Design of Miniature Antennas for Biomedical Applications

¶ … Miniature Antennas for Biomedical Applications

Most of the studies on microwave antennas for medical applications have concentrated on generating hyperthermia for medical treatments and monitoring several physiological parameters. The types of antenna implanted depend of the location. Besides the medical therapy and diagnosis the telecommunications are considered as significant functions for implantable medical devices those needs to transmit diagnosis information. The design of the antennas catering to MEMS and NANO technology therefore should be smaller enough with cost effective, low power consumption etc.

Research is going on since long in the field of development of wireless interfaces for environmental and biomedical sensor devices. CMOS and RF MEMS circuits, miniature antennas and sensor networking are now being explored. Complete process involving such elements is developed and is being experimented. Wireless interfaces are now being devised for neural probes, cochlear implants and for development of other biomedical devices like arterial stent monitors etc. Further different techniques are being explored for development of moderate range, moderate rate, and wireless communication to environmental sensors. Research activities are being continued for development of wireless circuits on the basis of RF MEMS and nanometer CMOS.

The experimentation of low-power CMOS radios for the Zigbee 2.4GHz sensor network standard is now considered to as a medium term objective. Introduction of RF MEMS assures radical developments in terms of power efficiency of RF circuits. RF MEMS enhances the high-Q of micromechanical devices. The functioning of RF MEMS devices is presently considered closer to that of off-chip quartz components. There are explorations of the circuit and process techniques that make possible the integration of RF MEMS and CMOS wireless circuits, along with signal processing and miniature antennas. Further activities are involved in the development of many projects for devising the many devices involving such techniques and for developing a low power wireless receiver for sensor applications. (Wireless Interfaces)

Discussion

The applications like home automation, industrial control and biomedical and environmental sensors necessitate low-power transceivers for short distance and low data rate wireless communications. Currently, the IEEE 802.15.4 standards body has prescribed a wireless communication standard that is beneficial for such applications. A project was undertaken by Wireless Interfaces Thrust for development of a transceiver design for low power wireless sensor networks. The objective of this project was to develop a low-power and cost effective receiver IC that is in compliance to such standards. Such a receiver has its application in the sphere of the environmental test-bed platform that is under development in the WIMS ERC. Such project had the motive of enquiring efficient power receivers and data telemetry circuits. This was significant for the applicability and dependability of both environmental sensors and implanted sensors. This involved implementation of two prototype transponders in 0.25µm TSMC CMOS.

The performance of entire transponder was monitored. The refinements in respect of designs were going on for a final generation of the transceiver that will incorporate on-chip data processing. The application of standard CMOS refers that the circuitry is well-matched with the broadest range of sensors and systems. This project was sponsored by the Engineering Research Centers Program of the National Science Foundation. Another project was undergoing for development of low power transmitter for Sensor Networks. The objective of this research was to devise a wireless transmitter for a low power sensor node compliant with IEEE 802.15.4- Zigbee Wireless standard. It concentrates on the low power, small area device for a remote sensor. Till now, the primary concentration of the wireless industry was on communication with high data throughput. This however, has ignored a wide range of applications like remote sensors that necessitated simple wireless connectivity with relaxed throughput and latency. (Wireless Interfaces)

Such transmitter architecture functions direct modulation with variation of a phase-locked loop -- PLL divide ratio. Any of the frequencies between the two or more divider ratios can be selected. With a view to ensure an effective application in neural prostheses with larger number of stimulating sites a wireless neural stimulating micro-system has been developed. Such micro-system can effectively utilized in applications like auditory, visual, spinal cord, and deep brain stimulation prostheses to restore peripheral and central nervous system. The objective is to develop a modular 1024-site wireless 3-D micro-stimulating array with 128 simultaneous stimulating channels, each capable of sourcing ±100A. The project applies full integration, new current driver circuits, low power circuitry, and novel modulation/demodulation techniques for the purpose of low power high rate data transfer.

