.​​ 

Parallel plate dialyzer

It is like a​​ multilayer parallel​​ capacitor​​ like,​​ it’s basically made of semi permeable plates,​​ in which​​ the blood circulates in the alternate plates and the dialysate to another plate. This is due to maximize surface area to volume ration.​​ 

Hollow fiber kidney

According to its name it’s made of fiber of about 10000-15000 hollow fibers (external diameter of about 0.2mm and a length of about 150 mm) that’s basically connected in parallel to make the blood flow in the lumen of the fibers (semipermeable material); surrounded by the dialysate (which is being pumped).​​ Much like the dialysate delivery system; the principal is explained in the figure 1.9​​ 

The pumps are metered in a way that allows it to mix the concentrated dialysate with the water, another pump aids it to circulate in the exchange chamber. The dialysate is made up of many solutes that are soluble in the water concentration. Much like the cardiovascular prothesis; the hemodialysis apparatus doesn’t require much external electronics.​​ Although some of the external electronics like that who control the concentration of the dialysate. If the blood is detected the machine is switched off until the problem is detected.​​ 

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Figure 1.9. The dialysate delivery system in this unit mixes dialysate from a concentrate before being pumped through the exchange chamber​​ 


1.5 Lithotripsy​​ 

The kidney stones are of great discomfort is caused when the kidney stops working properly.​​ Lithotripsy​​ is basically an open incision noninvasive or minimally invasive surgical technique​​ which is less dangerous than lithotomy which includes all the risk and complications;​​ causing disintegration of the stone allowing it to​​ pass through the urinary tract or some other external probes so it doesn’t cause severe discomfort or complication.

Extracorporeal​​ shock-wave and​​ Percutaneous lithotripsy​​ 

In this noninvasive procedure​​ a mechanically produced shock is used, that is basically controlled by ellipsoidal reflector; the waves are made to focus at one-point centimeters away from the reflector. In the demineralized and degassed water, the patient and the reflector are submerged on a gantry support.​​ 

A biplane X-ray system is positioned to detect the focal point and then high voltage of about 20kV at the spark gap produces a shock wave while the X-ray monitors the disintegration of the stone and patient’s position being adjusted by the operator. Approximately 2000 shocks are needed to disintegrate a stone into 1-2 mm pieces. It’s effective enough for the patient to continue normal activity within 2 days.

A probe is inserted in the human body by monitoring it through X-Ray fluorescent, mechanical shock wave is delivered to disintegrate the stone; these waves are produces through electric discharge or ultrasonic waves. Pieces are removed by another probe that is introduced into the body or through the urinary tract. ​​ 


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Figure 1.10.​​ In extracorporeal shock-wave lithotripsy, a bi-plane x-ray apparatus is used to make sure the stones​​ are​​ at the focal point of the shock waves by the ellipsoidal reflector ​​ 


  • Permanent kidney damage

  • Internal bleeding



1.6 Ventilators​​ 

To aid the respiratory system of the human body ventilators or respirators are used. The unit contains a controller that controls the respiratory ventilation of a patient and an assister that basically assists the spontaneous ventilation in the patient.​​ 

It uses the mechanism of negative pressure where the patient is basically sealed in a compartment; the pressure of the compartment or chamber is reduced hence the negative pressure in the thorax, when inhaled causing the pressure to be equal to the atmospheric pressure causing lungs to recoil and exhale. The positive pressure ventilator blows air in the​​ body increasing pressure in the trachea; hence making patient to inhale and recoil when pressure is removed.​​ 

Ventilators are time cycled specified in a period, it doesn’t cycle until a specific amount of air is available. There’s a safety valve to monitor pressure if it exceeds the specified value; the air is administered until a patient reaches a limit.​​ 

Much like the ventilators used normally,​​ continuous-positive-airway-pressure device​​ for​​ newborns​​ like​​ ventilators, used to aid the infants and monitors the amount of air inhaled and generated an alarm in its feedback. The semi-conductor strain gauge senses the pressure and a thermistor​​ sense​​ the temperature.​​ This basically uses an ADC and microprocessors.​​ 

High Frequency ventilators​​ 

Ventilation frequency is much higher than the normal respiration rate; basically, uses a high frequency audio speaker to generate the frequency waves. The waves are sinusoidal or rectangular of frequency of about 60-3600 min-1 or 1-60Hz; causing the inspiration ration to be 1:1-1:4. The volume of air per breadth is made to reach every anatomical dead space of the pulmonary system. It basically works on a principle of sudden jolts or turbulence causing molecular diffusion at high frequencies; mixing gas and reaching it to alveoli.

​​ High Frequency​​ jet ventilators, uses a tube inserted in the trachea giving short pulses of oxygen at a high pressure at high frequencies. A piston pump can alternately compress and depress a column of gas that is coupled to the airway through an endotracheal cannula and a motor rotating valve is used to connect the airway to a high-pressure and a low-pressure source of gas.​​&n