 Let us resume our lecture series on bioelectricity. So, now we are into the last two lectures 39 and the 40th one. So, as of now in all our 30 almost 30 up to the 34th or 35th lecture we have talked about the overall animal electricity. When we talked about the plant electricity in the form of photosynthesis where the electron transfer takes place. Then we talked about the movement of the plants form of a venous fly trap and touch me not plants. Then we talked about dye sensitize for solar cells followed by this we had a brief exposure about some of the very ancient molecule iron pyrite or iron disulfide and their electrical properties. So, we will close in this section or this course with some of the lesser known electrical phenomena which are observed in nature and one such material or biomaterial which is getting very huge prominence in last two decades a little more than that is silk. Silk as such the way it has been you know understood does not have when it is dry does not have certain like does not really carry charges, but it behaves in a very different way. So, what we will do today is we will devote two lectures now talking about the silk what is really silk is and how it has evolved and what are the different forms of silk and what we know about its bioelectrical properties and how this could be used for some useful applications and what are their implications in nature. So, let us start the 39th lecture on the broad heading of silk biomaterial and electricity talking about silk what really is silk. So, whenever we think about silk there are what the first thing you get is about silk saris or silk clothes or you know silk garments nothing else really comes in your mind, but this is a kind of a mass conception among the general audience that is not really silk silk is a broad name for all those insects which exude certain things from their mouth and form a protein or network or mesh of protein or proteinaceous network that is called silk. And as a matter of fact if you really go through the plethora of different silk we have talked about we will be talking about the spider silk which is also a silk we talking about hornet nest that is also a silk we will talk about bombics and plus our silk which is also a silk. So, just to give you a kind of feel what silk is. So, if you look at this slide which shows a spider net you are seeing this wonderful net. So, this material of net which is forming that specific net is a silk. Now, I give you a second example hornet nest you all of you have seen this kind of nest the honey bee is hornet you see this net some of the see this nest some of these nest if you see this picture you will see some of the nest which are closed with a white cottony coating or a cap and whereas, the other ones are without any cap this cotton cap what you are seeing is also silk. And then the most traditional ones which most of us understand is a silk cocoon a bombics and thread I have just given you two such examples these are also silk. Now, coming back to the world of silk now if I have to come back. So, how really what how the silk is produced first of all let us answer this question. So, there are series of insects which goes through a four stage life cycle. So, the adult lays the eggs these eggs just put it on the board to make more sense. So, these insects the adult lays the eggs and these eggs hatch and form something called larvae like this all of must have seen this kind of you know these larvae eat very gregariously they eat on plant material upon eating the plant material from their mouth from this part they secrete a series of kind of you know sticky materials. And these materials is essentially what we know as silk and there are almost 4000 to 5000 such species which produces silk we only know handful of them. And eventually this larvae goes into a dormant phase called pupae and this pupae is an inactive phase of their life and this pupae has to be protected from the surrounding. So, that protection is offered by this material because what it does essentially it covers around this larvae something like this which leads to the formation of called cocoons silk cocoon or you know in the case of hornet nest you see that white covering on the hornet nest like this that hornet nest covering that is also a silk material. So, this is the coating what we talked about the silk cocoon in the hornet nest if you look at the hornet nest hornet nest there is something like you know those hexagonal things and you see that white coating on top of it those white coating is also silk. And then you have seen those python net something like this this is also silk. So, this is the basic thing what I wanted to clarify those insects which goes through this four stage life cycle the egg followed by a larvae which I have shown here followed by a formation of the pupae or which is also called the dormant phase of their life. And then this emerges out as a butterfly. So, this four stage life cycle during their four stage life cycle during this phase they secrete the silk material and this material is nothing, but a rich matrix of proteins embedded with lot of elements and few other organic molecules depending on where they are being grown if they are rare under a specific kind of tree then there is a consistent material, but if they are you know evolving in the wild depending on what kind of tree they are feeding the protein quality changes not only the protein quality the quality of the silk changes. And as of now why the silk becomes so prominent with garments is there are two or maybe you know maximum there are four or five species which have been which have been domesticated very thoroughly one of them is bombics moly and from there we get the pure silk apart from it most of the other species whether they are found in Africa or you know India or Asia likewise wherever they all