BioMEMS Fabrication: Soft Lithography, Micro Molding & 3D Techniques

[00:00:03]today's lecture is going to be about this third topic the last two lectures we covered photolithography and then we covered a traditional machining for hard materials like silicon and glass and today's lecture we're going to cover soft lithography and micro molding so these techniques are more applicable to soft materials and can even be used to pattern biological cells by soft materials i mean things like polymers and plastics and we've mentioned last time that the reason you'd want to use plastics is just because they're cheaper those materials are cheaper to work with cheaper to buy than traditional microfabrication materials which are used to make electronic devices or microelectromechanical systems all right so let's just uh jump ahead here i actually bought a 3d printer in the lab and unfortunately it was it was quite a disappointment because i i bought one of the one of the early stage ones and they advertise it very well on the website as a company in in uh 3d up it's okay but the thing is it was quite unreliable the nozzles for broke down we can still use it but it just tends to break down quite often the newer models by makerbot and 3d systems those those systems actually have really nice um nice features they're more reliable yeah a couple grand now you can even buy uh 3d printers for a few hundred dollars now enough of them are being made for they're they're being sold incredibly cheap everyone could have one in their home that was the idea behind the industry yeah but some of the very the consumer grade the few hundred dollars it's not going to give you a very fine spatial resolution but it's okay for printing you know if something breaks in your house you want to print a 3d part to you know to fix it you can do that military did something like that over basically yes oh yeah yeah yeah that's that's interesting that that was kind of the idea like if you're not if you're in a um in a region where you can't get parts easily just print the parts yourself pretty interesting so we'll start off today's lecture by comparing polymers in glass okay glass is a traditional hard material that we showed you can fabricate using some of the more traditional techniques like wet etching photolithography and so on glass is also a traditional material in the sense that the chemists have used glass for years and it's a very good material but it's it can be a little bit expensive and more difficult to work with compared to polymers and plastics so with polymers and plastics we're going to talk about ways that we can use soft lithography to machine those types of materials uh so before we do that it's useful to sort of get a motivation for why we want to work with polymers compared to glass so this slide from a 2009 article just talks about some of the advantages and disadvantages of the two materials so let's take a look at a few of these things here uh so the first thing manufacturing costs okay we showed that with glass you know whether using powder blasting techniques or wet etching techniques a lot of them involve a photolithography step the photolithography has to be done in the clean room and so these processes typically have a higher cost a glass chip will typically cost much more than a plastic chip polymers plastics they have very low cost you know that the plastic toys can be made for made for pennies using a technique called injection molding which we'll talk about in terms of fabrication complexity if fabrication with polymers is typically simpler than glass operational temperatures this is an area where glass typically wins polymers and plastics tend to melt at low temperatures glasses go up to several hundred degrees so if you're working with high temperature applications glass is typically better optical properties in fluorescence glass has superior properties in that sense one of the problems with plastics and polymers is that they tend to auto fluoresce fluorescence is when you excite a material with light and then it gives off it just glows a certain color of light okay it turns out that when you're doing biological acids you don't want any kind of autofluorescence you don't want them the channel or the substrate material to fluoresce you only want your biological material to fluoresce all right so glass has very low autofluorescence so that's and that's a nice nice feature about that it's also very optically transparent you know plastics aren't always completely transparent they can appear kind of cloudy so uh glasses has a nice optical transparency and certain types of glass like quartz is all transparent even in the ultraviolet range when we're doing biological assays a lot of times we use ultraviolet light to to stimulate fluorescence in biomolecules with regards to bonding so when we talk about bounding we want remember we want a sealed micro channel right so we typically will make the channel structure in one and then we'll have a flat cover slip on top and we'll bond the two together to seal the channel so with regards to bonding there's a bunch of different bonding techniques available for polymers using adhesives thermal fusion ultrasonic welding and so on glass is a little bit more difficult to bond you can still bond it using fusion techniques and otic bonding techniques where you apply voltage i would say bonding with polymers is typically a little bit easier surface treatments with glass there's very well established surface treatment methods based on organocytes because chemists have been using glass for years there are surface treatments available for polymers as well a little bit less established but more and more service treatments are coming out all the time the reason we'd want to surface treatment material why the surface properties are so very important is because at the microscale surface effects tend to dominate other types of effects surface driven physics so for example if we want to attach dna molecules to the uh to the surface if we want to attach if we want to cast cells to attach to the surface and grow there we have to functionalize the surface meaning the surface has to have certain desirable properties so we have to be able to treat the surface of of the types of materials that we're working with so we're comparing polymers and glasser because polymers and glass are the two most common materials used for biomedical mems materials for like things like microfluidic devices and so on uh the next thing is compatibility with organic solvents or strong acids whenever we're doing chemistry we we want to have some type of resistance to acids or solvents not necessarily this is not needed for biological assays because biological assays are always done in some type of saline solution the physiological fluid which is not really that harsh but uh um if for some types of processing we need to have we need to have compatibility with with solvents and acids so generally polymers tend to be less compatible with solvents and strong acids and and glasses tends to be very good with it not all but it's resistant to many solvents and acids a glass polymers typically have low thermal conductivity and glass glass has a little bit better thermal conductivity so it's less subject to heating the next one is very important it's electrosmotic flow if you want to do something called capillary electrophoresis where you're separating molecules by size you use a technique where you can pump fluids through a microfluidic channel using something called electro osmotic flow electrosmotic flow only works if the material has negatively or positively charged groups on the walls of the channel okay so glass has this very nice property that when you expose when you fill a glass channel with water the surface of the glass is negatively charged and that negative charge can be used to actually pump fluids through the channel using electric fields that's called electro osmotic flow we'll talk more about that later so you get better electrosmotic flow in glass because it's very negatively charged a geometric flexibility and we'll talk about this in this chapter there's a bunch of different types of polymer processing techniques which allows you to make curved curved devices it allows you to make three-dimensional devices we talked about 3d printing multi-height channels and with glass typically we're limited to two and a half d designs two and a half d means basically like what we showed earlier where you have you know two-dimensional structures that are extruded into the three dimension so when we do a glass actually we have a two-dimensional pattern on the surface and then we drill down from that two-dimensional pattern all right so an ex it's an extruded two-dimensional pattern rather than a true three-dimensional pattern the last point here is the permeability to gases um can anyone does anyone think why why gas permeability will be important in uh in a biological essay if you're culturing if you're doing a cell culture in the microfluidic device why would gas permeability be important that would be that would be an argument to not have gas permeable systems but that's true in some cases we we want to retain whatever the cell whatever gases the cell is secreting but the opposite is also true many times if we have cells growing in inside a sealed microfluidic chamber we want we want gases to be able to flow in freely in and out