It has also developed a miniature system for reviewing the movements and inertial forces associated in sports and sports equipments by measuring all acceleration and angular rotation components during their application. Such a tiny wireless system involves commercial MEMS inertial sensors for measuring inertial data about al three axes. Sensor data is recorded amplified, multiplexed, and transmitted out to an external receiver applying FM telemetry. It involves application of two 2-D accelerometers -- ADXL210E made by Analog Device having a sensing range of +/-10g and three 1-D gyroscopes -- KX210 manufactured by Kionix having a range of ~700°/sec to quantify the fast rotation of a golf club in 0.1sec. An analog multiplexer accords sensor signals through low-pass filters with 50Hz bandwidth to deter from the interactions between the sensors and multiplexer. (Wireless Interfaces)

There is the process of transmission of multiplexed signals by wireless means to an FM receiver and the received data is fed to a laptop and assessed by MATLAB. The developed system is calibrated and several experiments applying the system have been implemented. The measured performance of the system is then measured with the same parameters applying a commercial optical device. The result predicts the system to be much dependable. A project is undergoing for development and demonstrating of a relatively wide band miniaturized slot antenna that can entail a high-impedance match to the bank of micro mechanical filters chalked out for the front end of the wireless interface. The antenna is so designed as to be in line with the Zigbee standard and also adaptable to the high impedance or standard load.

Moreover, it remains to be comparatively, high efficiency and small size, embracing an area no larger than 1cm2 at 2.4GHz. The designing of such an antenna involves three essential phases. Firstly, a technique for designing high-efficiency miniaturized slot antenna capable of conserving as much power as possible is devised. As the second phase it involves the design of the bandwidth and efficiency of such an antenna when it is placed above a ground plane. In the third phase in input impedance of the antenna is enhanced so that it can be matched to a micro-machined disk-resonator filter. The first phase is attained through design and fabrication of a number of inductively and capacitive loaded slot and printed wire antenna. (Wireless Interfaces)

MEMS the acronym of micro-electromechanical-systems technology has been infused into varied fields including RF, optoelectronics, and biomedical applications, MEMS research and development has been evolving for decades through out the globe. The pharmaceutical industry is applying the MEMS devices increasingly for experimentation of new drugs. The blood screening sensors that are applied for complete lab tests at bedside are considered another potential medical application. The biomedical applications of MEMS technology include networks of channels, pumps, valves and mixers for analytical devices. MEMS can also be applied as molds for plastic microfuidic parts. The MEMS devices are thought of for designing miniature surgical tools fluid dispensing heads and drug delivery and implantable sensors. The MEMS also miniaturize the RF components. (Lilliputian Machines Set To Revolutionize RF, Optoelectronics, and Biomedical Applications: MEMS in Biomedical Applications)

The wireless industry is confronted with a number of tough design challenges. A3G smart phone, PDA or base station could necessitate as many as five radios for TDMA, CDMA, 3G, Bluetooth and GSM. Such supplementary features generate and enhancement in component count. However, at the same time the industry must satisfy consumer demand form factors, low costs and reduced power utilization. The MEMS-based RF switches utilizes the proprietary membrane process to generate a low loss, low-power device. The RF switch contains a movable metallic membrane. With application of an electrostatic force the membrane is pulled down to complete the circuit.

Microfluidics a MEMS technology facilitates the fabrication of networks of channels, chambers and valves to regulate the flow of liquid in amounts as small as one picoliter. Such systems have less moving parts and necessitate little assembly. They are benefited by the physical phenomena like electro-osmosis, dielectro phoresis and suface interaction effects. Micralyne makes the microfluidic Tool Kit, a user configurable instrument which is being applied in the corporate and academic research laboratories for desired bio-analytical applications in protein, DNA and cellular analysis. Data Knife, a set of surgical tools has been introduced by the Verimetra that involves sensing and measuring devices. The Data Knife incorporates sensing and data gathering capabilities on the edges of several surgical tools. (Lilliputian Machines Set To Revolutionize RF, Optoelectronics, and Biomedical Applications: MEMS in Biomedical Applications)

Such instruments are capable of differentiating tissues like cartilage, bone, muscle, and vascular and also are useful in measuring tissue properties, inclusive of density, temperature, pressure, and electrical impulses. The MEMS technology has developed several devices for biomedical applications. The Microfluidics makes it easier the design of networks of channels, chambers and valves to regulate the flow of liquids in amounts as minute as one picoliter. Such systems have few moving parts and necessitate little assembly. They entail the potentiality to miniaturize analytical equipment that applies expensive chemicals and DNA samples. They take the benefit of the physical phenomena like electro-osmosis, dielectro-phoresis and surface interaction effects. It involves generation of a Electrokinetic flow when the electrodes attached to computer driven power supplies are placed in the reservoirs at each end of a channel and activated to produce electrical current via the channel.