are partly wild or you know semi domesticated species like anthria myelata which from the tassar silk then you have anthria perinite there is a bunch of such species which are still in the wild and still considered as a wild or semi wild species, but they are all silk likewise the spider silk, but then now again let us revisit this. So, this is also silk if you look at this like so this is also silk this is also silk this is also silk then what is the difference between these silk they must be a structural difference. So, that is what we are going to come first what is the basic difference between these silk. So, I told you silk is nothing, but a protein measure and these proteins are something like this it forms a measure of proteins. So, this these proteins consist of two different kind of proteins. So, if you really take a cross section in the case of silk cocoon silk then you will see this protein is made up of something like this inside one chamber you see inside one big cylinder there is another cylinder and this big the cylinder which is inside is called fibroin protein F I B R O I N and outside where I am putting the hatch line this is called sericine this sericine protein is a gummy protein kind of a gum or sticky protein. So, whenever one such so these two can stick with each other because of the gummy nature of it. So, this is what we talked about the regular protein fibers are. So, they have a fibroin protein which forms the core and then they have a outer shell which forms the sticky protein called sericine. So, thereby several of these fibroin cylinders are attached to each other and form the very complex structure of a silk cocoon something like this what you see out here, but if you see this one here also the same thing it is a fibroin and the sericine coating which is forming, but here the story is slightly different in the case of a spider silk in the case of spider silk they do not have these sericine. Sericine is missing instead they only have fibroin. So, they do not have any sticky things that is why you see those thread fibers they are separated from each other they are separated from each other because there is no sticky material and the protein what is forming the core or called the fibroin core I am putting my F this protein is a beta sheet protein extreme beta sheet protein. So, that much what you needed to understand about the basic structure of silk, but then why we are interested in this problem there are and where this whole electricity comes into play this basic introduction was very essential for you to understand let us summarize what we have talked just now. So, silk is a broad class of protein which is essentially secreted by at least five to seven thousand or eight thousand species of insects during at a specific phase of their life and most of these insects are four stage life cycle insect they have a egg larvae and the dormant phase called pupae and after the dormancy is over they become an adult butterfly and this dormancy phase last from 10 to 20 days to even eight or nine months depending on the species and the kind and where they are. There are species found in United States where the dormancy phase goes all the way up to you know eight or nine months whereas, most of the species which are found in the tropical and subtropical wells in and around India, China and all these places they last you know 2021 days likewise. So, this thing varies third thing what we talked about is the difference between the different silk in terms of their fibrin and the serocin. The silk what we understand really like from which the clothes are being made it is basically originally that silk consists of two proteins there is a fibrin core and there is a gummy coating called serocin and these fibrin cores are attached to each other side by side or crisscross whatsoever way because of the gummy coating which is present outside them. So, this is what we discuss now why we are getting into this problem now in order to understand this let us get into this slide. So, this is the first silk what we are going to discuss today the hornet nest the whole idea about you know trying to understand the properties or electrical properties of silk comes from a very fundamental question which has been raised by the entomologist for a long period of time. How most of these insect nests which are present in the environment could regulate their temperature they indeed have a thermal regulation because these hornet nests are living at a temperature say you know 50 degree centigrade. So, if the 50 degree centigrade remains inside this goes a small structure what you see the pupae won't develop there will be defects. So, there is a way by which the temperature inside the these nests are being regulated and this is the genesis of this asking this question. Then who is helping it because in the case of hornet nest if you see this hornet nest if you see this slide very carefully you will see there is this hexagonal structures which are formed here they are made up of clay whereas, that upper coating is made up of your silk. So, who is really taking care of it. So, since 1920s and 30s this remained a very interesting question and till date this is very interesting question and people started to you know address this question. So, today we will be talking about some of those pioneering discoveries or first attempts and I will be referring to these two papers which are produced which were published by Jacob late Jacob is he was working in Israel he was in Tel Aviv University. So, he was working on understanding how the thermo regulation takes place in the hornet nest this was his basic idea. So, there are two very nice papers which he published one of them is the thermoelectric effects in the hornet that is the vespa oriental cell and thermo regulation in the hornet and the second one is a review paper