like oxygen and carbon dioxide because without those gases cells can't live so um having gas permeable devices is nice so a lot of plastics and um a lot of plastics and elastomers for example are gas permeable they'll allow liquid they'll seal the liquid inside the channel but they'll allow gases to freely flow through the channel walls to polymers tend to have a high gas permeability and glass typically does not so so there's some advantages to polymers obviously okay so polymers and plastics elastomers how can we then fabricate devices using these types of materials the techniques that we use will differ depend depending on the type of polymer or material that we have there's three categories that are shown here which are quite common now one is the what's called the thermoset polymers thermoset polymers are often often made using injection molding so what differentiates a thermoset polymer is that during cross-linking the material during curing the material is cross-linked and the structure is fixed the curing can be thermal chemical or photochemical so examples of thermoset polymers are polyurethanes and epoxies now if i can just go back here and remind you that um at the very abstract level a polymer consists of a chain of organic molecules okay for example a very common type of chain is a carbon chain carbon atoms are bonded to other carbon atoms around it and then then there are different side groups hanging off the end here of the carbon atoms now i don't want to get into all the details of the chemistry because that's that's something that we could spend an entire semester talking about the point i just want to make here is that polymers tend to often be long strings of molecules that's why they're called polymers now this would be what's what would be called a monomer or one uh one one string of the polymer and then in a solution you might have many of these uh chains like floating around okay when you cure the material you actually form chemical bonds which cause different side chains of the polymer to interact and bond with one another so this molecule will then become attached to this one okay so i'm describing this in very abstract terms so i apologize for those of you who are chemists out there what happens is that the the when the monomers are separate in solution or when when these polymer chains are separated in solution when they're not cross-linked the the material tends to be um flexible it can even be in liquid form all right and when the polymer is cross-linked those chains attach to each other and then the material hardens so getting back to this the thermoset polymers is when the material is cross-linked by just applying heat to the system okay it can also be done with chemical or photochemical means a chemical example of a chemical curing would be epoxies have any of you work with two-part epoxies it's a you can buy a glue and the glue has part a and part b when you put part a and part b together and you wait overnight the glute cures and it hardens overnight there's also ultraviolet curable glues you know you take the glue and you when you expose it to ultraviolet light it hardens okay those are examples of polymers which are cured using chemical or photochemical means there are polymers that you can also cure using heat all right but once the material is cross-linked the structure is fixed at that point that's one of the things that defines a thermoset polymer thermoplastic polymers these polymers become soft when heated above a glass transition temperature examples include a polymethyl methacrylate some types of polycarbonates and and polystyrene so one of the features i would say about thermoplastic polymers is that they can be reversibly um you know when you increase the temperature you can soften the material and put it into sort of a liquid state when you bring the temperature down the process is reversible so again it hardens so when we're working with thermoplastic polymers something that we would uh you can do is just use an embossing technique meaning you just heat the polymer up it softens and then you can imprint a pattern into that polymer and you let it cool down that's called embossing or imprint lithography uh injection molding we we skipped over but i'll get to that in the next slide so elastomeric polymers these are polymers that have uh elasticity due to weakly linked weekly cross-linked side chains so going back to this this diagram here when these side chains cross-linked to each other if those cross links are very strong the material is going to be hard but if the cross links are relatively weak then these chains can actually slide up against one another and the material becomes more flexible and rubbery so that's what an elastomer is and these classes of materials we're just bringing up because they're commonly used in microfluidic devices with elastomeric polymers you can use a technique called soft lithography to make microstructures out of those materials so elastomers i mentioned that the elasticity is due to weakly linked side chains but these materials can be thermoset or thermoplastic meaning they can either be cured using thermal chemical or photochemical means that be a thermoset or they can also have thermoplastic properties as well the thermoplastics ones you can use these types of embossing techniques so example a common thermoset elastomer is polydimethylseloxine pdms it's a it's an optical elastic optically transparent elastomer which is similar to bathroom caulk you know the stuff that seals your bathtubs and but turns out it's it's a very good material for microfluidic devices so the thermosets we can use techniques like injection molding we can do stereolithography as we talked about last time what else um we can also do like in-situ polymerization okay so there's many techniques we can use to work with these um durable plastics often we use some type of embossing do you technique a question oh sorry you just you're just scratching your head resharpen that's one of the differentiating features between thermal sets and thermoplastics in a thermostat material once it has been cured then it's it's difficult to melt it down again it's difficult to reshape it once you cure it once those cross links form the material is hardened okay so but you can still you can take a thermostat material and if you heat it up to a high enough temperature then it'll start to you know other things may happen you know it could it could still melt you know you it could go under thermal thermal decomposition so it can be damaged by heat at certain temperatures so the the difference between the two is that in thermal plastics the process is reversible you can heat it up to a glass transition temperature it softens reshape it and then bring it back down and let it harden that's less that's less feasible with thermoset polymers so since we'll start off by talking about um injection molding so the reason i'm starting off with this is because you know before we we always start talking about microfabrication techniques it's useful to know actually what you know if you're fabricating a plastic part at the macro scale larger device what techniques would you use injection molding is the technique that's used to make most plastic parts today you know like and it's used to make plastic parts very cheaply and they cost like pennies so folks in the in the micro fabrication world have attempted to take this injection molding technique and miniaturize it so we'll start off by talking about micro injection molding so micro injection molding allows one to mass manufacture polymer chips with very low cost and and most commercial microfluidic chips are are in fact injection molded so there are some commercial companies we went over them on the first day of class the companies that sell plastic chips typically do make them from injection molding because when when you have to mass manufacture something in the hundreds of thousands of parts then injection molding is is definitely the way to go it's the cheapest way to make a part when when you have lower volumes then you may consider other techniques because the the cost of making the first device using injection molding is high is higher because you have to make something called a master so let's let's get into that so how does the technique work this is a typical setup for injection molding system so injection molding you're you're putting in this raw plastic material again that raw plastic material is melted and that melted material is goes through this screw system the screw system is basically a pump it's serving to push this melted plastic material into this chamber this chamber has a mold in it okay so the molded part is here on the left so the mold would be some type of like a metal a piece of metal for example that has a shape on it and the plastic will be the melted plastic will be flowed over that mold and will take the shape of the mold so the mold is reused once you make the mold you can reuse it and the cost of making the part is very cheap but you do have to make that mold so that's the expensive part and then you also have to tool the injection molding machine so uh this is the plastic can often be a very viscous material so in order to get the plastic into this small crevice that the system has to be at a high pressure you put you're pushing the fluid the plastic through at a high pressure so you need to have like a