Powering up Micro Mano robotic system is one of the biggest challenges faced by the Micro Nano scale research industry. As these are very small in size either we have to supply the power wireless or find a method to harvest energy from any within the system. These sources can be vibration energy, kinetic energy, thermal energy, light energy, radiant energy etc. One of the methods is using induction coupling at the micro machined antenna. Instead of providing a supply source for the micro robot we can use a high voltage integrated circuit to deliver power. One of the advantages of this method is that this enables both powering up and asynchronous control of micro robots. An alternative for this is using a photovoltaic cell in order to activate an electromagnetic field which can be further altered to transmit information. We also discuss about the micro robot in capillaries using the chemical power from the reaction between glucose and oxygen. In this paper we also discuss a way to harvest energy from the flow of blood using a Nano turbine.
1. Introduction
Micro/Nano robotic systems are robotic systems which required very low power for its operation which ranges from µW to µW
due to its tiny size and high system response [1]. Harvesting energy for Micro/Nano robotic system is challenging. In large robotic systems, Commercial batteries are used to power the system, but it's not suitable to use it in Micro/Nano robotic systems due to its large size and weight of these sources and also it negatively affects the system reliability. Therefore we need to find some other source for energy harvesting which should be able to give sufficient power to Micro/Nano micro system for its operation should be able to give long term power for the system and must be able to give good system reliability and efficiency. Also the structure related to the particular method of energy harvesting should be of small size, since it needs to be implemented within the micro/Nano robotic systems.
Some of the sources for energy harvesting in Micro/Nano robotic systems are [1] -
light
thermal
radiation,
vibration& kinematics(mechanical energy harvester)
The voltage produced by these methods is only of few micro volt or milli volts which are sufficient for the Micro/Nano robotic systems for its operation.
Light Energy- Light is a readily available resource which can generate electricity and it works according to the principle of photovoltaic effect [2], where an incident light is directly converted into electricity. Here the photons from sun light are absorbed by the micro photovoltaic cell. This produces an imbalance in the cells and as the result electrons from the cell is released and thus electricity is generated. Instead of sunlight we can also use laser light of different wavelengths which are more efficient. Some of the advantages of this type of energy harvester are, there are no moving parts, no emission of noise, simple integrity and modularity. However, it need an extra circuitry for its operation which occupies more space in the system and also it is expensive [5]. Its energy conversion efficiency is about 10-16% [3]
Thermal Energy- Generating electricity through thermal gradient is another source of harvesting energy and the principle behind is the so called Seebeck effect [4]. According to Seebeck effect, electric current is present in series circuit of two different materials, provided the junctions of two metals are at different temperature [5]. The current generated depends upon the temperature and the purity of the metals. It has got limited conversion efficiency and therefore the voltage generated will be of few micro volts or milli volts if a high temperature is applied [5]. It is limited to MEMS application due to its short of large thermal gradient in small devices volume. This source is very much effective in nanotechnology since one dimensional material such as Bi and BiTe have small thermal conductivity but high electrical conductivity and therefore it very much useful for improving the thermal power [1].
Radiation Energy- Energy from radiation is used in various applications and it can used to power the Micro/Nano systems. Basically there is a transmitter to transmit the radiation and a receiver to receive the radiation from the transmitter. Transmitter is situated external to the system and the micro sized receiver is within the Micro/Nano system. The fabrication is complicated since the energy conversions efficiency needs to be relatively high.
Mechanical vibration- Vibration and kinetic energy harvester comes under mechanical energy harvesters. Energy is obtained from these sources of energy harvester through any one of the transduction mechanism- electromagnetic, piezoelectric and electrostatic transduction mechanism [6]. Kinetic energy is typically in the form of vibration, random displacement or forces and is typically converted to electrical energy using any one of the transduction mechanism [6]. Basically we use spring mass system or a cantilever beam structure to implement the device within the system [7]. These are low voltage sources that generate few hundreds of milli volts that is sufficient to power the Micro/Nano robotic systems.
For each energy source, a structure need to be designed and a required micro fabrication process is followed in order to implement them within the Micro/Nano robotic systems In this review we focus on various sources for harvesting energy in Micro/Nano robotic systems mainly on energy harvesting by induction coupling, photovoltaic cell, chemical energy etc. It also explains about the current status and achievements, possible future and current applications and future challenges related to harvesting energy for Micro/Nano robotic systems. Also we suggest a method to harvest energy from the flow of our blood in veins, arteries and capillaries using Nano turbines.
