A conversation with our Microsystems Engineer Tijn van Omme who developed the Stream Heating chips
DENSsolutions introduces the latest development in Liquid Phase Electron Microscopy (LPEM). Our Stream solution was successfully introduced to the market late 2018 as the first LPEM system capable of controlling the liquid flow across a sample, as well as the liquid thickness.
Except for the Stream system, other LPEM solutions have always depended on a pocket structure where two chips are sandwiched to form a closed chamber, resulting in a “bathtub” configuration, which causes the liquid to bypass the chips.
This means that the user has no control over the diffusion speed of the liquid flowing in between the chips with no guarantee that the viewing area would even be wetted. Thanks to the patented Stream design with on-chip in- and outlet, and a liquid flow channel defined by the on-chip spacers, these problems have become a thing of the past. At its introduction, the Stream system was equipped with biasing chips to enable electrochemistry research. Now with the introduction of dedicated heating chips, we are ready to expand and serve other fields, like life sciences.
What is the benefit of controlling temperature during an LPEM experiment?
The main reason for researchers to use Liquid Phase Transmission Electron Microscopy is to see dynamic behaviour in a liquid environment at the nanoscale; they want to see stuff happening. Now, in order to make sure that something will happen we need a trigger, for instance, to start a chemical reaction. One of the possible ways to do that is to use temperature.
Ultimately we want to mimic environmental conditions as accurately as possible, for example the conditions in industrial processes or physiological conditions inside the human body. By controlling the temperature we can reproduce this environment.
What kind of research has already been done in this area?
So far there has only been a handful of research examples in which people used liquid heating inside the TEM. Most of this research has been focused on materials science, especially on the growth and nucleation of nanoparticles. Scientists have for instance managed to grow bismuth and zinc oxide particles link1 link2. In these research examples, the temperature is used to cross an activation barrier for the desired reaction, starting the nucleation of a nanoparticle for example. The results of this research shed new light on the growth behaviour of the particle and tell us which conditions are needed to start the nucleation. Eventually, such information could be very relevant for industrial processes, for example involving catalysis.
More recent examples in materials science include research on galvanic replacement and corrosion link. Eventually, it will be very interesting to use the LPEM system also for the life sciences; to mimic the physiological conditions inside the human body. This could be interesting to see how certain biological samples respond to new drugs in pharmaceutical research, for example, or to understand the interaction among biomolecules (e.g. proteins).
All in all, the amount of research that has been done using the combination of liquid and heating inside the TEM is very low because there are still a lot of challenges in this field. We hope to see these numbers grow thanks to the introduction of our Stream Liquid and Heating system. Because this system, with its high level of controllability, makes it a lot easier for researchers to get meaningful results.
What are the biggest challenges when introducing heating to LPEM?
It’s safe to say that any liquid cell experiment is already quite tricky. There are a lot of things that have to go well and by adding heat it becomes even more complex. First of all, you need a very thin liquid cell in order to get a good image. So we have to make sure that the liquid stays very thin throughout the experiment. But you can imagine that when you start to heat up the sample, everything around it will start to expand because of the thermal expansion of the materials that we use. Automatically you get a thicker liquid layer, so that is the first challenge that we had to cope with.
The second challenge is to keep the cell wet. We want to image our sample in liquid so it shouldn’t become dry which could happen easily when bubbles start to form. Introducing heat can initiate the forming of bubbles and can make existing bubbles grow in size.
The third challenge is to accurately determine the temperature of the sample or the liquid close to the sample. For that, we need to be able to measure and regulate the temperature close to the sample. Some people have used holders in which large temperature gradients are present. That makes it very difficult to know the exact temperature of the sample and the liquid surrounding it.
How did these challenges influence the design of the Stream heating chip?
The advantage that we have with the Stream system is a very well-defined liquid channel on the chip. We force all the liquid to go from the inlet to the outlet between the chips, so all the liquid has to pass the viewable area. The liquid is unable to flow around the Nano-Cell, which is possible in other “bathtub-like” LPEM systems, like our Ocean system. This gives us a lot more control on the liquid flow and direction. This is combined with active control of the pressure at both the inlet and the outlet to make sure that we know the pressure inside the Nano-Cell. That way we can independently vary the pressure and flow inside the cell. This way we have a much better control over the parameters that play an important role: flow, pressure and temperature. This is especially important in order to flush out unwanted species that are created by the electron beam or by the reaction that is being studied.
This fluidic design helps a lot to be able to address the challenges that we discussed. With the help of the liquid channel, we are able to push bubbles out and make the cell wet if it dried out. This can also be done on purpose, to do high-resolution imaging in gas, for example. After that, new liquid can be introduced to continue the experiment. And to prevent the liquid from becoming too thick, we can use the pressure control on the inlet and outlet to influence the bulging of the windows. Due to the vacuum in the microscope, the windows are bulging outward, but by reducing the pressure inside the cell, this bulging can be minimized. That way, even at high temperature, we’re able to image through a thin layer of liquid.
How do you ensure that your sample has the desired temperature?
We use MEMS devices that are made from silicon, which is a very good thermal conductor. We heat up the entire chip stack because the conductance of the silicon ensures that everything reaches the same temperature so we can get a very uniform temperature throughout the whole Nano-Cell. We did finite element analysis to make sure that this temperature distribution is uniform so that we know that the sample, the viewable area and all the liquid surrounding the sample are at the same temperature. This temperature is controlled accurately through the feedback loop in the software.
So when can researchers start using these chips?
We recently finished the first batch of production and went through an extensive quality control process to ensure the best standards. After the successful rounds of tests, we started (and currently are) working on developing exciting experimental protocols that can be used to demonstrate our new solution to potential customers. We expect to offer people the opportunity to request demos in Q1 2020 to see the capability of our new solution.
Thank you for reading
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