Sensors and lab-on-chip
The design and implementation of small and smart wearable and implantable data acquisition systems in e.g. medicine and robotics require cost-efficient and energy-efficient, yet reliable, sensing and communication solutions. The overall research directions include microfluidic-based lab-on-a-chip (LoC) systems for point-of-care and need-of-care applications (e.g. for the detection of pathogens, mastitis) (Click here for the lab-on-chip page), and sensors based on e.g. wideband technology, wired (galvanic electrode systems) and wireless (radio, radar, eddy current) excitation systems with the development of electrical circuits for high sensitivity differential electrical impedance spectroscopy (EIS) measurement and for amplitude limiting in the current-to-voltage converter part of the EIS measurement setup. Click here for the impedance page.
Moreover, these above topics are to be strategically combined with innovative techniques for sensor signal processing like generation of excitation signals, e.g. the analysis of the dependence of the electrical model fitting accuracy on the number of the excitation frequencies as well as enhanced iterative algorithm for the crest factor minimization of multisine, and IoT-based communication, specifically the smart and compressive signal processing as well as the cognitive communication topics mentioned in the two previous points. Time-frequency domain Fourier and wavelet/chirplet binary-ternary transforms.
Lab-on-Chip webpage: https://ttu.ee/institutes/thomas-johann-seebeck-department-of-electronics/labonchip/
Examples of PhD theses related to this research area:
- "Development of multiscale smart surfaces for neuromorphic applications" by Rauno Jõemaa. Expected 2022.
- ''Development of Bioimpedance Based Method and Low Cost Instrument for Non-invasive Tissue Characterisation with Eddy Currents" by Hip Koiv. Expected 2021.
- "Methods for Agent Detection in Lab on Chip Applications" by Kaiser Pärnamets. Expected 2021.
- "Development of sensors based on wide bandgap heterostructures" by Udayan Sunil Patankar. Expected 2021.
- "Numerical Characterization of Interface Layer Between Metal Film and Wide Bandgap Semiconductor" by Mehadi Hasan Ziko. Expected 2021.
- "Simulations of Silicon Carbide (SiC) and Graphene based Novel Semiconductor Devices for Power Electronics and Gas Sensor Applications" by Muhammad Haroon Rashid. Expected 2021.
- "Microheating Solution for Molecular Diagnostics Device" by Tamas Pardy. Defended in 2018.
- "Wearable Solutions for Monitoring Cardiorespiratory Activity" by Margus Metshein. Defended in 2018.
Development of multiscale smart surfaces for neuromorphic applications
PhD student: Rauno Joemaa
Supervisors: Tamas Pardy, Toomas Rang
PhD thesis defence expected in 2022
Neuromorphic engineering is a novel interdisciplinary field, focusing on the design of biomimetic artificial neural systems that emulate the electrical activity of biological neural networks. These systems can greatly help in the development of neuroprosthetics and help better understand the human nervous system. Neuromorphic circuits of the past employed VLSI electronics, using purely electrical signals to communicate to live neurons in a hybrid system.
Microfluidic artificial neurons on the other hand are a novel approach that can mimic the internal electrochemical processes of live neurons, and therefore offer a great interface between silicon-based electronics and living cells or can replace the live cells entirely. The development, and potentially, networking of microfluidic artificial neurons brings together the knowledge from various domains: primarily electrical engineering and neurobiology, secondarily physics and chemistry.
An artificial microfluidic neuron must have an ion-selective membrane similar to the neural cell wall and must model the ion exchange processes characteristic of neurons, that produce an action potential. As the modelling of different microfluidic solution for producing electrochemical effects comparable to those of the natural action potential are explored using the Finite Element Method (FEM), the results are best proven in direct contact with the live cells.
Stable cellular incubation to test biomimetic concepts or conduct pharmacological studies requires an application which falls in the bounds of an Organ-on-Chip (OOC). Such a device can be advantageous in the pharmacological studies by creating an option to test neurological effects without animal test subjects or bypass the animal stage entirely and use human neurons instead, broadening into personalization.
In order to determine whether communication between a biological neuron and a microfluidic biomimetic analog can work in both directions or one which requires the use of multiple technological solutions, will be considered as different smart surfaces are evaluated. The thesis will take steps to include exploring an automated microfluidic incubation system utilizing immobilization methods which enable keeping live neurons active and in short reach of continuous external influence and monitoring.
