Introduction to Thermoelectrics and Medical Applications
Introduction to Thermoelectrics and its Application to the Medical Field
What is Thermoelectrics?
Thermoelectrics is the collective application of the thermoelectric effect. The thermoelectric effect is made up of three different effects: the Seebeck effect, Peltier effect and the Thomson effect. The two most utilized are the Peltier and Seebeck effects.
Thermoelectrics can directly convert electricity into a temperature difference and heat flow. This is known as the Peltier Effect. The opposite is also true. Thermoelectrics can convert a temperature difference and heat flow into electricity. This is called the Seebeck effect. The Peltier effect has applications in cooling and heating and the Seebeck effect is used in thermoelectric generators (TEG) for power generation.
It was stated that thermoelectrics directly convert heat into a temperature difference and heat flow and vice versa. If this is a direct energy conversion process, what is an indirect energy conversion process? An example would be a power plant. Chemical energy from fuel is combusted into thermal energy, this thermal energy is converted to mechanical energy in a turbine and that mechanical energy is converted to electricity in a generator. This process is considered indirect because the thermal energy is converted to mechanical energy prior to being converted to electricity. With thermoelectrics, the heat is converted directly to mechanical energy without an intermediate step.
Thermoelectrics also are know by other names. These names are TEC (Thermoelectric Cooler), Peltier Cooler, TED (Thermoelectric Device), Peltier Tiles, Peltier Plates, TEG (Thermoelectric Generator), Electric Cooling, Peltier Chips, Solid-State Cooling and Heating, Solid-State Devices, Solid-State Thermal Management, Peltier Elements, Peltier Modules, and Thermoelectric Modules.
Discovery of Thermoelectrics
In the previous slide we noted the Peltier, Seebeck and Thomson effect. The names of these effects come from the names of the people that discovered them in the early to mid-1800’s.
In 1821, Thomas Seebeck discovered if two different metals are connected at their free ends to form a loop and one of the two connection points in heated, a nearby compass would deflect. This was initially thought to be a thermomagnetic effect. Later it was found that a voltage was induced in the loop (Seebeck effect) which induced current and then by amperes law induced a magnetic field.
In 1834, Jean Peltier discovered if two different metals bars are connected at their free ends to form a loop and an electrical current is applied within the loop, one connection between the two metals will absorb heat and the other connection will release heat. This is called the Peltier effect.
In 1851, Lord Kelvin (William Thomson) Discovered if electric current is passed through a bar of metal and the metal bar has a temperature gradient from end to end, heat will be either absorbed or released along the length of the bar. This is called the Thomson effect.
The Basic Building Block of Thermoelectrics: Semiconductor Couple
Today, rather that two metals as in the 1800’s, semiconductors combined with metal are used. The basic building block of today’s thermoelectrics is a semiconductor couple. One n-type and one p-type semiconductor are soldered to copper interconnects.
How Thermoelectrics Work
The two thermoelectric effects (Seebeck and Peltier) occur due to the movements of charge carriers in thermoelectric materials. In n-type materials, the charge carriers are electrons. In p-type materials, the charge carriers are called holes. Holes are vacancies in a crystal structure in which an electron could occupy.
With the Seebeck effect, charge carriers diffuse away from the hot end. Buildup of charge at the cold end produces a voltage potential. This voltage potential will drive current in a completed circuit and produce power.
With the Peltier effect, rather than having a resistive load in the circuit, a direct current source is used in its place to move the charge carriers by use of an electric field. When these charge carriers move, they take heat with them. Electrons moving to a higher energy level absorb heat and electrons that move to a lower energy level release heat. This movement of charge carriers can be used to “pump” heat against the direction it naturally flows and produce cooling and heating.
Both the effect can be revered. For example, if the hot side and cold side of a thermoelectric generator are swapped, the direction of current flow reverses. With a thermoelectric cooler, if the current flow direction is revered, the hot side becomes cold and the cold side becomes hot.
It should be noted that the Peltier effect happens during the Seebeck effect and the Seebeck effect happens during the Peltier effect, however although these effects are parasitic to the intended effect they do not dominate it.
What is a Thermoelectric Module?