Under such circumstances fluids of the suitable kind will move by a process known as electro-osmosis. Typical flow rates inside the channel are about a millimeter per second and the flow rate can be regulated by means of a high degree of precision. The occurrence of Electrophoresis in the microchannels is another electrokinetic phenomenon. This involves the movement of charged molecules or particles in an electric field. The Electrophoresis has its application to move molecules in solution or to separate molecules with very subtle differentiations. The molecular world is connected through the regulation by light. Fascinating quantum behavior comes out of the fact that molecular design is at the scale of wavelength of light, to illustrate, quantum dot lasers that emit light and band gap crystals that enable to switch light.

Arryx fabricates 10000 tweezers which are independently controllable and that can manipulate molecular objects in three dimensions like move, rotate, cut, place etc. All these are from one laser source which passes by means of an adaptive hologram. (Lilliputian Machines Set To Revolutionize RF, Optoelectronics, and Biomedical Applications: MEMS in Biomedical Applications) An ultra low power, high quality and high frequency micro-electro mechanical -- MEMS resonator for wireless communication and signal processing can be designed that has the features of smaller size, lower cost, higher reliability and integrability. This would have the advantages of avoiding the need for nm gap in electrostatic transduction, better power handling capability, and improved impendence which match with electronics, enhanced potentials to reach 10 GHz and above. (Low-Power RF Wireless Power RF Wireless Communication)

Passive telemetry is being adopted in biomedical applications, in that sensing and data acquisition electronics are implanted in animals to assess physiological parameters and transmit the data to the base station. As in tele-identification systems, the power of the sensors and electronics is provided to the implanted unit by radio transmission in the ISM band and got by a small coil on the unit. As a result of the confined physical size of the coil the received power is very low, in our case less than 1mW, for a reading distance of a few meters. Taking into consideration the data acquisition and transmission circuitry with low power consumption is hence extremely significant. As the available power is not adequate for conventional radio transmission, the digital data acquired by the data acquisition unit is transmitted by absorption modulation. (A 0.5mW Passive Telemetry IC for Biomedical Applications)

In other words the reflection index of the implanted coil is differed at the instants of data transmission by shorting the coil that results in a glitch in the reflected waveform visualized by the base unit. The very low power requirement indicates that the implanted unit must be meticulously optimized for low power at both the system level and the circuit level. Particularly, in the way sensors are powered, instrumentation amplifier and filter are implemented, type, speed and resolution of A/D converter along with an effective voltage regulator. The passive telemetry IC includes a "low noise low offset instrumentation amplifier, a low pass notch filter and a 9-bit A/D converter along with on-chip clock generator, elements of the RF-DC converter, band gap reference and supply voltage regulator and the needed regulatory switches and logic for powering the sensor and modulating the reflection index." (A 0.5mW Passive Telemetry IC for Biomedical Applications)

The data acquisition part reads the sensor output and helps it in converting into a 9-bit digital signal. One bit per system clock cycle is transmitted to the base unit by shorting part of the RF-DC converter. The base unit clock is phase-locked to the clock of the implanted IC, with the assistance of a synchronization pulse prior to transmitting each sequence of 9 bit data. The integrated circuit is achieved in a 2?m 40V BiCMOS technology primarily to take the benefits of the 12V zener diodes as protection devices at the input of the voltage regulator. The IC is designed for the purpose of interfainge one of the sensors which is known as magneto-resistive sensor bridge for blood pressure measurement and the bridge resistance is 1.7k? (A 0.5mW Passive Telemetry IC for Biomedical Applications)