One of the main approaches looked at is found in the largely underdeveloped area in microfluidic reactors - the patterning and deposition of electrospun nanofibers. Electrospinning is a method of nanofiber deposition, which is cheap, easily scalable and technologically simple to implement, but provides robust polymeric fiber meshes that intrinsically resemble the extracellular matrix (ECM).
Fig. Simulations of magnetic microfluidic valve actuation. Images A and B defining open-closed valve positions. The paired images below represent the corresponding measured electrochemical potential.
Development of Bioimpedance Based Method and Low Cost Instrument for Non-invasive Tissue Characterisation with Eddy Currents
PhD student: Hip Koiv
Supervisors: Mart Min and Alvo Aabloo (University of Tartu)
PhD thesis defence expected in 2021
Check out Hip's video here: https://youtu.be/HdiOXbGlDn4
High blood pressure (hypertension) is a primary risk factor for different cardiovascular diseases and keeping it under control is vital to reduce the risk of many dangerous heart conditions. Brachial cuff sphygmomanometer is widely used to assess the pressure parameters, but we have known for over a half century that brachial pressure is a poor surrogate for central aortic pressure (CAP). CAP represents the true load imposed on the heart and large arteries, but there are many reasons why we are still so stuck in old methods (i.e., lack of proper guidelines and standards for alternative technologies).
Our impedance workgroup is developing a wearable device that measures bioimpedance from the wrist (on radial artery) and estimates blood pressure in the aorta using transfer functions (Figure 1).
Figure 1. Bioimpedance variations are registered from the radial artery with four-electrode configuration. From peripheral radial pressure wave it is possible to get central aortic pressure (CAP) using transfer function (TF).
For the measurement of small bioimpedance variations, we need reliable electrodes that enable low electrode-skin impedance. For a wearable device, similar to a wristwatch or a wristband, electrodes have to be dry or semi-dry and reusable. Unfortunately, it is quite a challenge because bioimpedance method requires a small excitation current flowing through the body and two electrode-skin interfaces as well. Hydrogel between the commonly used gel electrode (Ag/AgCl) and the skin can reduce this interface impedance but these commercial electrodes do not have reusable nature.
My research focuses on soft, flexible and dry (and semi-dry) electrodes that are based on silicone (PDMS), carbon nanofibers (CNF) and carbon fibers (CF). Longer carbon fiber strands that stick from the base material reduce the electrode-skin interface and make a better contact with the skin than the smooth texture (Figure 2).
Figure 2. A and B – side view of the CNF/CF-PDMS electrode and carbon fibers sticking from the base material. C – top view of the electrode.
In addition to dry electrodes I am working out semi-dry electrodes together with Tartu University IMS (Intelligent Materials and Systems) lab. These novel electrodes are based on ionic liquids (as electrolyte) and carbon textile or carbon/silicone composite. Ionic liquids have very low vapor pressure making the surface hold its moisture for a long time. This means that the electrolyte layer does not dry out as usual hydrogel does, which is very beneficial when making reusable electrodes. In addition, to test and validate the new electrode material, custom tissue phantoms are developed. Phantoms are expected to have tissue similar dielectric properties and precision across the band of interest and reasonable shelf-time to mimic the real testing situation.
PhD thesis defence expected in 2021
Methods for Agent Detection in Lab on Chip Applications
PhD student: Kaiser Parnamets
Supervisor: Tamas Pardy
PhD thesis defence expected in 2021
Development of sensors based on wide bandgap heterostructures
PhD student: Udayan Sunil Patankar
Supervisors: Ants Koel, Tamas Pardy
PhD thesis defense expected in 2021
Heterostructures have become essential constituents of most advanced electronic devices. These structures are well suited for high frequency and fast switching digital electronic applications. Heterostructures are of great interest because; motion of charge carriers can be controlled by modifying energy band profiles of constituent materials. During the past few years, heterostructures based on silicon carbide (SiC) polytypes have come to prominence due to their promising physical and electrical properties. Very important efforts have been made in the last decade on the development of wide band gap materials. It is crucial for any industrial development to produce large-size materials with good quality at a reasonable cost.