robust assembly to do injection molding but the basic idea is that you have uh um you have a sort of a clamp a clamp device here and then you can flow the plastic in the clamp uh clamps down on the on the part so that the the plastic takes the shape of the mold and then you allow the system to cool the process cycle looks like this you clamp the two halves of the mold assembly together you inject the plastic material and when you inject the plastic material the the parameters are typically the shot volume like how much plastic you're putting in there how much pressure you're injecting it with and how much power you let the part in the mold cool and then you eject the part from the mold so this process cycle can take anywhere from two seconds to two minutes per part depending on how complex the part is depending on how large the part is but it can be done serially and it can be done you know a lot you can have multiple machines running in parallel so this is a very manufacturable uh process yeah when it's done at large scales it's it's actually much cheaper than glass fabrication you can get features down to the sub micron size amazingly people have shown that you can get very very small features but it requires high injection pressure and speeds to ensure the filling of the small cavities so when we're talking about the scaling of forces remember at the micro scale we were saying that the smaller you make your fluidic channel the amount of pressure needed to drive that fluidic channel goes up okay the hydraulic resistance goes up by a factor of the diameter to the fourth power all right so when you make these in microscale devices when you have these very very small cavities and very very small features you're going to need very high pressures to push liquid into those areas so it's micro uh micro injection molding is typically more difficult than standard injection molding when you're working with larger parts the mold insert also has to be able to be hard and and relatively durable because it has to withstand a high pressure and smaller molds like smaller freestanding parts thin parts typically are less durable and they can bend they can under pressure all right so that's something that has to be considered uh and to make the mold inserts you often have to use some micro fabrication techniques to do that all right so this brings up sort of a paradox right this is what many folks will wonder like what if we have to microfabricate a mold for this device then how is it a cheap process the the the fact that the the process that is inexpensive is because you can reuse that mold once you make it it might be expensive to make that metal mold but once you have it you can just you can keep on injecting injection molding parts into that and so the process becomes cheap that way okay so the mold there we go the mold inserts can be made with micro milling uv liga and electric discharge machining we're not going to go into all those techniques right now just for the interest of time a couple examples of things that have been made a lot of optics companies have made gratings and wave guides out of out of plastics gratings have like certain surface patterns that reflect light in certain ways you can make spectrometers out of that you can make light guides use for optical applications in the mems area you can make microsprings and switches biomems people have used them to make micro channels pumps reaction vessels and various types of mixing structures this is an example of a milled mold insert so before you even get into this injection process you the first thing you have to do is make a mold that mold will represent what type the shape of the part that you want to make one way you can make molds is by a cnc milling them now cnc stands for computer numerical control it's basically a little drill that's controlled by a computer and the the drill kind of goes back and forth in a three-dimensional pattern and it and it shapes uh the the mold or the shapes this metal part that you see here okay so this is a a popular and easy way to make molds if you have access to a cnc machine some desks you can actually buy desktop cnc machines anywhere between ten thousand dollars and fifty thousand dollars now uh but like an industrial scale one will cost it could cost several hundred thousand dollars so cnc milling and basically involves drilling right physical drilling you have a drill bit so obviously the the feature sizes and the resolution that you're going to get from cnc milling is not that great the feature density is about 200 micron minimum spacing between channels right so if you were to make a microfluidic channel you have to define these features on your mold and the spacing between those features will will determine the spacing between channels so that's a minimum spacing between channels is about 200 microns the minimum feature size is about 50 microns and the minimum feature depth is about 5 microns this all has to do with how the drill is actually cutting out cutting out spaces in the metal this is an example of what a metal mold may look like so you can get nice aspect ratios like this and reasonably defined structures you know this was 200 microns at 400 microns tall 100 microns wide you can also see that there's quite a bit of surface roughness okay when we want really smooth channels like if we use something like wet etching hf acid etching for glass we got really smooth sidewalls right when we're using a drilling process like this to make the mold the surface is going to end up being a little rough it really depends on what type of application you're going to be doing the maximum feature size feature width here is a you know four four millimeters uh you know a few more specifications here you know aspect ratio is up to two radiuses of curvature and you can see the surface roughness here all right these are examples from a company in europe called microfluidic chip shop they're a company that will actually if you tell them a design of a microfluidic channel you want if you send them a cad design they will make the part for you and ship it back to you there are a lot of foundry services like this popping up but the microfluidic chip shop is one of the better known ones in europe right now so in addition to cnc defined mil inserts you can also use photolithography and traditional micro machining techniques to make that metal mold if you use lithographically defined molds you can get much smaller feature sizes so whereas the the cnc milling could get 50 microns minimum feature size and 200 micron spacing between channels with with lithographically defined mold inserts you can get about a factor of 10 better actually a factor of 20 better the minimum feature size here is about 100 microns and you can still maintain a large height you can maintain a large aspect ratio so how does a lithographically defined mold inserts work well remember we have to if we want to use photolithography to define a pattern the techniques that i showed you in last lecture was just how to make structures out of photoresist right we can use you can develop a layer of photoresist and make a pattern on photoresist then we talked about micro machining if you have a layer of metal or a layer of glass underneath that photoresist you can transfer that pattern to the layer below it but those weren't very deep structures if you want to make a mold for if you want to make a mold for doing these types of processes then if like injection molding and or if you want to do imprint lithography you need to have taller features uh you can use a technique called liga and another one called demo to to do this uh i guess i'll just touch on these different techniques liga is a way to form metal structures on a substrate the process is uh the process is is relatively straightforward but it requires expensive equipment you start off with a substrate then you deposit photoresist on here and you pattern that photoresist okay the difference between liga and standard photolithography is that this photoresist layer is actually quite thick it's 100 micrometers thick whereas with traditional photolithography it's only a few micrometers thick you know less than 10 usually it's uh this is this is done on a sheet of metal so this is done on a metal sheet now if you want to get high aspect ratio structures and you're dealing with very thick photoresists you you can't use traditional optical lithography instead of using ultraviolet light which is used in traditional optical lithography you use x-rays x-rays as you know they're just it's just another form of light but it's high energy light very low wavelength high energy light it turns out that high energy photons they don't diffract as much they just go straight into the material so if we have if we had a mask defined here and we wanted to expose this region of the photoresist those x-rays would go into they'd go straight down the collimated beam would come straight down into the photoresist they wouldn't bend and diffract off which is what limits the aspect ratio of traditional optical lithography so x-ray lithography is great in that sense you can make very tall high aspect ratio structures but it's very expensive because you need an x-ray generator as opposed to just a standard ultraviolet light source so you use the x-ray source to expose your photoresist then you electroplate nickel in here electroplating is where you put