It was noted earlier that a couple is the basic building block of thermoelectrics. These building blocks are used to construct modules. When more than one couple is electrically connected in series (and sometimes parallel) the cooling and heating power for the Peltier effect and voltage for the Seebeck effect are increased directly proportional to the number of couples.
The couples are sandwiched between a substrate that is sometime ceramic. This provides structural rigidity, a very flat surface for good thermal contact and electrical insulation for the electrical interconnects between the p-type and n-type semiconductor blocks.
Typical non Medical Applications of Thermoelectrics
Because thermoelectrics have no moving parts, they can be extremely reliable. Thermoelectric generators are used for extreme environments. These extreme environments are places that no one can travel or it would be far too costly to send someone there for repair or maintenance. For example, thermoelectric generators are used to power spacecraft. Spacecraft like Voyager 1 and Voyager 2 utilize the heat generated from radioactive decay of Plutonium-238 as heat input to the thermoelectric generator. The temperature difference between this heat generated and the cold of outer space produces electricity to power the spacecraft. The Voyager spacecraft have been operating since 1977.
Other extreme environment applications include power generation for extreme remote terrestrial environments. These include power generation for well heads, offshore platforms, well sites and deserts.
A more recent application of thermoelectrics is that of cookware that can produce enough power to charge a phone during hiking or camping. The temperature difference is provided by burning wood one side of the thermoelectrics and boiling water on the other.
Another application of thermoelectric power generation includes waste heat recovery. Thermoelctric generators can utilize heat from exhaust pipes that would normally be wasted to the atmosphere. Using this heat to generate electricity and improve the fuel economy of vehicles. Heat from steel manufacturing and gas flares can also be harvested to provide energy generation that produced no pollution.
Further applications of thermoelectric power generation include micro generation for sensors and electronics. Some of these applications include body heat powered wrist watches and wireless transmitters.
Combined heat and power (CHP) systems utilize thermoelectric generators. With this application, power is generated with the thermoelectric generator from some heat source like natural gas or propane. Heat that passes through the generator under normal operation is used for heating buildings and for providing hot water. This makes for a very high efficiency system because minimal heat goes unused.
Solar heat recovery can also be utilized with thermoelectrics. The heat from the sun is focused on the thermoelectric modules hot side while the ambient air cools the cold side. This produces a high temperature delta across the thermoelectric device which in turn increases the devices energy conversion efficiency.
Cooling and heating applications include include refrigeration systems that can be smaller or miniaturized. Some applications include small refrigerators and mini in-vehicle refrigerators.
Electronics cooling applications include cooling of CPUs, telecom devices, kiosk cooling and battery thermal management.
Thermal comfort applications include heated cooled mattresses, office chairs and vehicles seats, 35,000,000 have so far been sold.
Thermal convenience applications include beverage heating and cooling as well as wine bottle coolers.
Advantages of Thermoelectrics
There are no moving parts with thermoelectrics. This leads to increased reliability and long life.
No Greenhouse Gases Required
Thermoelectrics use no refrigerants or greenhouses gases and this is a positive for the environment.
Thermoelectric cooling and thermoelectric power generation is very scalable from less than one watt of cooling power or power generation up to kilowatts.
Solid-state cooling systems can be designed for high COP. Efficiency gains can also be obtained by using the devices for spot or distributed cooling rather than cooling an entire enclosure.
Cooling and Heating in One Device
Other cooling methods require a separate heating system. With thermoelectrics, changing polarity changes cooling to heating and vs versa.
Precise Temperature Control
Electric current provided to the thermoelectric cooler can be precisely controlled. The temperature of a thermoelectric cooler is dependent on the current provided to it. This leads to precise temperature control within +/- 0.1C or better.
Below Ambient Cooling
With passive systems like fans and heat sinks, only above ambient temperatures can be achieved. However with thermoelectric cooling, below ambient temperatures can be achieved.
Solid-state thermoelectric systems do not create any noise or vibration like compressors other than that of a fan
Mountable in Any Orientation
Thermoelectrics can be mounted in any orientation which increases design flexibility over other cooling or power generation methods.
Fast Response Time
The response time of thermoelectrics happens at the speed of electrons.
