108108113_week1_mod3.pdf

Fabrication Techniques for Mems-based Sensors: Clinical Perspective Prof. Hardik J Pandya Department of Electronic Systems Engineering Indian Institute of Science, Bangalore Lecture – 03...

Fabrication Techniques for Mems-based Sensors: Clinical Perspective Prof. Hardik J Pandya Department of Electronic Systems Engineering Indian Institute of Science, Bangalore Lecture – 03 Introduction to Microengineering Devices Contd Keywords: VOC, Microfabrication, Diabetes, E-nose, Micro-Machining, Bulk plus surface Micro Machining Welcome to this module, this is module number 3, for our lecture 1 that is Fabrication Techniques for Mems-based Sensors Clinical Perspective. So, until now we have seen several sensors and we have also talked about how sensors can potentially solve important problems in medical diagnosis or in clinical research. So, let us see other sensors in this particular module and let us understand how we can use our understanding of microengineering and technology to fabricate those sensors. Now a very important point is that the most prevalent disease is diabetes. And we have seen that for understanding the blood glucose level concentration, what we have to do? We have to puncher the finger. And then we have to put a drop of blood on the glucometer to understand the blood glucose concentration. Now, this is a minimally invasive technique because you are puncturing your body. How about you find a non-invasive way that means, without puncturing your body can you detect diabetes? Or a matter of fact, can you detect any disease can you diagnose any disease without puncturing your body? So, you assume that every time you want to measure blood glucose, you have to puncher your finger and it is painful. So, what is that an alternative technique? And that is we can detect or we can diagnose the particular disease from the concentration of the gases that we exhale and inhale. So, when you exhale from patient to patient or from person to person, there are several VOCs that we exhale along with the things that we already know which is carbon dioxide CO 2, but along with CO 2, what else or what are the other VOCs we exhale. 57 Now, when I talk about VOC, what does VOC mean? VOC means Volatile Organic Compounds. So, when you say a volatile organic compound, what does that mean? Organic compounds that are volatile. Can you give me an example? VOC, think about organic compounds that are volatile, have you thought about it? Is it petrol? Is it diesel? Is it ethanol? Is it acetone? Methanol, butanol, isopropyl, alcohol kerosene; so, all these are volatile organic compounds. They will evaporate at room temperature volatile. So, when I talk about ketones or alcohols. Then a certain amount of these VOC’s is exhaled from our breath and the concentration of the VOC changes if a person is suffering from a particular disease. So, if we can selectively delineate or separate out that particular VOC from the group of VOCs from the breath of a person, then we can have a non-invasive way of diagnosing a particular disease. So, it is an exciting field of research; so, how can we do that? Now you see we are blessed to have a lot of sensors. If you think about my hand I can feel it, I can feel the surface I have touched the sensors. When I have these force sensors, I can understand how much force I am applying. Then sound, then taste sensors, then optical sensors, etc just to name few, there are thousands of sensors within our body. So, now if we have these many sensors, let us take our nose. So, let us say if I have blindfolded myself. So, I cannot see and if I put something into my ear. So, I cannot hear and I cannot taste if I put tape on my mouth hence the only thing is I can smell. And if there is a lemon or an orange, can I distinguish just by smelling the lemon and orange? And 90 percent of people can, how? By the smell of it, that means, there are sensors within our nose in a form of an array and that sensors give us the sensitive data that is concentration or at least qualitative analysis; that this is the smell of lemon this is the smell of orange, but how we process this? How can I distinguish between just by smelling a lemon and just by smelling an orange? So, you should further understand how our nose functions, Our nose is connected with some other part of our body and you will say yes it is connected with your brain ; that means, brain has that pattern that whenever your nose gives you that particular smell it has a signature that immediately tells you that this is the smell of lemon.; that means, our nose is connected with your brain. And that means if I want to mimic the same function in the form of a sensor because you see a nose; there are so many sensors within the 58 nose, all are microsensors; so small even nanosensors. So, if I want to design a nose using my microtechnology, what can we call it? It is an electronic nose. It is an electronic nose or it is an e-nose. Now in our body, our nose is connected to the brain; here also we need to connect the nose to the brain which is your artificial neural network. Or we need to understand or recognize the pattern that is why pattern recognition. So, you no need to design the sensor, you need to fabricate that sensor and collect the data by inserting or you know you put the sensor in a chamber and let different VOC’s pass through the chamber and you collect the data. Once you have a good amount of data you can you can train your neural network and then you can test it. Now it looks so simple is it not? because now there is a factor of humidity. But if you study well and if you understand, then the research articles say that the person suffering from diabetes will exhale a higher concentration of acetone compared to other volatile organic compounds. So, what does that mean? That means, compared to the VOC’s that a normal person exhales, the person suffering from diabetes would exhale acetone in a higher concentration which our sensor would sense. Then not only should it sense, but also has to selectively delineate from the group of other VOC’s, from the group of other volatile organic compounds. So, can we design the sensor? So, when you want to design this kind of sensor or group of sensors, one should know microengineering one should know microtechnology and you can design this array of sensors of course, which are bigger in dimension; larger in dimension, but for the smaller sensors for the micro for the nanosensors, you need to go through this particular technology. 