of all his work what you see here silk produced by hornets thermo photovoltaic properties. So, the basic idea was about these papers even before you get into it is silk membrane has certain thermoelectric properties. So, let me just get on the board for this just to give you an idea what is the basic premise to start these kind of studies is. So, you see the hornet nest something like kind of structure side by side and on top of this you have this and the pupae is growing out here for this pupae to grow there has to be an ambient temperature how this ambient temperature is being maintained this was the fundamental question there are two possibilities somewhere or other this membrane this clay structure is helping it or silk cap which is covering it is helping it or both of them are helping together the both the silk cap this one and this clay both of them are helping both the silk cap and the clay both are playing the role. So, the hunch was that it is the silk cap which is playing the role. So, how to study it? So, one of the basic idea which was kind of tinkered by Ishey and his colleagues was the silk cap. So, if I kind of you know put this it is a cap like structure which is nothing but a mesh work of silk protein out here. So, this is structure is somewhere or other has the ability to you know change these two things thermal to electrical vice versa and they added one more thing they say based on the light they respond. So, what was the kind of beginning of this is of this kind of study was this membrane is thermosensitive membrane this is light sensitive membrane. In other word it is almost like a T T V thermo photovoltaic membrane will come across this and one more thing I wish to request if I go to the slide I wish you guys please download these papers or I will try to add these papers at the end. So, that go through these papers. So, this membrane is acting as the thermo photovoltaic membrane which is helping this is structure to maintain the ambient temperature. So, this was the premise with which all these things were started how to study this this is a very challenging problem this is not something. So, easy you know first of all you have to collect these hornet nest. So, professor E. Shea collected all these things from the in the Mediterranean region and after collecting. So, the hornet nest that the silk top comes like this. So, then what he has to do is in order to understand this he has to connect electrode like this and electrode like this across this particular membrane just is having this silk cap like this electrode is connected on this side and an electrode is connected on this side. Now, this is exposed to different kind of light because you are studying the photo effect as well as different kind of temperature and what you are monitoring is depending on the temperature and the light on this silk cap electrode 1 this is electrode 2 you are monitoring the voltage this is the basic premise what they wanted to do. Now, think of it now if I show you the picture again. So, this is what you are doing you are removing this white part what you see in the slide and on either side of it you are connecting those electrodes. Now, I will show a real picture. So, this was the whole idea was this thermo regulation is regulated by charge transfer. So, it has been observed this hornet nests maintains a temperature. So, if you see the middle line at 28 degree centigrade irrespective of anything whether the air temperature is changing it may go up it may go down across the day, but the hornet nests temperature remains at around 28 to 30 degree centigrade there is no shifted. And as a matter of fact, this is the same thing which is being observed in the silk cocoon and I will give you the reference in the silk cocoon also they could do the same thing and we will talk about talk later about that experiment. So, it means and as a matter of fact, let me just take the liberty to explain that experiment. So, this experiment was done like this. So, here is a silk cocoon and this is a pure cocoon now it is completely there is no and this was done with the anthria myelita or this is also called tassar cocoon. So, what was being done is you kept a thermocouple inside. So, here is this thermocouple and now you are just changing the temperature. So, you keep this cocoon. So, for example, at a temperature of a 50 degree centigrade and vary it all the way to say 5 degree centigrade, but what you will observe inside the cocoon is without the by the way when you are doing these kind of experiments realize you have to remove the dead animal you have to kill it and you have to remove the dead animal because you are only studying the property of the membrane. Now, if you measure the temperature inside it you will see at the 50 degree this is will maintain at around 30 degree centigrade and when you are keeping for long period of at 5 degree it will maintain around 20 degree centigrade. So, from starting from these two I was showing you these two pictures whether it is a silk cocoon or whether it is a hornet nest the temperatures are being regulated they all have certain thermo regulatory mechanism and this reference I will add this reference for you people at the end you can go through that paper this is a much more recent paper this was published in 2012 coming back. So, this is what inspired Ishii at that point of time this was the slide which this was from Ishii's paper itself that it maintains the temperature inside it and how it does so. So, what Ishii pretty much claimed is that this particular membrane transfer transform the electrical energy into thermal energy and thermal energy into electrical energy and same with the light light to electricity likewise which essentially is a photovoltaic property now we will see what are the results you obtain and how far we have understood about this