Medical Applications of Thermoelectrics
Medical applications of thermoelectrics are divided into two categories. The first category is cooling and heating and the second category is power generation.
We will look at both commercial medical applications of cooling and heating and cooling and heating applications that are currently undergoing research.
For power generation applications, there are currently only applications undergoing research.
Next, we will look at medical applications of thermoelectric cooling and heating
Let’s start with the commercialized medical applications of thermoelectric cooling and heating
Commercialized Medical Applications of Thermoelectrics: Refrigeration
The best applications of thermoelectrics are those that utilize one or more of the advantages of thermoelectrics (slide 10).
The two applications of thermoelectrics shown here utilize the high reliability as well as the precise and accurate temperature control and the scalability.
High reliability and precise and accurate temperature control are needed for medical applications of thermoelectrics to prevent spoiling of vaccines, medicine, and experiments.
Since thermoelectrics are highly scalable, miniature refrigerators can be manufactured in sizes that are not possible with other cooling methods. This make possible miniature and portable refrigerators for medicine and insulin storage.
Commercial Medical Applications of thermoelectrics: Cooling and Heating
In addition to refrigeration, thermoelectric chillers are utilized heat and or cool liquids. These liquids can be pumped through specialized blankets, wraps or vests to cool and / or heat the patient. This cooling and heating method is used to treat Hypoxic-ischemic encephalopathy (lack of oxygen at birth). This chiller / heater and wrap system is also used to perform Therapeutic hypothermia and to treat low grade tissue injuries.
In contrast to cooling and heating a liquid that circulates around a patient, thermoelectric cooling and heating can be applied directly to the skin (https://embrlabs.com/). This can give the body a sensation or warmth or cooling and in turn be used to treat the feeling of cold during chemotherapy due to Anemia.
These applications benefit from thermoelectric advantages of scalability to small size, accurate and reliable temperature control, cooling and heating in one device and fast response time.
Commercial Medical Applications of thermoelectrics: Cooling and Heating
Another important medical application of thermoelectric is PCR (Polymerase Chain Reaction). Developed in 1983 by biochemist Kary Mullis. The work was award a Nobel prize in Chemistry. PCR makes possible the generation of thousand to millions of DNA copies from a small amount of DNA
Small size, accurate and precise temperature control and fast response time are all characteristics of thermoelectrics that benefit this medical application.
Next we will look at Medical Applications of Thermoelectrics for cooling and heating that are currently in research.
Medical Thermoelectric Cooling: Research
Below is a survey of the research literature for medical applications of thermoelectrics. Specifically cooling and heating applications of thermoelectrics. Underneath each items is a short note about each article.
- Optimization Strategies for a Portable Thermoelectric Vaccine Refrigeration System in Developing Communities
In some countries, vaccines are shipped on ice which does not allow for temperature control
- Design of Portable Medical Cooler with Artificial Intelligence Control. An On-Site Thermoelectric Cooling Device for Cryotherapy and Control of Skin Blood Flow
Cooling patients directly with thermoelectrics rather than with an intermediate chiller
- Medical chilling device designed for hypothermic hydration graft storage system: Design, thermohydrodynamic modeling, and preliminary testing
Improving temperature control to prevent damage cells and tissues in vitro
- SkyPort: payload: medical cooler for the skyport UAV
Developing a small refrigerator to deliver vaccines by drone
- Thermostabilized photodiode for monitoring radiation of medical lasers
Cooling of laser sensors to more accurately measure and adjust the laser output
- Altec-7012 Thermoelectric Device for Cooling of The Human Head
Used to treat brain hypoxia (lack of oxygen to the brain)
- Measurement of temperature dependent heat flow rate from human limbs towards thermoelectric cooling device
Development of thermoelectrics for cooling injuries rather than ice packs that are too cold
Medical Applications of Thermoelectrics: Power Generation from Body Heat
Next, we will look at medical applications of thermoelectrics in the area of power generation from thermoelectric generators. These applications are currently undergoing research.