a voltage on you put you put it in a bath okay an electroplating bath and then you just put a voltage on this side and you put the other side in the solution you keep the solution at ground and you apply voltage to this nickel sheet here and metal ions from the solution actually electrode deposit into this region here and then you remove the photoresistor left with this so that's a process called electroplating another way you can do it is by using this approach which also involves electroplating some nickel and i'm not going to go into all the details of this process just in the interest of time the idea is the same you end up with a structure a metal structure with protruding metal structures on top okay this is what defines where your channel will be right this is the mold that's ultimately used in your injection molding machine so these molds can be expensive to make if you want to lithographically define them you can get very small feature sizes that's nice they are expensive but once you have the mold you can reuse it and you can make many parts out of it so that brings the cost of the the final fabricated part brings that down significantly let's go back here this is an example of a plastic injection molded chip that has integrated fluidic connectors and can it seals up to 10 megapascals which is which is decent it's pretty good so moving on to the second technique here so this was injection molding we talked about and we can talk about hot embossing right now this one's an easy one to understand just because it's a um it's pretty intuitive you heat up the plastic it softens and then you imprint a pattern into it that's pretty much it so the uh this is done in the macro scale world it's called embossing hot embossing and in the micro scale world it's called nano imprint lithography and in the nano imprint lithography you take a hard mold something like silicon nitride or micro machined silicon you heat it up and then you push it down into a soft material the soft material will be like a thermo just this type of plastic or thermoplastic and when you push that mold down the plastic will then deform it'll take a and when you let the system cool down and you remove the mold you're left with an imprinted pattern all right there's a good review paper on this if you're interested in nano imprint lithography um this is what the the syste what the patterns may look like amazingly you can get very tiny features using this technique there's no optics involved in here it's just pressing down a mold against against the substrate so there's no rally diffraction limits it's pure it's a purely mechanical process and it's incredibly it's simple in concept um it hasn't taken off it's there are nano imprint lithography machines out in many clean rooms but i would say in terms of as a it hasn't taken on taken off to the extent that that was initially thought that being said you can still make a lot of very nice features of it this is an example of a grading grading is a series of repeating line patterns in a plastic and those gratings can reflect light in specific ways that's used for optics type experiments you can make things like photonic crystals also used for optics you can make nanoscale channels i mean it's there's a lot you can do with there's a lot you can do with these types of uh things here the the feature size here you can see this is about a 500 nanometer scale bar so the size of each one of these lines here is about 200 nanometers really really small and in this case it's even smaller okay so now let's get into a pdms soft lithography or what's called replica molding so we're talking about this third technique now so pdms is an elastomeric material it's it's similar to bathroom caulk it seals water very well it's inexpensive it's optically transparent and it's people have been using it for biological assays for i would say since the mid late 90s early 2000s so the way that you can micro pattern poly dimethyl siloxane pdms is by using a technique called replica molding now a pdms is a thermoset elastomer so when you when you heat it up it cures it solidifies it doesn't it doesn't harden into like a like a plastic a hard plastic it hardens into like a rubber it's an optically transparent rubber essentially so this consists of four steps and this is the process that hopefully you saw in the uh the week i was away in the in our lab this is the you guys saw the replica molding process okay and we use this technique all the time the labs microfluidics labs always use this technique to make their devices the first step is photolithography now this gray region that you see here this would be a material for example like su-8 it's a it's a photoresist material that is optically defined after you develop it certain regions of the photoresist are washed away in the developer so you have this uh patterned photoresist layer now um you guys did not see how the mold was made in when you guys came to the lab the our mold was actually made in the clean room the microfabrication facility okay so this is the if if there's an expensive part of the process it's this part of the process the photolithography part with the same thing as injection molding once you have this mold you can reuse it and make as many devices as you need to until the mold like degrades and you can't use it anymore the second step we have this mold we pour the polymer precursor onto the mold okay now did you guys have a chance to to actually do this or did you just watch you just had a chance to watch it right so okay that's that's fine too so pdms comes in a liquid form there's two parts to it there's a part a and a part b part a is the um the pdms material part b is a curing agent when you mix the two together and you heat it up the material cures so what we do is we mix the precursors part a and part b and we pour that over the mold it takes the shape of the mold and then we put the we put this in a vacuum furnace the furnace heats it up we leave it overnight we allow the pdms to cure the reason why you need a vacuum furnace is because you may or may not have noticed this but there were bubbles in the uh pdms when you when you pour that material over the mold any liquid material can have bubbles in it right so if you have bubbles in it those bubbles will eventually get you know solidified not solidified but it'll they'll be encapsulated inside your microfluidic channel right so that's that's not a good thing so having it in a vacuum furnace actually degasses it pulls out all the bubbles from the solution from the pdms so after a few hours in the vacuum furnace the pdms cures it hardens or hardens or becomes a rubber essentially and then you can just peel it off so this has a bunch of relief features on the top and then if we flip this upside down we can bond it to a piece of glass which is shown in brown here or we could bond it to another layer of pdms okay this second layer here serves as a ceiling layer this defines a surface pattern so you can define a microfluidic channel on the surface here but in order to seal the channel you have to bond you have to you have to have a cover slip you have to cover the top and then you bond it to that a second second piece of pdms so the reason why this process has become so popular is because the chemistry associated with it is is well understood the bonding is well understood and it's very easy to do pdms if you take two pieces of pdms um and you you treat each one of the surfaces with something called an oxygen oxygen plasma or you can do a corona treatment on them where you basically like corona treatment is uh where you you have an ionized gas that treats the surface of the pdms it sounds complicated but it's actually very simple you can you can plug a corona corona discharge machine into the wall they're about like this big and you just run it over the surface and i and some of you may hopefully you saw that in the lab there's one in the left okay so when you put that corona surface treatment on there it creates these solenol groups in the pdms and those salad all groups can bond to bond to other sold groups so when you take those two pieces of pdms that have been service treated you you push them together they will form a bond with each other and they'll form a nice seal so it's a very simple technique because all you need is that corona is a treatment device it costs a few hundred dollars compare that to if you wanted to have a sealed glass channel to have a sealed glass channel you have to have a fusion bonding device which will heat up the glass to temperatures of above 600 celsius or if you use enotic bonding you have to apply several hundred volts or several thousand volts between the two devices as two sides of the chip as they're pressed together so those processes can be more complicated compared to the pdms process this is and researchers like to do the easiest thing possible to get to where they're going you know so that's why a lot of us uh use pdms for our work he showed us where you basically when you're actually one yes do you have more issues like let me show you the i don't think we have a separate slide on that so yeah it looks like i don't okay so let me just draw a quick diagram to show show you uh so in the first step uh you made the mold i'll just draw it from the beginning you know just be simple straightforward so you have a substrate then you then you code it with su-8 let's say