 A very good morning to all of you. This is the same experiment, the continuation of the experiment which we were doing in the earlier module in a different lab where we were talking about laser etching of PMMA sheets. The idea there was to be able to use that as a mold so that you could develop a device and the device could microfluidically operate and do various testing or calibrations for different processes at the microscopic lens scale. So, here in this module what I would like to demonstrate today is basically work on the next step of replicating the shape or the feature that had been produced by laser etching technique and the replication would be like a micro casting. So, therefore, you have to use a soft polymeric material in our case we will use material called poly dimethylsiloxane PDMS and this PDMS is nothing but a silicone rubber which is otherwise commercially available into two different forms. One of the components is actually a curing agent and another is a silicone rubber and you mix both of these components together in a certain ratio to obtain the requisite bond strength and the curing agent essentially helps the PDMS to develop cross bonding and eventually it results in a rubber like membrane which comes out. So, if this is poured on a small laser etched surface the surface kind of gets replicated and the negative of that is created on to the surface of the PDMS. So, I would just like to recall what we did in the last module where we actually made vise like this using CO2 laser processes on PMMA and the etching which happened therein resulted in this kind of a feature being imprinted on this surface of depth about close to 14 or 15 microns and various widths or various dimensions of the different parts that you can see on the central chamber the chambers which are on the sides etcetera. So, I am going to actually take a box where this is a commercially available plastic box and this is for the purpose of pouring the PDMS and containing it or confining it over the mould surface. So, whatever you saw here this particular chip that had been formulated I am going to first sort of paste this on the top of this box at the centre and make it immobile and then use that as a casting platform as a mould platform for micro casting the PDMS. So, I am going to first try to fix this using double sided tape over this box and before doing that you ensure that you have another identical box here. So, that whatever amount of PDMS is needed on the top of this mould box can be calibrated using this other spare box of identically same nature these are very easily commercially available in the market. So, now what I am going to do is to take double sided tape like this interestingly double sided tape commercially available in the market comes in this form where one of the sides has a yellow covering on the top and the other side also has adhesive. So, basically the adhesive side is lifted up like this and you can measure how much you need for completing the or cover the whole mould surface and try to fix it. So, using normal shear pliers I want to take small measurement here like this to be able to kind of estimate how much length is needed to cover the whole surface and I am going to cut a piece of this length all the way up to here. So, that it can cover half the surface of this feature very well. So, I am going to actually now place this feature almost half way through in this particular tape lie you can see here. So, the other half can have another piece which I will cut out by doing the same kind of size measurements and what I am actually going to do is try to cover the whole base of this mould in a similar manner. So, that one thing you have to really ensure is that you make it, but tightly to the other tape. So, that is one continuity you can see from the back you can either is no gap between the two tapes that you have put and then on the top surface there is some protection which is in outwardly manner and I am going to just shear that off. So, that it becomes of the same size as the mould so, we will do this shearing action you see roughly now in the size and shape of this double sided tape is such that it is covering the entire lower portion of the mould box and the etching the etched feature is available somewhere on the top surface which you need to micro replicate. One has to be really careful about which side you are pasting because this feature itself is very small. The only way to feel it is to either rub your nail over it and see if it produces a scratching sound because of the laser etching which has happened in this area or if the features are too small just try to ensure with your hand whether there is a feature of a certain depth and the planarity of the surface is not uniform and that goes on the top side the other plane surface or planar surface goes on the bottom side. So, I just need to pull this tape off from the lower surface exposing the other side of the tape like this similarly like this and you are left with the adhesive part now on the lower side which you want to go ahead and paste on the top of this particular box right about in the center of the box as this ok. So, this is how it now looks and you can see that because of the tape it does not go anywhere and when you pour something which is liquid in it there is always bow and forces through which the mould might come up unless it has been fixed. So, this we did for ensuring that the mould does not float up because of the bow and see of the liquid in any even PDMS is a highly viscous thick liquid which will have to offer more bow and forces and we need to somehow be able to hold this mould on to the place so that the PDMS can go on the top without making it to lift the mould off because of bow and effects. So, I am now going to go ahead and make the PDMS solution as I told you there are two parts of the PDMS in which it normally comes this particular PDMS is from a company called Doconic in Midland and the brand name is known as Selgard 184. So, you have two components one is a Selgard 184 silicone rubber which comes in this tube and then this curing agent and I am going to now sort of go ahead and mix these two in a manner. So, that the PDMS comes all the way to the top of the box here so it can actually cover what is there on this particular mould surface. So, for that we need a chemical balance like this and we will just try to first measure the amount raciometrically through weights and then we will just do 10 is to 1, 10 parts of the silicone rubber and one part of the curing agent and then mix properly using a pipeter so that it can uniformly disperse the two phases can infer you disperse throughout. So, doing that I first need to cancel or tear the weight of the box machine and once it has been tear I want to take it out and put little bit of this phase 1 here on the surface of this to ensure that it goes all the way to the top you need to be little careful while handling this and the other part which is in this particular solution here also curing agent. So, first of all we need to weigh this and find out how much this would be so this happens to be about close to 30 grams. So, we need exactly 3 grams of this other material to keep it going and so as I told you that the chemical balance here measures the quantity of the PDMS at least the silicone rubber part which we have put inside this small box and then we have to mix the curing agent but before doing that one has to ensure that whatever mold you are using will release the PDMS after the heat curing action etcetera is over. So, the idea here is that I would need to treat this with some kind of vapors or some kind of chemical which will formulate a mono layer on the surface of this material PDMS otherwise is highly hydrophobic in nature the contact angle is about 108 degrees. So, one has to have a hydrophobic surface so that there is a mutual repelling action between the two and there are no as such