Medical Applications of Thermoelectrics: Power Generation from Body Heat
For medical applications of thermoelectrics, the Seebeck effect is utilized. With the Seebeck effect, body heat is converted to electricity to power electronics. Two main areas of research in thermoelectric Medical Applications are that of powering implantable medical devices and that of powering wearable medical devices.
Wearables are a rapidly growing area of health care. These wearables provided real time health monitoring of patients. Using thermoelectric power generation for these medial wearables can reduce the need for or even replace the use of batteries. When these miniature thermoelectric generators replace batteries, the data transmission will be uninterrupted by repeated removal, changing and charging of batteries.
Another important medical application of thermoelectrics in the area of power generation is thermoelectric generators for implantable devices. The benefit to the patient elimination of the need for surgery to replace batteries. Without these surgeries, the psychological and physical pain of the patient can be reduced. This can also reduce the financial strain on the patient and health care system.
Medical Applications of Thermoelectrics: Power Generation from Body Heat – Wearables
The below list of medical sensors are prime candidates to be powered by thermoelectric generators powered by body heat, thus eliminating wires and batteries, and allowing for uninterrupted data transmission. They can also be connected to wireless sensor networks (WSN) for continuous remote patient monitoring.
- Electroencephalography – EEG (Electrical activity of the scalp)
- Electrocardiography – ECG (Electrical activity of the heart)
- Electromyography – EMG (Electrical activity of the muscles)
- Pulse Oximeters (Oxygen saturation in the blood)
- Ambulatory blood pressure monitors (Continuous blood pressure measurement)
- Thermistors or Thermocouplse (Skin Temperature)
- Accelerators (to measure Patient movement and orientation)
Medical Applications of Thermoelectrics: Power Generation from Body Heat – Implantable Devices
Previously we discussed wearable devises that can be powered by thermoelectric generators. Now we will look at implantable devices. Examples of implanted medical devices that are candidates to be powered by thermoelectric generators are pacemakers, defibrillators, drug pumps, cochlear implants, muscle stimulators, neurological stimulators, wireless real-time monitors, blood pressure, implanted microelectrodes (Intramuscular electromyographic (EMG) signals)
Powering these Implantables with thermoelectric generators that convert body heat to electricity rather than batteries benefits the patient by removing the need for surgery to change the batteries.
Medical Applications of Thermoelectrics: Power Generation Research (Wearables and Implantables)
Below is the research literature found regarding the medical field and thermoelectric generators
- Thermoelectric Energy Harvesting for Energy Autonomous Active EEG Electrodes
- A Prototype of an Implantable Thermoelectric Generator for Permanent Power Supply to Body Inside a Medical Device
- Electronic Medical Thermometer with Thermoelectric Power Supply
- Kinetic and thermal energy harvesters for implantable medical devices and biomedical autonomous sensors
- Energy Harvesting from Human Body Using Thermoelectric Generator
- Energy-Harvesting Methods for Medical Devices
- Feasibility of Energy Harvesting Techniques for Wearable Medical Devices
- Revolutionizing Medical Implants through micro Generators
- Assessment of MEMS energy harvester for medical applications
- CMOS-MEMS Thermoelectric Generator for Low Power Medical Devices
- The Effect of Aluminum Nanoparticle on the Seebeck Coefficient of Biomedical Thermoelectric Devices
- Flexible Thermoelectric Generators for Biomedical Applications
- On Getting Energy for Medical Equipment from Human Body
- Power Approaches for Implantable Medical Devices
- Energy Harvesting for Wearable Wireless Health Care Systems
- Monitoring of Vital Signs with Flexible and Wearable Medical Devices
- Battery less thermo electric energy harvesting generator for implantable medical electronic devices.
The majority of thermoelectric power generation research related to the medical field has been focused on wearable sensors and implantables devices.
There has also been a great deal of thermoelectric generator and wearable research that has not been directly focused on the medical field.
Thermoelectric Cooling: Our Design Process (High Level)
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Medical Applications of Thermoelectrics: How can we help you with your medical device project?
Thermoelectrics are a technology with many benefits applicable to the medical field. There are many applications currently on the market and in research. Future thermoelectric applications are only limited by our imagination. Consider the potential applications in your area of expertise. How can we help you bring your application to reality?
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