Nano-electromechanical systems entail the benefits of small size, lower cost, lower consumption of power, low mass, higher reliability and lower maintenance costs on both the system along with the component levels. Mechanical machines have been devised and fabricated those are integrated with microelectronics at the micron scale. New device concepts incorporate but are not confined to the "integration of micro-optics components, miniature signal processing devices, biomedical/genome processing devices, miniature electromechanical wireless components, miniature opto-electromechanical devices, miniature biosensors and environmental sensors and microfluidic devices." (Creatative ideas and exciting applications with challenging scientific breakthroughs) At a miniature scale, the prospective applications of Carbon Nanotubes -- CNT for biomedical instruments are infinite. CNT features unique properties that incorporate very high mechanical strength, high thermal conductivity, effective chemical and thermal CNT/nanotechnology research thrust concentrates on developing novel designs and fabrication concepts based on CNT/nano technology for next generation devices. (Creatative ideas and exciting applications with challenging scientific breakthroughs)

There is a growing inclination towards development of ultra-miniature and low-power sensor Microsystems for application in medical diagnostics, environmental monitoring and other industrial applications. The ultra-miniature sensor micro system is required to contain a large diversity of complex electronics equipments, inclusive of "sensor interfaces, signal conditioning, a microprocessor core, digital signal processing and wireless transmission technology." A system on-chip technology caters to the design and implementation methodology that entails low cost and low form factor and low power consumption that facilitates rapid design of many intellectual property blocks. The system leads to development of a multifunction micro system associated with micro-electro mechanics, laboratory-on-a-chip, micro-fludics and biochemical sensor technology. The sensor micro-system consists of an application specific integrated circuit with sensor interfaces analogue and digital system, a radio uplink to a base station and power source. The micro-system has therefore a simplex communication link to a base station that can manage data from various capsules. The figure given here depicts function blocks of the prevailing prototypical micro-system. (An Integrated Sensor Micro-system for Industrial and Biomedical Applications)

The capability of measuring physiological parameters is considered a crucial element in the sphere of effective medical diagnosis and treatment. The correct measurement of biological values like pulse rate, respiration, blood oxygenation and glucose levels are resorted to by the physicians to accurately diagnose and detect most of the illnesses and conditions. Such quantified values are normally applied in the course of treatment sometimes to assess the conditions of the patients or to guide a practitioner's hands. The sensors have since long been applied in respect of quantifying and monitoring a broad range of physiologic parameters. All the sensors have the same primary functions of converting one type of measurable quantity into a varied but equally quantifiable value, normally an electrical signal. Even though the basic function continues to be the same, the technologies applied to perform that function differs greatly. The prospects of sensor technology are much dependent upon the present development in miniaturization and micro-arrays. Development of 'smart devices' and intuitive systems for patient treatment are also dependent upon the progress being attained in the field of sensor technology. The micro-fabrication processes of non-silicon, non-traditional materials need to be developed or improved based on silicon-based processes. It is worth emphasizing the significance of adopting array for detection is desirable for biomedical applications. (Sensor Advances Spur New Diagnostic, Therapeutic Tools)

Biotelemetry indicates a remote method of quantifying the biologic information through cable, mechanical means or wireless. Normally it represent transmission by radio wave ever though it involves applicability of every portion of the electromagnetic spectrum. Till now the activities are to transmit the data from units external to the body using implanted or external sensors. With the advancement of technology it has become possible to devise the integrated circuit techniques and improved packaging and construction methods, systems are being developed that may be completely implanted within the body or swallowed. By means of biotelemetry growing cases accurate measurements are being made in animals and man without confinements from encumbering wires or interference with physiologic function. The recordings have been attained in man while at work, performing normal functions, sleep, watching astronauts, electrical surveillance of the sportsman and critically ailing patients. (Telemetry Methods: Animal and Man)

Micro miniaturization and integrated circuit designs were at first adopted in systems for the simple registration of ECG or deep body temperature. However, recently more sophisticated units have been made "to measure blood pressure, blood flow, flow velocity, dimensions and gaseous or chemical content of tissues." (Telemetry Methods: Animal and Man) The completely implanted units with data transmission by means of the intact skin have their greatest benefit with long-term experiments and have entailed intermittent or contined recordings of single or multiple variables. Some systems like swallowable transmitters entail data unavailable by any other means. It provides enough liberty to the implanted animals and also range of motion and necessitates no post-implant attention. The physiology can be followed over the course of each single day and during any important event along with eating, sleep, normal course of social interaction or change in environment. With persistent improvements in design and construction they assure to become the primary means for adopting the course of certain induced or naturally occurring disease process like hypertension or coronary artery disease, documenting drug physiology and pharmacology and recording the mechanisms of adaptation to different stresses, like long-term space flight and the manner of recovery after any sort of exposure.