van der Waals forces which develop between the surface and the PDMS and there should not be any stickion why particularly when you are pulling it out that is the purpose of a release agent when you talk in castings you use powder on the walls of the mold so that the metal comes off without damaging or breaking the mold. In this case breaking is not possible because the PDMS is a softer material but yes it has to be coated properly so that it should not be that the features and trap a part of the PDMS and it gets broken off that resulting in a bad finished casting. So, mold release is very very important so what we are going to do is to use a vacuum desiccator right about here and we are going to use something called HMDS which is commercially otherwise available in Waals and this material has to be handled very carefully the HMDS is normally coming in a closed vial it has to be taken to a fume hood something like this where you can break open the cap of the HMDS and transport the bottle all the way into the desiccator and then evacuate it and before doing that you ensure that you put this into the desiccator so that the environment given by the vapors sort of does a preliminary treatment of the surface by giving a small coating on the surface. So, as you can see now the processing of the vial is done there is a vial inside which is broken cap and the material that you want to coat with that is the mold everything is inside here in this particular desiccator and there is a pipeline which goes into otherwise evacuating system small mechanical pump and that takes care of sort of close to 10 to the power of minus 1 or minus 2 torr vacuum within this particular chamber. One reason why the vacuum is also engaged is that this material HMDS is highly volatile in nature and the idea is that it should actually come and evacuate or come because of the negative vacuum pressure and formulate an environment over this surface including the plastic box the mold etcetera which is kept inside for a very long amount of time so that there is a small layer and in fact you will see that the layer which is formulated will be visible as a small whitish layer on the top of the surface. So, this process will take about 15 minutes beyond which we will have to examine whether the mold is fully coated and then if we feel that it is fully coated we will go ahead and stop the process otherwise we will have to again recoded so that that small thin whitish layer and be seen on the surface of the mold. So, this right here is demonstration of how the oven functions there is a controller for this oven where there is a set value which you need to set for heat curing PDMS. PDMS as you have seen before has been mixed into two different parts component A and B curing agent and the silicone rubber and your heat curing it to a temperature of about 85 degrees Celsius for about 40 to 45 minutes and this has to be very precisely controlled otherwise if the temperature is higher you know the plastic container it might just warp and it might just deform and change the shape of the containing mold which is really not desirable. So, what we do is to preset the oven as you are seeing here at 85 degrees and wait for the oven to achieve a temperature all the way up to the set point value as you can see a little bit quick glance here that temperature is changing quite a bit and basically the temperature once it achieves the 85 degrees Celsius you have to keep the oven in closed condition. So, that the temperature is maintained at that particular level now once we are sure that the temperature has been reached the other important aspect is to take out the mold which has been cured prior to this or coated prior to this using the HMDS or the hexamethyl disilazane layer and a lease agent layer. So, what I am going to do is to sort of take out the mold from the vacuator and see the mold surface clearly it is indicative that there is a layer on the surface. So, we pour the 10 is to 1 ratio of the curing agent to the silicone primer from the other box that we had earlier taken provision for this purpose onto the box containing the mold and so once this is thoroughly poured and the whole PDMS is transferred it is a very viscous substance as you are seeing like molasses and therefore it needs to you have to really hold it for some time for the material to be completely loaded onto the other new box containing the mold. So, once this is finished you have to really check what is the level of the PDMS on the on and over the mold pasted on the surface of the box because mind you the mold height has now changed because of the thickness of the double sided tape which has been inserted below the box. So, you have to actually see now what is the level of the PDMS on the box if you are happy there is another issue about setting up the evacuator again because mind you when you are actually trying to do this process of mixing agent curing agent along with the silicone polymer there is a issue of entrapment of bubbles and dissolved air which comes in so that air has to be removed because otherwise when starts heat curing and there are convective currents within the medium there is a tendency of this bubbles to formulate a two phase system and bubbles in this manner if they get entrapped into the cured silicon matrix would have a lot of problems regarding optical visualization of the PDMS matrix. So, we place this PDMS now on the vacuum desiccator and wait for few minutes at high pump pressure for the desiccation action to sort of pull out all the air which is entrapped in the PDMS matrix. In the meantime you also should check the conditions of this gravitate convection oven which has been sort of now you know stalled at about 85 degrees Celsius and the way you normally handle is to actually keep a sort of elevated platform which is already prebalanced and there could be an issue of proper leveling of the mold box that you are eventually going to place into this oven. So normally it is pre leveled by using a spirit level such ovens also have the possibility of tilting shelves. So therefore you can actually using a spirit level in a crisscross manner do both x and y in the x and y direction you can check whether they are completely leveled with respect to gravity so that the overall PDMS surface can be flat. Now the it is high time now that this desiccation has already happened rocker and bubbles must have come out of the solution. So we are going to replace we are going to just sort of take out the mold box along with the PDMS on the top and see what is the condition of that mold box you can see here that there are bubbles in a way coming off the surface of the PDMS which actually is suggestive of that the dissolved air has really come out. So a few more minutes are needed so that you can individually bust these bubbles and you have to see that over this mold really are there any bubbles or not in this case over the mold you have relatively a clean layer of PDMS which you can see and so the idea is that once this has been formulated you can put this into the oven in the level plate there is a level plate here in the bottom corner as you have seen. So you just put this over the level plate and the same can be done with the shelves as illustrated earlier so that the level of the PDMS over this mold is completely flat in the x and y direction parallel to the gravity and close the oven and wait for about 40 minutes till the PDMS actually achieves the necessary curing action because of the opening of this oven for loading and off loading there is a sudden change in temperature as you can see here it again needs to go all the way to about 85 to 86 degree Celsius which it has now reached the PIV control and we have to wait for 45 minutes so on that we can actually have good cured sample of PDMS which will later separate from the mold which has been formulated you can see here actually the sample has come out after 45 minutes.