this is where your channel is going to go let's make this larger this is where you want to define your microfluidic channel remember this would be defined in in three dimensions then you're also going to have some inlet ports here like this and your substrate would look like this so i'm going to try to draw this in three dimensions yeah that's better okay so you're going to have after your you photolithography photolithographically define your su8 layer you're going to have a mold that looks something like this this is the part that's made inside the and made in the cleaner and it's just a one step photolithography process so this green material is we use something called su-8 so i'm just going to walk you through the process as well as the connector connection part that you were talking about so in the cross section if we just take a cross section of this device we just have the su-8 layer and then we have the substrate underneath it just holding it we pour the i'll put the pdms in blue so we pour the pdms over this so this is the pdms layer we pour it over the su-8 layer and we just let it harden right so afterwards you're left with this pdms layer that's looks like this you can peel it off the mold uh then you will then what we do is we drill holes through here so we can just take a small drill bit and then just push it through this region here so then we end up getting a hole in the pdms that looks like this go like this these are our axis holes so that we can connect connect tubing to it we bond this to another piece of pdms flat piece okay and this part here becomes our sealed channel so from here we can inside these little axis holes we can connect the tubing to drive drive liquid into the channel and the other tube would probably be waste in this case so these circular regions that i showed you earlier this is these are the regions where the access holes will be so these regions of the pdms you can't see the circular regions in this cross section but basically like these are the regions where you can you can drill down into into there and make an access hole does that answer your question but even with the high pressure system oh it's a very good question yeah if you get very high pressures do you get leaking yeah there's i can refer you to a paper on this i don't remember the number off the top of my head but this seal that pdms forms with another piece of pdms it's a good seal it's not perfect though if you go to very high pressures it would this it will start to leak you know the this pdms layers will delaminate from one another so i know you can't go to like several thousand psi you may be able to go to i've seen at most like 100 psi it depends on the quality of the bond that's formed here yes yes that's a that's a very good point you know last time in class we were talking about microfluidic chips 3d printed chips with integrated fluidic connectors now when you have integrated fluid connectors you can tolerate much higher pressures because when when you make just a chip prototype using pdms you know you have your tubing you just get by plastic tubing and you insert it into this region so you can get leaking from there because literally you're just it's all this seal here is just a friction seal now you can also have leaking from here because this bond is not it's not perfect right so when you one of the downsides i would say you brought up a good point that pdms is it's not good for high pressure applications for high pressure applications you'll want to use you can use materials like glass if you have like good fluidic connectors on them with very high pressure applications you typically have to use the three dimensionally defined microfluidic parts that have integrated connectors all right uh there's quite a bit of material i'd like to get through and i would like to finish this up today so let's um we'll pick up the pace a little bit here these are some pdms structures that were uh that were formed using a soft lithography this is that photoresist material this is the su-8 material i was showing you in the drawing this is what you define lithographically using um you know photo traditional photolithography when you pour the pdms over this it's going to take the negative of whatever the photoresist pattern was so you get a negative replica like this that's why it's called replica molding and you can see you can get nice structures like this in pdms this is 30 micron scale bar if you try to make your pdms structures too tall and too thin then uh you know then these structures will tend to bend they'll collapse so obviously you don't want to do that and if your channel is too wide this effect can happen too so let's say you want to define you want to define a very wide channel like this so you would create a pdms structure that looks like this and when you bond it to the substrate you'd have a sealed channel but if this is too wide any pressure on the pdms can cause the whole thing to sag and then close off your channel so pdms being a flexible material there's some downsides to that a perfectly rigid material you wouldn't have to worry about these types of issues like lateral collapse and sagging uh this chart goes over some of the properties of pdms it's it's an inexpensive material it's quite elastic and soft being an elastomer it has optical properties that are suitable for microscopy it's generally transparent as i mentioned it has low autofluorescence the surface is hydrophobic but treating it with o2 plasma creates reactive hydroxyl groups which make the surface temporarily hydrophilic this is useful for bonding pdms so when we treat the pdms with that corona treatment we're you know essentially doing the same thing as as an ode to a proper o2 plasma treatment this oxygen plasma treatment would be done in a in a vacuum chamber with an oxygen plasma machine but we can approximate that with the corona bonder it's generally inert and biocompatible but it can swell when it's exposed to some acids it can self seal by conformal contact in principle if you take a piece of pdms and you push it up against another piece of pdms or a piece of glass it forms it forms a seal it's just not a very good seal the bondi can be strengthened with corona treatment so i mentioned it it's it has a high permeability to gases and fluids which is good for cell culture there are two types that are available we don't need to go into the details of this right now but this is what the material looks like it's it's it's become a very popular material for microfluidic devices so uh this talks about pdms soft lithography micro molding is only one of the techniques that you can you can use with pdms there are other techniques you can do as well micro stamping we'll talk more about this in the next module this is for basically inking materials to a substrate you can also do something similar with microfluidic patterning where you have a channel that's attached to a substrate you fill the micro channels with some type of liquid and then you remove the top and you're left with these patterns here right this slide compares traditional photolithography with these soft lithography processes now this is where you might want to use these suppose you want to pattern some proteins okay you want to deposit some proteins on a slide and you want to deposit those proteins in a pattern matter doing that with photolithography photolithography would be very difficult because photolithography involves photoresists and you have to develop the photoresist and then you have to strip the photoresist afterwards they all involve strong acids and strong for solvents strong solvents and strong acids not good for biological materials so how can you do micro patterning of biological molecules that's what the next chapter is about so one of the very popular methods is by using microstamping you make a pdms stamp right so this this stamp you can make using the soft lithography as we showed so it has a relief pattern on there and just like the rubber stamps that you can buy you know if you've ever seen the the rubber stamps for for stickers and so on if you want to stamp it you you dip it into the the ink and then the ink soap sponge and then you stamp it right the same idea with the pdms stand okay except you know as we showed with soft lithography you can make very small features on the pdms stand you can get very very small features even down into the tens of nanometers because this this stamp the stamp was originally made from photolithography all right you can define very small features in there another way you could pattern proteins is by taking this stamp sealing it temporarily to the substrate just by applying some pressure to it and then flow the a solution of proteins through through the channels the proteins will absorb to the surface here we'll talk more about that in the next chapter and then you remove the micro channels that's another easy way you can protein materials so when we when we say the word soft lithography it refers to this replica molding process often and it also refers to techniques that you can do with the stamp once it's been made so this slide shows how you know how fine a resolution you can do this shows the process i'll go over it again just because we have a nice diagram here the first step is that you'd have a substrate and you deposit su-8 on it su-8 is that photoresist you photolithographically define that resist