Unfortunately, crystal growth of these refractory materials is difficult. The SiC has the potential to be obtained using very high temperature sublimation, PVD or CVD techniques. Silicon carbide (SiC) material possesses excellent physical robustness, chemical resistance and multiple options for smart devices through its electrical, chemical and optical properties It is also an ideal surface for developing another important material like graphene, with superior physical, chemical and electrical properties. Silicon carbide heterostructures fabricated from their most popular polytypes, 3C-SiC, 4H-SiC and 6H-SiC have high value of breakdown voltages and hole mobility. They are extensively used as power electronic devices, sensors and light emitting diodes.
Attractive properties and a wide range of applications of SiC heterostructures are attracting researchers to explore the use of SiC heterostructures in the field of sensorics. The objective research work is to understand the different characteristics of a wide band semiconductor heterostructures devices through thorough device simulations ie how they behave electrically in different conditions and problems associated with its fabrication. Investing in heterostructures can improve sensing capabilities of wide band SiC semiconductor like particle sensing, vibration, UV light sensing, emitting possibilities etc. under different conditions.
Figure 1. (a) (b) Device Structures and (c) Energy band diagram of SiC / Si nn Heterojunction
Figure 2. Device IV characteristics
Figure 3. CV Measurement of 6H-SiC 3.2mm 2 Substrate 80K to Room Temperature
Numerical Characterization of Interface Layer Between Metal Film and Wide Bandgap Semiconductor
PhD student: Mehadi Hsan Ziko
Supervisors: Ants Koel, Tamas Pardy
PhD defense expected in 2021
One of the research tasks in the semiconductor industry is inexpensive manufacturing of heterojunctions and reliable contacts. It is not very common technology to join the different material layers (like different SiC wafers and metal contacts to them) using direct bonding. Traditional sputtering and evaporation technologies have a number of drawbacks, naming for example cost, many processing steps, time-consumption, realization of metal layer with homogeneous thickness over a large area of contact, restrictions for the general thickness of the metal layer over the whole contact, etc.
This direct bonding will reduce the fabrication cost of the devices, improve their electrical properties, and solve so many processing problems. Direct bonding is possible with diffusion welding (DW) technology that has been proposed as the first topic in my research work. Generally, the DW technology gives the possibility to improve some quality features of contacts and improve the realization of metallization process. The drawbacks unfortunately also lie in quality - achieving defect less surfaces of two hard materials (like two SiC wafers). Shear micro deformation can take place during DW (at applied high pressure and temperature), in a subcontract surface layer of SiC.
During the development of metal contacts for p-SiC substrates, it has been observed that about 5 nm - 25 nm amorphous layer develops between the metal film semiconductor surfaces, which seems to influence the electrical characteristics of the Schottky structures. These traditional contact challenges in electronics and semiconductor structures are influencing the performance due to contact quality and reliability and finally influence on the device performance.
So, it is necessary to develop well-accepted understanding of the mechanism and hence the need to investigate the reasons missing behind this influence. Therefore, one of the main tasks (and method) is in the development of numerical models for such a multi-layer interface and try to develop the acceptable physical explanation of the behavior of for Metal-p-SiC interfaces.
Additionally, realization of large area contacts with experimental Schottky barrier diodes for power semiconductor structures is technologically enough tricky activity. The influence of such a layer improves on the base of our measurements specifically the forward characteristics of the structure.
Fig. Simulated and real Schottky barrier diode structure and its current- voltage characteristics.
- Ziko, Mehadi Hasan; Koel, Ants (2019). Theoretical and Numerical Investigations on a Silicon-based MEMS Chevron type thermal actuator. 2018 IEEE 18th International Conference on Nanotechnology (IEEE-NANO)
- Hasan Ziko, M.; Koel, A. (2018). Design and Optimization of AlN based RF MEMS Switches. IOP Conference Series: Materials Science and Engineering, 362: 2018 International Conference on Smart Engineering Materials, ICSEM 2018
- Ziko, Mehadi Hasan; Koel, Ants; Rang, Toomas (2018). Numerical Simulation of p-type Al/4H-SiC Schottky Barrier Diodes. Proceedings of Baltic Electronics Conference BEC2018: 16th Biennial Baltic Electronics Conference BEC2018
Simulations of Silicon Carbide (SiC) and Graphene based Novel Semiconductor Devices for Power Electronics and Gas Sensor Applications
(Formerly known as Characterization of Novel Photovoltaic Materials and Devices)
PhD student: Muhammad Haroon Rashid
Supervisor: Toomas Rang, Ants Koel
PhD defense expected in 2020
The goal of the project is to work in two different regimes to develop novel semiconductor devices for power electronics and gas sensing applications.