process. So, now if you look at the ultra structure I was telling I will show you the ultra structure this is how the ultra structure of the silk looks like it is a mesh work. So, basically what you see here is a mesh work and this is basically taken a scanning electron microscopic picture which will give you an idea that it is almost like a porous membrane on the right hand side of the picture you will see the hornet nest in black and white and those white what you have done is we have taken those white caps and these sections were made and this pictures were taken this is how the ultra structure looks like and this ultra structure consist of both februin and the sericin protein. Now, if you look at the photo properties of hornet nest exactly the same experiment. So, experimental setup is in front of your eyes. So, if you look at the photo properties of hornet nest. So, this is what you will see is if the cocoons are excited at around 249 or 290 or 312 nanometer you will see the emission spectrum. So, emission spectrum is coming at you know at around depending on what you are if you are if you are exciting at 290 nanometer you are seeing at seeing an emission spectrum. So, at around you know near to 350 and there is a slight shift at 249 nanometer there is a slight shift. So, it is pretty much it is all concentrated around 350 when you see the emission there is something very interesting depending on which side is exposed whether the inner side say for example, what I mean by inner and the outer side and you have to again go back to the structure this is how it is covered. So, it has one face which is facing downward another face which is facing upward. So, if you take the downward face the one which is facing inside the nest it has different luminescence property as compared to the face which is outside. So, this is the surface which is exposed to the direct exposure to the sun or to the light the other one is kind of underneath it is something like this is the one which is getting exposed to the sun the other one is different. So, if you see these two plots now on the slide you will realize that they show a different kind of intensities in terms of it, but in terms of their emission spectra they pretty much remain constant. Now, moving on to the next slide this is how I was trying to explain you people through this diagram. So, here if you go through the slide you will see how these connections in the real life connections are being made again these are all from the issues paper what I have already described you people if you see this. So, you will see this is the cap like structure what has been removed from a from a hornet nest after anesthesizing it. So, that all the life insects are not biting you or whatsoever and then you connect using colloidal silver you are connecting a wire on either side of the membrane. Now, let us see how the electrical properties are changing. So, we talked about the photo and thermo electrical properties of hornet nest. So, if you look at this slide this is this is you have to really look at this side very carefully. So, look at the light and the dark situation while there was light. So, there are two symbols here one are the filled circles what you see here and the under ones are the empty circles the empty circles are the one which is indicating the voltage which is on the left hand left hand side on the left hand axis. So, there are two y axis here on the right hand side you are seeing the current on the left hand side you are seeing the voltage. Now, if you see it during the light the voltage was very high as it dark as you are moving. So, it is a diaruminal. So, during the day. So, there is a light phase then there is a dark phase because it is a day and night during the night the voltage drops down and in between while the voltage drops down there is a spike of current which you see there is a spike in the center there is a sharpest spike and then it falls down. So, if you go through it is a measurement of electrical charge and cell the white dots typically the voltage measurement of the cell cap initially exposed to light at which time 15 to 200 millivolts were recorded. Now, then exposed to darkness where upon the voltage sharply this decrease to a very low value that you could see here that it has fallen down. Whereas, the black dots which is the current measurement which shows that during the exposure to light the current was almost 0. However, as soon as the light was switched off a current appeared whose maximum value attained 1.6 milli ampere, but gradually in the course of several hour reduced to 0 and this all this measurement was done at 30 degree centigrade. So, now what we are talking about these natural materials what you see out here response to temperature will sorry temperature response to light as well as temperature will coming to the temperature, but definitely they respond to light and as we are dark as the dark approaches darkness approaches as we are day to night the voltage changes. So, it means what it looks like as if you know they are just inert cover they are not inert cover they are electrically active cover and this is the picture which is taken from all those two references what I gave you given you this is from the second the review paper by late Jacob is a shame. So, for his time these were some of the very very seminal studies what he conducted like you know this was never done before. So, this material has some very interesting thermo photovoltaic property. Now, next we come about the dark current and photoelectric effect look at this trace this is a very interesting study the dark current and photoelectric. So, what you see out here you look at carefully there are spikes there are you see this underneath line what you see. So, the result of the exposure of the light cocoon cap to intermittent light at 365 nanometer with the