so you develop certain areas away you pour the pdms over it you cast it and then you cure it and then you remove the pdms and you're left with a stamp this stamp is what you'll then use for you'll ink it with your proteins and then you'll just push it up against the surface and you'll essentially just deposit or stamp those proteins on the surface now the the resolution of the technique is determined by the the resolution of your pdms stamp if this pdms stamp were to have two nanometer features on it or 10 nanometer features then in principle you should be able to ink two to ten nanometer patterns of protein so where is that going with that so one of the ways that you can make the patterns much smaller is instead of traditional photolithography you can use e-beam lithography we talked about that in two lectures earlier if e-beam lithography can be used to pattern your photoresist into much finer features and then on top of there you can spin coat what's called hpdms which is a hardened pdms so it's less less buckling and then you cast regular pdms on top of that just as a carrier so this is this stamp is made up of two layers of pdms a hard layer at the bottom and a softer layer on the top the hard the hardened pdms is more rigid it doesn't bend as much so it allows you to more faithfully reproduce very small patterns in this case they were doing sub 500 nanometer features here you can see in this diagram here and we'll go over this one really quickly you can also use flat stamps too if you want to take a pdms stamp which has a relief pattern on it a deposit ink on the top of it you can take a flat piece of pdms or other material and ink and transfer the ink to the flat stamp like you see here flat chemically patterned stamp and then take that stamp and then transfer it to the substrate all right this is if you want to just to be able to preserve your pdms stamp and you want to use an intermediate flat stamp to do the the transfer these are examples of proteins that were deposited using micro contact printing these proteins were fluorescent so everywhere you see the bright line here that's where the protein was deposited you can see that you can get very fine features of proteins on here you can also use hydrogels this is another creative way of using micro stamps now i mentioned here in this slide we talked we said that one of the properties of pdms is that it's hydrophobic hydrophobic means it's water repelling so one of the challenges sometimes with pdms is that if if this is water repelling it's hard to get aqueous inks on there the inks won't want to stick to the pdms now once once the ink is there that's a good thing the the hydrophobicity is a good thing because if you were to take if you were to deposit the ink on a hydrophobic stamp and you were to push that stamp up against a hydrophilic surface if you were to take that ink inked pdms and push it up against glass glass is hydrophilic so that ink will transfer to the glass layer and it'll stay on the glass rather than staying on the pdms that being said sometimes the hydrophobic nature of pdms makes makes a micro contact printing a bit of a challenge so this is an example of a microstamp that's made from a hydrogel a hydrogel is a hydrophilic material that that absorbs water like a sponge okay you can use the same replica molding techniques that i showed you earlier pour pour the hydrogel over an su8 layer and you can get this pattern on your hydrogel so like the hydrogel again it's it's a it acts like a sponge it absorbs water it also absorbs cells and molecular solutions so this an example is that they actually used a hydrogel stem to ink bacterial colonies you can take this stamp and dip it into a solution of containing bacteria and then you can stamp out patterns of bacterial colonies on a substrate that's very useful when you're trying to do high throughput studies of different colonies and how each of the colonies interacts with how the bacteria within the colony interact with each other another example is if um if this stamp contains an etchant you could take a foil of copper and just stamp the etchant onto that foil and wherever you stamp the copper will actually get etched away so this was an example of this is an example of how you could just pattern a copper foil by this micro contact printing basically they had this hydrogel stamp made of material called agarose they inked the stamps so the the the stamp absorbed all the copper etchant copper etchant is a it's a fairly common type of it's an etchant material that you can buy um for for patterning printed circuit boards readily available and then you put the copper foil on on top of the agarose like this or you you know the agarose has the uh the etchant in it and you can basically transfer the patterns of the stamp onto the onto a copper foil pretty interesting so there's there's many ways that you can use microfabrication techniques to make you know make small structures and you can transfer those structures to a wide variety of materials this is another example but i think just in the interest of time we're going to skip over this this shows how you can use micro contact printing to actually make a tube a tubular structure where you have copper foil wrapped around microscale copper foil wrapped around a glass tube that was that was pretty interesting let's talk a little bit about microfluidic patterning so again the idea here is uh that you can flow a liquid solution through a micro channel and then remove the micro channel so wherever the liquid flowed through the channel you'd end up with a surface pattern so there's a a famous paper that was published in 1997 where they showed what you could do with this so we were talking about that a lot of times like biologists are interested in patterning proteins on a surface you want a pattern one type of protein here and another type of protein right next to it when you have patterned layers of proteins on a surface for example if those proteins were cell attachment molecules the cells will preferentially migrate along certain types of proteins they will avoid other types of proteins so there's a there's interesting biology you can do if you can engineer the surface of of a device this famous paper showed that you could pattern immunoglobulins it's an immune proteins that are part of the immune system using these microfluidic networks so in this example they had two channels here that were running parallel to each other so they fabricated two parallel microfluidic channels and they flowed protein a into one micro channel they flowed protein b into the second micro channel and then then they basically removed the microfluidic channel and they left the pattern proteins on the surface and this shows this shows a fluorescent image so what they did was after they deposited these proteins on the surface then they put a solution of antibodies on top which were tagged with red or green fluorescent fluorescent dye to show where those antibodies attached to the surface so this is the reason why this paper was very popular back in the 1997 is that it was showing you know in 1997 a lot of these techniques were these techniques for patterning cells and proteins were not did not exist at that time this is one of the early works that showed that oh you can use microfluidics to actually pattern biological molecules on a substrate on a surface in a relatively robust manner another way you could do this is by using pdms stencils a thin membrane with holes can be used as a mask for depositing proteins and cells we'll get more into this in the next module but i just want to talk about how you could actually make the stencil here so it's done in the same way as i showed you earlier you have some type of mold here okay this would these little gray posts would be sua that photoresist so this is the mold you pour the pdms over it the only difference that you do here is that you press you press the pdms down you take a piece of glass on top of it and you press the pdms down so that um so in this step here we poured a layer of pdms on top of the mold so this was the this was the su-8 we poured pdms on top of there and we just allowed it to cure all right that made just this blue structure that you see up at the top if you were to press down if you were to take a just a piece of glass here and just wild the liquid while the before the pdms is cured if you just take some type of surface and you push down hard enough then instead of getting this thick layer of pdms you'll get just a thin layer of pdms it'll just look like this it'll just look like this now what will happen when the pdms cures the regions where you had the su-8 there's going to be a hole in the pdms it's going to be a thin membrane of pdms with holes in it so that's how that's what a pdms stencil is right this is an example of what you can make with that it turns out stencils are nice for patterning proteins also if you if you take the stencil and just put it up on on a piece of glass and then you pour the proteins everywhere proteins will only go through regions where there were holes you can pattern the proteins that way as well so that's a pdms stencils now going back to this so in this example of microfluidic patterning we flowed liquid through the micro channel sense we flow proteins through there and the proteins themselves deposited themselves on the surface now this is through a mechanism it