Methods for harvesting energy in detail
2. Induction coupling
The basic principle used here is electromagnetic induction. A change in flow of current through one coil induces a voltage in another coil. We use induction coupling for transmitting both power and data. Here the first coil is the transmitter side and the second coil is the receiver side connected to the IC and the MEMS actuators.
Fig.2.1 Autonomous robot concept [8]
Here the remote powering is achieved by inductive coupling at 13.56 MHz Digital data is transmitted using amplitude modulation. The carrier's amplitude is modulated by 25% [8]. The modulating signal is an 8 bit serial sequence which is the command for the actuators.
Let us consider different blocks of the setup in detail
Emitter board-
In the emitter board we have a pc interface to a micro controller, a 8 bit digital to analog converter, a carrier generator, amplifier and an antenna. The PC generates the signals which are interfaced to the microcontroller. The micro controller produces an 8 bit digital sequence which will be the command for the actuators. These digital signals are converted to analog signal by a digital to analog converter which is our modulating signal. Our carrier is 13.56 MHz signal [8]. The carrier undergoes amplitude modulation. The modulated signal is amplified and the output buffer provides a signal up to 1 A to the emitter antenna [8].
The emitter antenna is a coil made of four 7 cm diameter loops. The wire used is copper which is 1 mm thick. The impedance matching is done with the help of the RLC network [1]
2) Receiving antenna
The receiving antenna is the crucial part of this design. The size of the antenna should be in micro size but should be able to couple enough energy. The power transmission mainly depends on the Q factor of the antenna [1]. Higher the Q factor lower is the rate of loss of energy.
Fig2.2. inductive link between transmitter and receiver [8].
Consider the loop antenna L1 carrying a current i1 of radiant frequency w , and a secondary inductive loop antenna L2 realizing a RLC tank circuit a parallel capacitor C2 and a load resistor RL. The induced voltage u2 across L2 is given by [8]
Where k - coupling factor
r2- resistive loss
ω - Frequency of the tank circuit
From the above equation it is clear that we have to increase the coupling factor, reduce the serial resistance r2 maximize the ratio L2/C2.
The antenna is manufactured using the micro fabrication techniques. As the size of the system is a crucial factor we cannot increase the size of the antenna beyond a limit. To increase the value of Q factor of a planar coil we have to reduce the gap between the two turns (to increase coupling factor) and increase the width of the conductor (to reduce ohmic loses). When we increase the width of the coil, there is an increase in the frequency and three by increase in the serial loses. The parasitic capacitance is found to be increased when the spacing between turns is reduced.
Due to the above problems the antenna is a square coil without the inner turns. It is made of electroplated gold and is fabricated on an insulating material[8].
3) High voltage controller IC
This acts as the link between the receiving antenna and the actuators. The main functions of this block are [8]-
Amplification, rectification and limiting of the induced voltage
Clock recovery
Demodulation, digital processing of the received signal and driving of the actuators.
Fig.2.3- microphotograph of the IC [8]
3. Embedding a wireless transmitter using a Photo voltaic cell
The principle used here is the photo voltaic effect. Here we can transmit both power and data wireless. In the previous method of harvesting energy it was very difficult to fabricate the receiving antenna as the size of the antenna would be more than the micro robotic system itself. The receiving side is a photo voltaic cell. The light source that is the transmitter side is tuned to the monochromatic light frequency. According to some of the recent experiments we know that the 830 nm wavelength light is having the greatest penetration in human body without damaging the tissues.[9] The exposure time should also be calculated because over exposure can damage the tissues.
Fig 3.1- wireless transfer using local electromagnetic generation [9].
The above figure shows how we can transfer both data and power wireless using the photovoltaic cell. The light containing the photons falls on the photovoltaic cell which is converted into electrical energy. This electrical energy is used for the generation of a local electromagnetic field. This electromagnetic field is varied by the signals from the sensor. These signals are transmitted using the transmission pad.
In order to improve the efficiency of this design we use the energy from the photovoltaic cell to power up both the sensor and transmitting the signal. This can be done using a multiplexing technique [9]. At a time either the sensor or the transmission pad is functioning.