intensity of 100 mega hour per centimeter square the light pulses induces a current flow between the two electrodes up to several nano amperes and if you see the time constant which is shown by tau tau a and tau b you see 18 seconds and 30 second. So, you see when you are exposing you are giving a light the. So, if you if you remember the previous light when there is light there is no temporary there is no current and whenever there is a darkness there is a spike of current. So, light dark light dark and you will see whenever there is a dark there will be a spike of current whenever there is light the current will fall down. So, this shows that this membrane what you are what we are talking about this self membrane is a light sensitive membrane it has photo properties because we saw about the photo characteristics about its absorption and emission. Now, we are seeing the functional properties that it indeed shifts its electrical it there is a charge movement based on the exposure to light. Now, let us move on to the next light which is the thermo current. So, as of now we talked about the light we have not talked about the thermo current. Now, let us talk about the thermo current. So, if you see the membrane has been varied from a temperature of say 20 degree to 30 degrees. So, here they are using 33 degree if you look at the slide 20 to 33 degree. So, if you are if you observed it carefully you will see as you are going to 33 degree centigrade there is a increase in the current as you are bringing down the current falls down again there is a increase again it falls down again there is a increase again it falls down again there is a increase you see there is an cyclic fashion. So, it means just like the light if you see the previous light just like the light as the light increases current falls down as there is a dark the current rises. Same way as a temperature goes up there is you really can you know shift it as it falls down you see. So, this is how there is a cyclic fashion this is happening. So, this is very interesting that they have another thermoelectric property which is fairly unique, but you have to keep on shifting this temperature there is to be a shifting temperature without if you do not shift the temperature this you would not be able to observe this kind of effect. So, now moving on there is a temperature dependence resistance in cell cocoon. So, this is what is being shown here. So, if you see follow this graph very carefully. So, see at on the y axis sorry in the x axis you are seeing the temperature. So, if you see the temperature very carefully at around 20 and 30 degree centigrade there is some the whole resistance goes down. So, the typical temperature dependence of the electrical resistance of the pupil cell cap plotted versus the absolute temperature can be seen. There is a sharp drop approximately 3 orders of magnitude in the resistance with temperature in case of between 1 to 21 degree centigrade. Following that there is a plateau between 21 to 32 degree centigrade that roughly coincide with the optimal temperature of the hornet activity. If you remember if I go back to this slide the optimal activity could be seen at around 28 to you know near about 28 to 30. Now, if I follow this one that is exactly what you are seeing at around 20 to 30 near about 30 you see an optimal temperature. So, there is a temperature dependence. So, the membrane itself changes its property. It is thermo active membrane it is a light active membrane and there are lot of things to be understand that you know how with temperature the current is changing how with light along with temperature the properties are varying. Now, yet there are several things which it is believed that this is what is regulating this kind of temperature regime. What we do not know as of now is is it what we are seeing is a biological semiconductor thermo photovoltaic membrane or what are the charge carriers we really do not know how to correlate it with traditionally accepted N and P type of semiconductors. We know this far that it indeed shows a thermo photovoltaic membrane it acts like that, but what we do not know what are the charge carriers that needs much more better understanding. So, in order to summarize this. So, we talked about today we started with the basic problem of thermo regulation how thermo regulation is being conducted and we saw the ultra structure of the hornet self. Then we talked about the photo properties of the hornet self followed by the electrical properties and electrical properties we talked about in the presence of light the current is less, but as you take it to the darkness the current increases. So, there is a light and dark situation, but in case of temperature what you see as it as you increase the temperature to certain optimal value the temperature kind of goes as the current goes up and then it falls down. So, there is an optimal range for this kind of material to function and that is why I told you that and then we talked about the temperature dependence resistance in cocoon and that at a specific temperature range 20 to 30 degree centigrade you see there is a significant fall in the resistance of the membrane and there is a sharp rise in the current followed by these some of these interesting questions which in future will be able to answer is this a biological semiconductor what are the charge carriers how to correlate with traditionally accepted n and p type of semiconductor. So, I will close in here and we will go to the part two of this where we will be talking about the other membrane what we have not talked about yet is this membrane we will be talking about the bombics and the anthrae amylata which we have not talked yet we have only talked about a wild species which is this one. So, I will close in here in the 40th lecture we will talk about this section. Thank you.