could be physisorption it could be chemical chemical absorption we'll talk more about that in the next module there's some chemistry involved there but another thing you could do is let's suppose you filled the micro channels with um with a liquid and that liquid is curable it's meaning it's it it can be cured in the presence of ultraviolet light so if you flow liquid into a micro channel that has a certain shape and then you subject it to ultraviolet light it will harden inside the micro channel into whatever shape the micro channel has that is called micro molding in capillaries and they call it mimic for short so what they did again they had the microfluidic channel they flowed liquids through there and those liquids were uv polymerizable materials once the channel is filled you expose the entire channel to ultraviolet light pdms is transparent down to about 300 nanometers so it is transparent to ultraviolet light and the ultraviolet light polymerizes these materials it hardens them it cures them so you can make nice structures like this you can make sheet structures you can even make multi-layer structures if you take these sheet type structures and you stack them on top of each other you can make these types of multi-layer structures or if you had a multi-layer microfluidic channel network multiple layers of microfluidic channels then you could also do this type of do this type of thing some of the advantages is that it's a relatively inexpensive process to do the channels the microfluidic channels can be reused you can make complex multi-layer structures some of the disadvantages is that the patterns have to be connected with one another because you're only flowing liquid once through the channel in order for the liquid to fill all the microfluidic channels all the channels must be interconnected with one another so there's some just you have to set up your labs that you can have the pressure sources to fill the channels you have to be careful that the material doesn't stick to the channel walls and a few other things here okay this is a nice uh nice and in it relatively inexpensive process for making structures flow liquids into a channel and just polymerize them in situ a cooler version of this is what's called stop flow lithography now this was a lot of this work was done by patrick doyle's group at mit and it's it's very interesting interesting way to make uh customized particles now there are many applications where you might want to have custom particles of specific shapes specific sizes it's easy to make those particles in a large scale you can use like 3d printing and so on to do that but if you want to make those part those objects very very small and still have like well-defined features then it's harder to do that so um he came up with this process called stop flow lithography which uses microfluidics and ultra use it uses ultraviolet light to create these very small particles the way that it works it's kind of cool it's like a microfluidic assembly line uh i think this this one describes it well so we can look at this diagram here so let's say you have let's say you have fluid flowing through a microfluidic channel all right so this is a pdms channel shown in blue here and you have some liquid flowing through there and then just like the multi the micro molding and capillaries approach you you will expose some of that liquid to ultraviolet light when it's exposed to ultraviolet light that material that liquid will harden into a solid that's what's happening here so you have ultraviolet light here and then it's hardening into a structure now that's the same idea as micromolding in capillaries but what uh what doyle's group did was just kind of interesting is that they patterned the light into a specific shape so they were not polymerizing everything in the channel they were only polymerizing a certain shape inside the channel right for example if they want to make the shape of of a heart or heart-shaped object then they would project um they had a digital micro mirror device which could basically project light patterns you guys have learned about that already and the digital micro mirror device was projecting a light pattern of ultraviolet light and that ultraviolet light goes through a microscope objective so the this is the light pattern patterned light goes through the microscope objective and it's focused down into a small area so whatever image was generated by the digital micromirror device was shrunk down by the objective lens and that pattern was polymerized the liquid inside the channel so with the digital micro mirror device you know like you can connect to computer to it and generate any type of shape or image that you want right so you can project any arbitrary pattern of light into this microfluidic device and therefore get any arbitrary shape and in fact that's what they showed you could get circular objects square objects triangular objects they made you know you can make any shape object you want you can make letters so this example here they actually made particles that had all the letters of the alphabet okay it would be very difficult to make these type of custom particles with well you can make them using traditional photolithography but it's it's more expensive so this is really a an assembly line method of making particles so they call it stop flow lithography because you have an inlet you have a micro channel you have liquid flowing in into the micro channel and then you have this polymerization region light source objective so somewhere in the microfluidic channel the ultraviolet light is being focused and in that region the particles are being formed so you flow some liquid into the channel you stop the flow and then you turn on your ultraviolet light you polymerize one particle and then again you turn on the flow that particle is pushed forward and then you stop the flow again you make the next part it's like an assembly line so this paper they're sort of showing that you know you could um make a bunch of particles and sort of stack them up into different structures but i think the real interesting part about this is just the fact that you can just make arbitrary shapes and particles just one at a time inside a microfluidic channel now this particular microfluidic channel had like a recessed structure at the top so these particles also end up having a recessed structure at the top and they show that you can make all sorts of like custom custom sized particles this is really a neat neat process so inside the microfluidic channel they are flowing a material called pegda and with two percent ergo cure photo initiator a photo initiator is something it's it's a chemical that when you add it to a polymer material and you add ultraviolet light to it it's a curing agent you add the photo initiator and that makes the material sensitive to ultraviolet light when you shine the ultraviolet light there it will it will harden and polymerize this is another slide on stop flow lithography showing the lock and release technique the crux of it is right here so here they use ultraviolet light to make to make two particles next to each other and there was a feature in the microfluidic channel that that kept those particles in place and when you apply pressure to the channel that moves up and it releases the two particles so they were able to make more complex particles using this lock and release technique it allows you to add multiple layers of chemistry to the same object so that was a very creative way to do a customized lithography making particles we talked about other 2d lithographic approaches but some of the limitations here of 2d lithography all the techniques we've talked about or most of them we've talked about whether we're talking about replica molding soft lithography traditional micro machining traditional photolithography they're they're mainly 2d techniques so you have a two-dimensional pattern that's defined by a mask and and so you end up getting essentially two-dimensional structures you can make three-dimensional structures by stacking layers together but that gets more complicated so there are some limitations of the lithographic approaches so that lends itself to some of the 3d approaches so how do we make true three-dimensional structures we talked about some of these things before we talked about like 3d microfluidics via stereolithography and we talked about 3d printed microfluidics so i'm not going to go over those again but the reason those techniques emerged why they've come about is because there was an interest in in creating truly three-dimensional structures okay there are some other examples here and we can make channels of varying height using something called grayscale lithography the way this works again it uses microfluidics in three microfluidic channels here you flow a dye through them and the dye has different levels of the dyes have different concentrations the thing about the dyes is that they absorb some of the ultraviolet radiation so when you expose the photoresist layer the blue photoresist layer here when you expose it to ultraviolet light these microfluidic channel layers are really what are acting as a mask and because the dyes have different concentration they allow different amounts of ultraviolet light to pass through and so you end up having after you develop the photoresist you have different thicknesses of photoresist all right