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Wet and Wild: Our Favorite Fluidics Design Challenges

Liquids can be messy. Product design and manufacturing are always more challenging when the wet stuff is involved. At Bresslergroup we know this well, because fluids factor in to many devices we design.

We’ve worked on a wide range of fluidics challenges including dosage, mixing and dispensing, controlled delivery, storage, pumping, sealing, and flow rate control. Here are a few of our favorite fluidics design quandaries (and solutions):

Fluid Dispensing: Precision Dosing

Getting the correct, repeatable dose of liquid can be crucial, and not easy. One of our favorite examples of the evolution of dosing solutions is the toothpaste tube. Original toothpaste tubes were metal. You’d roll them up with a key, flattening and squeezing out the paste. The problem was one of control; it was easy to extrude too much or too little, resulting in waste.

Now imagine you need to mix three different toothpastes to get a custom flavor. You want equal amounts of the strawberry and banana, but only a tiny percentage of licorice to give it that extra kick, 49:49:2. The flavors change and taste terrible if they’re mixed in advance, so they have to be combined on the spot, right as you’re about to brush your teeth. That’s a pretty hard mixing problem.

fluidics design challenges

Modern toothpaste manufacturers solve this in two different ways. The first is a pump, a rigid tube with a traveling piston that gives you a repeatable dose with every squeeze. But pumps are hard to seal off between uses to isolate the flavors, and changes in atmospheric pressure can change the dispensing rate, so your toothpaste might taste different in Denver than it does in Philadelphia.

Getting the correct, repeatable dose of liquid can be crucial, and not easy.

The second solution is a bag on valve. Each flavor paste is in a bag inside a pressurized container. When the valve sealing each bag opens, paste flows out. The size of the valve controls the dispense rate, and all the bags are in the same pressurized environment, so changes in altitude shouldn’t matter.

Three bags on valve is the solution we used for D7 Systems’ BDAS, a spray used by first responders to clean up areas contaminated with nerve gas or biological weapons. The 49:49:2 ratio of ingredients was critical to the performance of the BDAS application system, and it had to work equally well in extreme temperatures and altitude, even if the product container (or the first responder) was hanging upside down. But that wasn’t the only complication we encountered with BDAS.

Flow Management: Rate, Turbulence, and Mixing

Getting the three ingredients — each with different densities and viscosities — to mix thoroughly and precisely as the user sprays was another BDAS quandary. Flow rate — the amount of fluid that passes through a specific volume in a specific time — is a key design parameter in a fluidics project. It’s easy to calculate for tubes such as a toothpaste dispenser or a syringe, but not so easy for other shapes, or if you’re mixing multiple fluids of different weights and volumes. For BDAS, we used a specially designed spray bar with different orifices to control flow and dispensing rate, and then introduced irregularities to create turbulence.

fluidics design challenges

Turbulent mixing can be very useful. For example, on a different project, we used intentional turbulence to enhance the rinse fluid’s ability to clear mucus and pollen out of a stuffy nose. A nasal rinse product we developed was a high-tech version of a Neti pot, which allows you to pour liquid into one nostril and let it drip out the other. The final shape of the nasal rinse product used a simple design of angled holes based on a shower head to create a spinning vortex of fluid that scoured mucus out of the sinuses.

Flow rate — the amount of fluid that passes through a specific volume in a specific time — is a key design parameter in a fluidics project.

This same innovation inspired the solution for a hose end fertilizer applicator for Central Garden Products. Our client had challenged us to design a better way to mix liquid fertilizer. Liquids are convenient to apply with a sprayer, but inconveniently heavy to ship and carry. Solid pucks of concentrated fertilizer are much cheaper and easier to deal with. So Central Garden wanted a hose-end applicator that could take a solid puck and dissolve it at a slow, even rate through a spray nozzle. To do this, we used angled holes in the puck chamber to create a vortex as the water flowed through-similar to what we did for the nasal rinse device. The vortex dissolved the puck evenly over time, putting out a known dose of fertilizer through the spray nozzle.

Fluidics Control: Isolation and Separation

Sometimes the hardest part of working with liquids is keeping them separate. When we worked on the FSC PathOne Analyzer, it was critical that the liquid sample be isolated from the liquid biosensor until the user intentionally combined them. No leaks!

Essentially, the PathOne Analyzer has a cartridge that houses a fluid that to be tested for the presence of bacteria. It could be a sample taken in a meat processing plant, or a pharmaceutical lab. The user wants to be absolutely sure that this batch of meat or medicine is clean, and they want to do it fast. They put the liquid sample in the cartridge. Then a reagent is pumped into the sample. If the sample contains bacteria, it emits light. If it stays dark, it means there’s no bacteria present. To make sure there are no false negatives, a second reagent is pumped in after a negative result to force the first reagent to emit light and demonstrate it’s working.

fluidics design challenges

Obviously, if any of those liquids mix too early, the reaction either won’t work or won’t be detected. Hence the need for perfect sealing. We used a molded plastic part for the cartridge, heat sealed with a plastic film, to keep all the compartments in the PathOne Analyzer isolated. We worked with our client’s biology engineering partner to build dozens of rapid iterations of fluidics designs with a 3D printer to move quickly from theory to prototype. And then we built a custom heat sealing fixture with a custom heated die to make sure the molded plastic stayed perfectly flat and the film adhered with no gaps. Our client wants to make thousands of these devices and have 99.9% of them work as expected.

Cross-pollination of Fluidics Techniques

Often, challenges we’ve encountered have already been solved — just not in the realm at hand. A number of years ago a client asked us to develop an oxygen concentrator for folks with Chronic Obstructive Pulmonary Disease (COPD). The air we breathe is about 78% nitrogen and only 21% oxygen. Our client wanted the device to provide 99% pure oxygen in, and remove the carbon dioxide the patient would breathe out. And it needed to do it with fairly low power consumption and without the explosion hazard of a pressurized tank of gas.

It turns out the chemical and energy industries already had a solution: zeolites, a type of mineral with a lot of surface area that preferentially grabs carbon dioxide and nitrogen while letting oxygen flow through. It worked.

fluidics design challenges

A few years after that, we were working with the Miller beer company to develop a mini-keg for the home. We needed to keep the top of the beer covered with carbon dioxide to keep it fresh. From our medical air product, we knew about zeolites. Zeolites are food grade! So we ended up charging a container of zeolite with carbon dioxide. By opening a valve to dispense beer into a glass, the beer flowed through the zeolite, which in turn released carbon dioxide into the space left in the keg.

Stay Fluid and Find Your Flow

As the examples above illustrate, there’s no shortage of interesting fluidics design challenges — and the solutions can come from seemingly unrelated products. Staying playful allows you to note the parallels, whether they’re between a tube of toothpaste and a biological decontamination sprayer, or a nasal rinse and a garden hose.

Link that creativity with rigor (engineering analysis, rapid prototyping, and testing) for execution, and you’ll really find your flow.

Read about our Rugged Product Design expertise.

(Engineers Jason Zerweck, Daniel Pfautz, and Mark Clark also contributed to this post.)