The first part of the thesis is the simulations of SiC based heterojunction devices (power diodes and LEDs) considering a novel technique called diffusion bonding. Heterostructures are of great interest because they give rise to interesting electrical and physical properties in the resultant semiconductor devices due to change of the semiconductor material at the heterojunction interface.
During the past few years heterostructures based on silicon carbide (SiC) have gained significant importance for power electronics application due to their high value of breakdown voltages and hole mobility. They are extensively used as power electronic devices, sensors and light emitting diodes. Currently, the most promising techniques with predicted results and stable yield for growing epilayers of SiC polytypes for the fabrication of electronic devices have been chemical vapour deposition (CVD), liquid-phase epitaxy (LPE) and molecular beam epitaxy (MBE).
These techniques are complex and require sophisticated apparatus and skills. Fabrication of SiC based heterostructure device with diffusion bonding will simplify the fabrication efforts and reduce the cost of the resultant devices. Micro- and nano-scale SiC based devices have been simulated with Silvaco TCAD Software and Quantumwise Atomistix Toolkit(ATK) respectively.
The second part of the project is to simulate graphene based gas sensors to detect a large variety of hazardous organic and inorganic gases in domestic and industrial environments to avoid accidents. The change in the electric current through the device in the presence of the target molecules is used as a molecule detection mechanism for the simulated sensors. A nano-scale semiconductor device simulator, Quantumwise Atomistix Toolkit has been used to simulate graphene based gas sensing devices.
Figure 1: Geometry of 3C/4H-SiC based NN-heterojunction diode simulated in Silvaco TCAD Software
Figure 2: 3D-view of 4H-6H/SiC nanoscale device with semiconductor electrodes simulated in ATK
Figure 3: ATK view of simulated structure of gas sensor
Microheating Solution for Molecular Diagnostics Device
PhD student: Tamas Pardy
Supervisors: Toomas Rang, Indrek Tulp, Ants Koel
Novel molecular diagnostics devices, primarily nucleic acid amplification tests (NAAT) mandate the use of precise temperature control. Labon-a-Chip (LoC) devices are self-contained molecular diagnostics devices, meaning they rely on no additional external instrumentation, thus the term noninstrumented NAAT (NINAAT). Integrating microheating into a noninstrumented molecular diagnostics device is challenging due mainly to the restrictions on cost, space and power. This limits commercialization efforts and therefore widespread use of rapid tests that would otherwise help decentralize clinical diagnostics, reduce waiting times for patients and healthcare costs in general.
This PhD work proposes a workflow and methodology for the development of temperature control for non-instrumented molecular diagnostics devices and demonstrates its application to a LoC NINAAT device, called the InTime NINAAT. In its first part the work details the evaluation process for temperature control options as well as describes the evaluation of four microheating options, namely chemical heating, self-regulating electrical heating, thermostat-regulated electrical heating and thermoelectric heating. For the evaluation, physical and simulated thermal models are constructed. These thermal models are simplified representations of the molecular diagnostics device in development and are used solely for thermal analysis. The work proposes self-regulating heating and thermostat-regulated heating for use as integrated microheating candidates and demonstrates that both are capable of temperature regulation in the specified target range with 0.5 °C steady-state error (SSE) in the LoC NINAAT system being developed.
The second part of the work describes the process by which the proposed microheating candidates are developed further and prepared for integration with the molecular diagnostics device. This part describes how to evaluate the candidates for multiple assay targets (demonstrated in the work through isothermal NAAT protocols) as well as in-detailed thermal analysis for a single assay target (the LAMP protocol in this work). Both microheating candidates developed for the NINAAT platform were demonstrated compliant with assay requirements and could maintain target temperatures for the LAMP protocol in 95% of the reaction volume in steady state as calculated from simulated thermal models. The self-regulating heating candidate was chosen as the final solution due to its simplicity and lower cost compared to the alternative. A microheating solution was proposed based on a polymer resin PTCR self-regulating heater for use with the functional prototype of the NINAAT microfluidic chip. The heating solution was demonstrated to be capable of maintaining reaction temperatures in the required 60-63 °C range for 20 minutes powered by 2xAAA alkaline batteries and to reach the target range in 10 minutes. From the simulated thermal model, it was calculated that about 85% of the reaction volume was in range in steady state. In a final experiment the functional prototype NAAT chip along with the microheating solution were demonstrated capable of executing the LAMP protocol and successfully detected the target DNA.