so this really isn't this isn't three-dimensional lithography but at least you're getting channels of varying heights using this what's called grayscale lithography grayscale lithography is where you're varying and varying the amount varying the dose of ultraviolet radiation in different parts of the photoresist this can give you structures of varying heights you can also do channels of varying heights using pneumatically actuated replica molds so in this example you you have these structures here i shouldn't go in too much of the details here we're going to talk more about deformable membranes and pressure driven channels later on i just want to point out that if if you have a membrane which can deform and the different membranes deform by different amounts and you pour a layer of pdms over it you can end up getting different structures different heights of structures here depending on how much the membrane deformed so pdms will basically take the shape of whatever you pour it on is the point i'm trying to make here it's so if the mold has varying heights the pdms will then also have varying heights and just two more slides here we talked about stereolithography 3d printed microfluidics another completely different way of creating three-dimensional structures which is is very innovative actually very interesting and it's it's what's called biomimetic it mimics nature is this approach of self-assembly if you want to make a three-dimensional object using legos right you take these small blocks of legos and you basically connect them together right that's how a kid would make the 3d structure um the way that the way that this this was proposed by george whiteside's group is by using what's called self-assembly you have small particles small building blocks that can be connected together to form three-dimensional structures the elegant thing about this was that they used what's called self-assembly there's nothing that's actually putting the pieces together the pieces are actually assembling on their own that's the really beautiful part about this now in in nature self-assembly happens all the time and we'll talk more about that in the next chapter so this is a segue into the next module for example in nature if you have a positively charged ion and a negatively charged surface that positively charged ion is going to go to the negatively charged surface and stick there right if in that case like the chemical molecule if you think about the chemical molecule as a building block that building block just found um a complementary the piece that it's supposed to stick to right so in nature like molecules are self-assembling all the time because electrostatic forces are very significant you know compared to the size of the molecule all right so at the mesoscale at the millimeter length scale capillary forces are pretty significant we were talking about the scaling of forces right so if you had particles if you if you created particles that have defined features so for example um this particle has a pattern of solder on it solder like a liquid metal right and those solder patterns are defined in specific regions of the object you have one type of particle here you have another type of particle where the solder patterns are defined on the edges of the cube you have another particle where they're on the edges of a tetrahedron and here there are just on the corners of the cube now if you take those if you were able to first make those objects and then you put them in a heated bath when you put them in the heated bed the solder melts just a little bit and these regions where there's solder are going to attach to other regions where there's solder because due to a phenomena called capillarity and solder patterns will find the other solder patterns so depending on what kind of patterns these particles have on them they will actually self-assemble into larger scale structures those solder patterns will basically find each other when they're in this flask and you're just stirring around the flask and the flask is heated to melt the solder just a little bit so this is it's inspired by nature but the the idea behind this is to show it's to show a philosophy the philosophy is that you could actually get structures to form three-dimensional uh aggregates on their own through a process called self-assembly right so i think in in one demonstration they actually um they actually self-assembled leds on a on a circuit board so that assembling circuit boards is pretty difficult it requires machines that pick up one piece here and literally puts them here if you when you're dealing with really small objects that becomes a problem so if you had a way to doing or particles would automatically find themselves where they're supposed to go that that can be very um very useful so um this was some initial work done by uh george whiteside's group there's a faculty at the university of washington who does uh some interesting work carl boringer he does work on the assembly of particles on substrates that tell this piece to find to register itself in one location and self-assemble there it's pretty cool um but we don't have time to talk much about that right now since we're at the end of class just want to summarize the main points from this module so the first thing we talked about was lithography that's the patterning of materials via light so lithography can be applied to photoresists and other photosensitive materials we talked about a few different types of photoresist the standard ones then there's the the su-8 we talked about other photosensitive materials that are that can be patterned using stereolithography we talked about three-dimensional stereo lithography after that we talked about traditional micro machining techniques if you make a pattern with lithography how can you transfer that pattern onto a thin film material underneath it so under traditional micro machining techniques we talked about thin film deposition and thin film etching techniques this is well suited for hard materials like silicone glass and thin metal films so these are traditional semiconductor materials these are these techniques emerged out of the semiconductor microelectronics industry today we talked about um soft lithography micro molding stereo lithography micro embossing micro injection molding these techniques have come out of really have been inspired by just the the plastics industry the the techniques that we use to pattern plastics and make these make plastic parts which are much less expensive than silicon or glass parts you can use some of those techniques to make small objects small materials too then the soft lithography and the pdms processes those some of those processes are really designed for micro nanoscale structures and you can get very very tiny and fine structures with things like micro contact printing replica molding and so on and the last point is that a lot of these fabrication techniques are 2.5 d 2d would be just a two-dimensional surface 2.5 d is that you have means you have a two-dimensional pattern that's sort of extruded into the third dimension all right so that's why they call it 2.5 d and we talked about a few 3d techniques like true three-dimensional techniques like stereo lithography and 3d printing so those things are emerging right now some of those techniques have limited spatial resolution and and limited speed the ones that do have very good spatial resolution are still costly and the fact is a lot of these 3d techniques are building things serially each device is being built one device at a time so that can slow things down as well so these are just some some things to talk some things to think about so this module is is really giving you a spattering of a whole bunch of different fabrication techniques a little bit of this little bit of this little bit of this and that's really how you know it's it's good that you know these different fabrication techniques because when it comes to building a device you can borrow ideas from these different techniques when you come up with your own process to make it a device that you're interested in just to expose you to the different areas so next module we're going to go into patterning cells and proteins we'll start on that on monday that module will involve some chemistry so just a fair warning but it's an important uh chapter related to biomems because a lot of times with bio biomes devices we are patterning cells and proteins uh on a surface and we need to engineer the surface properties of the of the device that we're working with all right so we'll probably spend um one to one and a half lectures on surf engineering surface properties and then we'll go on to the physics of microfluidics how you can make pumps and valves and things like that okay so uh next tuesday let's plan on turning in the assignments if you find that you need extra time and just need an extra couple days let me know that's that's okay too i'm okay pushing back the deadline in a couple days so just let me know if if you'd like to but i'd prefer that if you can have it done by tuesday that'll be good so any any questions monday sorry sorry i have another class tuesday thursday so yeah monday so i'll see you all next next monday then just email you or i'm sorry just email you oh just if you can turn in the assignment just bring in hard copies to class that would be best yeah that would be the easiest way