59 (Refer to Slide Time: 10:47) So, generally, you see the effect of pollution on our health. For example, if you see air pollution, the air pollutants such as carbon monoxide, particulate matter, ozone, lead SO 2, N O 2, NOx, VOCs volatile organic compounds. So, we have to see this particular group of VOCs. In water pollution, we talk about bacteria, parasites, chemicals; I was talking about antibiotic resistance in the last module and how do these VOCs affect? which kind of organs? you see respiratory illness. It causes cardiovascular illness and just not VOCs, but by total air pollution, but if you just talk about VOCs it causes cancer risk, nausea, skin irritation. It is harmful. It clearly states that VOCs can cause cancer. So, if you have seen a newly painted wall then that will smell differently. And people should not go close to a newly painted wall because it will emit VOC. So, now VOC sensing is very important and you can also sense other gases. So, there are two techniques that we will learn in this particular course; one is called VOC sensor, a sensor that can sense volatile organic compound; second is called gas sensor, sensor that can detect various gases. So, we will see how we can design this VOC sensor or gas sensor using microfabrication. As I stated VOCs are found in an individual’s breath in 100 PPM range and only a small amount of VOCs are common to everyone. The concentration of VOCs exhaled by patients for different diseases is different than that of healthy people. So, VOC sensing is 60 important in environmental monitoring and in medical diagnosis, very important. So, when you talk about VOC we had to see sensors and now in front of you can see two types of sensors, sensor one here, we will see in detail how we can fabricate the sensor (Refer to Slide Time: 13:25). This sensor is fabricated using bulk micromachining. While you can see sensor two which is this particular sensor, this is used or fabricated for detecting VOCs and it is fabricated using bulk plus surface micromachining. Sensor one is fabricated using bulk micromachining, and we will see how we can fabricate these particular types of sensors that can be used for detecting volatile organic compounds or it can be used for detecting gases. And of course, the main part of the sensor is the sensing film here you can see we have used indium tin oxide here we use zinc oxide and you can use several metal oxide semiconductors or you can say conducting semiconducting oxides or semiconducting oxides. There are several kinds of semiconducting oxides and how the sensing works, we will discuss in detail when we talk about the sensors in the following lectures. Now, if we talk about a flexible force sensor or flexible sensor you need to first design a flexible sensor and you should know what kind of substrate I should use. So, my sensor will not crack or it should not get destroyed. 61 Even though it is not really important or necessary for a micro engineer to learn schematic tools such as Solid Works or CATIA. It is always good to learn, as most of us when we study electronics or electrical engineering, we kind of ignore the design and design which is a very important aspect of fabricating any equipment including sensors. How well you can show your schematic representation because that will not only help the person who is fabricating but will also help a person who is understanding or reading the research article. (Refer to Slide Time: 16:22) So, if you want to fabricate a flexible force sensor, let us see the design on the screen now what can we see? If you see this particular sensor; in the center here, you can see this is the magnified view of this particular image here. So, what can you see? You can see a few sensors here, still, it is difficult to see. So, we have further magnified this area, now it is a little better. What does it have? It has a strain gauge. Let me draw a little bigger strain gauge so that we understand and this strain gauge is made up of piezoresistive material, we can use conducting polymer P.PSS. On this strain gauge, there is insulator, what is that there? There is an insulator. On the insulator there is an electrode. I am just making it thicker. Mind that there is an insulator between the strain gauge and the electrode. Now, insulators have the strain gauges, how many points? It has 2 points; 1 and 2 electrodes. This is what you can see in this particular image. You see this one or you see 62 this one or you see this one or this one or this one. There are 3 points going out 1 3 1 2 and the pink one is your third one, two from strain gauge, one from the electrode. There is an electrode in the centre and this electrode is made up of gold. For Strain gauge, I told you that P.PSS is used and this is the flexible substrate; that is why you can see here three pads 1, 2, 3. There are three pads; these are contact pads for the sensor. So, what will you do with this flexible force sensor? We can connect these flexible sensors on the robotic arm, we can connect on a surgical device, we can connect this on a catheter, we can connect this particular sensor with a 3-D printed cone to measure the property of a material. For example, the elasticity of the material, to measure the electrical and mechanical properties of tissues; it has many applications. (Refer to Slide Time: 20:08) Now, if a housefly is sitting on a flexible sensor, you can see that there is a sensor at the bottom and there is a cube on which the housefly is sitting. And if we want to measure the weight of the housefly; our sensor is sensitive enough to measure the weight of the housefly. You can see that this is an SEM image of a housefly and of course, the fly is dead, otherwise it will fly. So, this house fly is dead and we have placed the housefly on the sensor. 63 So, without the housefly and with the housefly we can see the change in the resistance of this particular sensor and from that we can correspond to the weight of a housefly or how good our sensor is. Before we can measure the fly’s weight, we have to first characterize our sensor. We have to compare it with the commercially available sensor and see how that responds to the different forces. Have you ever thought that such a small thing that flies all around can have a beautiful structure? What is the structure of the housefly? How can it see? How can it feel? What are the sensors on the housefly? When you really try to observe these small tiny beautiful things in nature, you should appreciate the beauty of the smaller structures, smaller insects, smaller organisms around you and really understand how beautiful nature is. And from that design of nature, we can get lot of ideas for designing our sensor. This is inspiration that you can see, so tiny and still it can do so many things with its small structure, is not it? Is not beautiful when you see it in SEM? So, anyway, the point is not to just appreciate the beauty that nature has, but also to understand the design that nature gives us. When you really understand the design of nature, you can implement it in your sensors and that is how we can come up with novel designs of sensors that can help us in solving very important problems in the clinical domain. So, with this, we will stop this particular module and we will see in the next module in the next lecture in fact, that how we can fabricate a particular sensor and to understand fabrication we should understand lithography, but before you move on to lithography. We will first see what do you mean by bulk and surface micromachining. So, I will see you in the next class, till then just look at the sensors that we have been discussing in all the three modules. And now we will go on to understand how we can fabricate these sensors and what is the application of each of those sensors. Till then you take care and I will see you in the next class bye. 64