However, it suffers from several disadvantages like high cost, necessity for careful circuit design to keep the power requirement to its minimum the concern for transducer and component reliability as the adjustments cannot be easily made after units are placed inside the body. Generally, a number of frequency bands are adopted for wireless transmission. The very low frequencies such as 500 KHz to 10 MHz do not propagate inside or outside the body. However, very short-range telemetry applying small laboratory animals like rats. The FCC has however, authenticated the adoption of the FM entertainment band with a range of 88 to 108 MHz for lower-powered telemetry applications, remarkably developing range and lowering cost since commercial equipment is easily available. Even though small 0.75-inch diameter antennas are implanted, low-powered implanted transmitters have generated ranges up to 100 feet. In common laboratory applications within a range of 5 to 15 feet these transmitters generate strong signals to come across all except the strengthened FM stations however, become marginal above 50 feet as a result of station interference. The back-pack transmitters supported by larger replaceable batteries and containing effective antennas could have ranges of several miles. (Telemetry Methods: Animal and Man)

A project is being developed to devise a new generation of a retinal prosthesis to become advantageous for the totally blind affected by Retinis Pigmentosa -- RP or Age-related Macula Degeneration -- AMD. The prevailing retinal prosthesis system is normally consisted of two units: extra ocular that is used for capturing imaging information outside the human eye and transferring this within the eye itself and intra-ocular which is responsible for processing the information that is received from the extra-ocular unit and thereby stimulating the retina by means of an array of electrodes. Research is going on for achieving highly improved data telemetry link by means of implantable miniaturized microstrip or dielectric antennas functioning at microwave frequencies. This proposed high frequency telemetry link will entail large bandwidth to transmit the data between external and internal units, thus permitting the increasing number of stimulating electrodes on the surface of the retina than is available presently. Solutions associated with hybrid systems that isolate the data transmission from the power delivery frequencies are to be found out. The system is proposed to be designed so as to attain minimal electromagnetic and thermal deposition in the human head and eye. (Wireless Integrated Microsystem for a New Generation of a Retinal Prosthesis to Benefit the Blind)

The specific objectives in this regard are to devise numerical electro thermal simulation models and methods for the design of the wireless system and the assessment of the performance of the whole implant. So as to devise and optimize new miniature data transmission antennas at microwave frequencies for implantation in the proximal region of the anterior chamber of the human eye, attributing to the effects of the insulating materials and heterogeneity of the human eye physiology. The project also aims at assessing the efficacy in terms of power and thermal deposition in the human body of the telemetry systems, involving hybrid low- and high -- frequency systems, so as to attain an optimal implantable device that will enhance the comfort of the patient. The research is to devise a device expected to benefit the life of the millions of people globally affected by blindness induced by Retinitis Pigmentosa or Age-related Macula Degeneration. (Wireless Integrated Microsystem for a New Generation of a Retinal Prosthesis to Benefit the Blind)

The propagation models have significant role to play in devising wireless communication systems. Present developments in the sphere of semiconductor technology have made it easier to implant a network of bio-sensors within the human body for purposes of health assessment. The tissue medium functions as a channel by which the information is transmitted as electromagnetic radio frequency for the wireless communication inside the human body. A propagation model is necessitated so as to find out the losses associated in the form of absorption of EM wave power by the tissue. Normally the free space formula is adopted with a higher loss coefficient in the sphere of fading channels in mobile communications. In order to estimate the propagation loss for wireless communications within the human body, varying the loss coefficient only varies the rate of decrease in power and does not assist in assessing the total loss in the form of absorption. (Towards a Propagation Model for Wireless Biomedical Applications)