The final part details the integration of the microheating solution into the functional prototype of the developed molecular diagnostics device. In this part the proposed microheating solution is integrated into the InTime NINAAT functional prototype device, complete with all fluidic and user interface functions. The InTime NINAAT device is a self-contained non-instrumented NAAT platform that carries a DNA amplification workflow from sample input to result output. After integrating the heater and a final compliance check, the device is demonstrated to work, as it would be used by the end-user.
Clockwise: Developing microheating candidates into microheating solutions for use in the functional prototype molecular diagnostics device; Thermal modelling for the core chip functional prototype indicated about 85% of the reaction volume was in the defined target temperature range; Thermal model for the core chip functional prototype (a) including a frame, electrical connections (b) and a heater (c); LAMP testing in core chip functional prototype (left) resulted in successful DNA amplification (right). Source: Tamas Pardy, Microheating Solution for Molecular Diagnostics Device, PhD thesis, Tallinn University of Technology, 2018
Wearable Solutions for Monitoring Cardiorespiratory Activity
PhD student: Margus Metshein
Supervisors: Paul Annus; Mart Min; Alvo Aabloo
The principle objective of this thesis is to propose the knowledge of the means for providing the best possible signal that contains the information of cardiorespiratory activity. Cardiorespiratory signal carries useful data of the condition and status of the subject: heart rate, respiratory rate, the condition of cardiovascular system, the presence of lung water etc. In order to gain access to cardiorespiratory activity, the goal of implementing the components of wearable experimental systems, based on capacitive, inductive and resistive coupling, was established and fulfilled.
The work is motivated by the need for wearable and unnoticeable solutions for monitoring the defined volumetric changes in an organism. The trend of continuously following the physiological parameters, either in everyday life for personal body condition monitoring, or as a medical observation of patients in the risk groups of some aggravated illness, is demanding the means for implementing the assigned tasks. Although the effect of motions is moderately researched, the techniques based on electrical methods for monitoring cardiorespiratory activity in remote are strongly affected, proposing an obstacle in the development of sensors. Currently, there is a lack of knowledge for the positioning of electrical sensors relative to body in order to monitor cardiorespiratory activity.
The principle of capacitive, inductive and resistive coupling is conceptually the same: by using some sort of electrical stimulation, the object is excited and the response in the form of an electrical signal is measured. The sensitivities and the relevant current distribution, affected by the volumetric changes, is expected to influence the measured response. The most vulnerable part is the interface between the sensors and the object i.e. coupling, which is capable of causing interferences that corrupt the interesting signal. This thesis deals with the following: the schemes of sensors for providing the signal of cardiorespiratory activity, the effect of positioning sensors on the surface of the body on relative organs, the effect of concurrent movements and the variations relative to cardiorespiratory activity.
The achieved results show the significance of the exact positioning and the need for aware choice of configurations and placements of electrodes. The solution of utilization of large area electrodes in capacitive connection for mapping the thoracic surface in order to measure the EBI for monitoring cardiorespiratory activity, constitutes a relatively new field and the results give grasp to the future of contactless monitoring.
The effect of two essentially different shapes of inductively coupled coils on the measured signal was experimentally determined, from which the Fo8 coil proposes a novel approach in focused field.
The most suitable positions and configurations of electrodes have been experimentally determined and proposed, showing the vantage of distal arrangement relative to radial artery in front of circular. The optimal pressure point for externally applied pressure on rigid electrodes, located on radial artery, was determined since which the modulation in the measured signal of EBI is not increasing significantly any more. As the resistive solution presumes the galvanic contact, the externally applied pressure contributes to further improve the quality of measured signal of pulse wave, designating a relevant outcome.
Clockwise: 4 Block diagram of the proposed idea of EBI measurement device; Attachment of the implemented prototype of EBI measurement device on ES1; Influence of the choice of EPC on the measured Sresp (parameter no. 1) and Scard (parameter no. 2); Influence of the choice of EPC on the measured Sresp:move (parameter no. 3) and Scard:move (parameter no. 4).. Source: Margus Metshein, Wearable Solutions for Monitoring Cardiorespiratory Activity, PhD Thesis, Tallinn University of Technology, 2018