4. Chemical power for robots inside the capillaries
The micro robots inside the capillaries use the oxidization of glucose for power itself [11]. These robots use an external magnetic field or flagella movement to propel through the fluids. These robots can even use the circulation method to reach to the desired position [10]. These robots will be containing tanks and pumps to store and collect oxygen respectively.
Fig.4.1-robots inside the capillaries [12]
Glucose is available in plenty in our blood. The limiting factor is oxygen [10]. The reaction taking place between glucose (C6H12O6) and oxygen (O2) for the power generation is shown below
C6H12O6+6O2→6CO2+6H2O.
The output of this reaction is water and carbon dioxide. We can see that, we need 6 oxygen molecules for a reaction. Suppose the rate of absorption of oxygen is 'x', then the power generation is proportional to x/6.
As mentioned before oxygen is the limiting factor. One of the methods to collect oxygen is using pumps at the surface to absorb it and pass it to the reaction sites or else we assume that enough oxygen will be diffused through the surface that is the 'no pump' concept [10].
Some of the limitations of this design are-
It creates a local heating which is harmful to the nearby tissues
There is a competition between the micro robot and the tissues for oxygen absorption [10].
From the experiments 1 micrometer sized robots were able to generate several tens of Pico farad of energy [10].
5. Current status and achievement
On the current status and achievement in some fields, the use of harvesting energy has fulfil the powering of Nanorobotics and create a new method for supplying energy and controlling the movement of Nano robotics [17]. For example, some scientists use the inductive coupling for powering wireless and remote control of electrostatic actuators[8]. This research shows how to powering and control of electrostatic micro actuators successfully [19], this method is organized by two distributed micro motion systems (DMMS) for create the wireless, and distribute it for powering and remote the Micro/Nano robotics by inductive coupling [8].
Another achievement in the development of Micro/Nano robotics is the use of nanowire arrays and modifies it as new Nano generator which is creating mechanical motion [15]. The principle of this can be used for powering the Micro/Nano robotics motion.
Fig5.1. Nanowire arrays [14]
Another current status of the Micro/Nano robotics is the use of a piezoelectric vibration based generator to produce the AC voltage output and use this power for the moving of Micro/Nano robotics [14].
6. Possible current and future application
There are some possible current and future application relating with harvesting energy and Micro/Nano robotics. The new researchers is trying to develop the micro-generator and Nano-generator for generate electrical supply [13]. This instrument can change the sufficient ambient vibrations which are available to be an electrical current [14].
Fig.6.1. Micro-generator [20]
In the medical field, Nano scaled piezoelectric energy harvesting devices which are called as hybrid Nano powered and self-power systems are commonly used as Nano sensor systems. New technology in communication channels between multiple devices has successfully collaborated the sensors for micro motion system in the area of electromagnet field [18;20].
7. Future challenges
By the success of the research, there are many future challenges is face by the researcher. In the medical field, there are some difficulties to deliver microscopic robots and manage them to travel inside the body for special purpose such as deliver drugs, and monitoring the harmful material inside the body and others [16]. Another challenge is to drive diode-powered scalpels wirelessly by radio frequency electric fields which are not absorbed by the body [24].
8. Our contribution
By the all current condition and achievement above, our team is trying to contribute the new research in harvesting energy by using Nanoturbine, to harness energy from flow of blood. From the facts that speed of our blood through arteries (500mm/sec), veins (150mm/sec) and the size of the arteries (0.5-8.5mm), veins (0.5-2mm), capillaries (0.0062mm) [22], we are trying to use the blood flow power to spin the Nanoturbine and deliver the energy. The size of the Nanotubine is very small (nearly 1nm) [23]. This Nanotubine is connected to a Nano sized conductor within the robotic system. We can supply an external magnetic field. From the electromagnetic induction principle we know that a small current is produced in the conductor. The power established by this method can be used for various purposes, such as monitoring the blood vessel, measuring the blood power level, sensing and imaging any damaged cells or to remove some blocks.
9. Conclusion
By this evidence, we are concluding that the method for harvesting energy for the Micro/Nanorobotics depends on the application. Accordingly, each method is associated with a particular micro fabrication technique and researchers are still trying to fabricate these microstructures and harvest the energy efficiently. From the previous explanation, it can be seen that harvesting energy is an important part of Nano technology. The use of harvesting energy has support the development of micro or Nano robotic systems.
