Category Archives: Engineering

3D Printing Hearts with Valves

Improving Valve Representation in Cardiac Stereolithography by Spatially Registering Magnetic Resonance Imaging and Echocardiography Tyler Robert Moore MDa, Erin Janelle Madriago MDb, and Michael Silberbach MDb Introduction Additive manufacturing, a means of fabricating objects layer by layer though extrusion or sintering, has the potential to impact biomedical research, patient imaging, and medical therapies. One example is personalized anatomic modelling, the creation of tangible models representing the anatomy of an individual patient.1 Of particular interest is the creation of cardiac models in patients with congenital heart malformations.2 The benefits of such models include physician education,3 operative planning,4 and procedure simulation.5 Patient-specific heart models are readily created from cross-sectional data acquired through computed tomography (CT) and magnetic resonance (MR).6 Despite improvements in cardiac-gated CT and MR,7 the temporal resolution of fast moving cardiac structures such as the valve leaflets is limited. Standard three-dimensional echocardiography can acquire volumetric data up to 20 Hz, allowing superior resolution of the valve leaflets.8 Spatially constrained volumetric data is a more recent development in echocardiography that allows for its use in creating heart models through additive manufacturing.9–11 While echocardiography can be used to create very accurate models of the valves, its limited field of view precludes representation of the entire heart, great vessels, and adjacent thoracic structures in a single model. Integration of multiple modalities allows for more comprehensive modelling of the heart by exploiting both the larger field of view inherent to CT and MR as well as the detailed valve anatomy acquired with echocardiography. Combining modalities requires a means to spatially register the data sets. If there are several anatomic fiducials, corresponding points that are readily identifiable in both data sets, they can be used to determine a linear transformation between the coordinate systems of the two studies.12 Preliminary results have demonstrated the feasibility of combining cardiac MR and three-dimensional echocardiography to create such models.13 Materials and Methods Subjects Subjects are less than 18 years of age. Cardiac models are created from cardiac MR and three-dimensional echocardiography performed in the course of the subjects’ care. The Oregon Health and Science University Institutional Review Board approves maintenance of a cardiac imaging data repository for the creation of heart models for pediatric subjects. Source Data MR is performed using a 1.5 Tesla Philips Ingenia with Philips REV5 software. Sequences used directly for modelling include a pre-gadolinium Fast3D sequence, a pre-gadolinium BFFE cine sequence acquired in the short axis plane that included the atrioventricular valves, and a post-gadolinium angiographic sequence. Gadolinium enhanced sequences are performed using a bolus of Gadavist at 0.1 mmol/kg. Subjects under 12 years of age are routinely sedated by a pediatric anesthesiologist as part of the institutional routine. Three-dimensional echocardiography data are acquired using a Philips iE33 xMATRIX with an X7-2 probe. Volumetric data sets are acquired in 30 temporal phases per heartbeat with 208 slices per phase. Visual inspection allows retrospective selection of the temporal subset corresponding to the appropriate phase of the cardiac cycle. Software Mimics Innovation Suite v17.0 (Materialise, Belgium) is

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Continuous Measurement of Fluid Density, Part 2: Parts and Plumbing

Continuous Measurement of Fluid Density, Part 2: Parts and Plumbing

Before the hiatus, I made an attempt at creating a Continuous Fluid Density Sensor. Here are the primary components: Two dip tubes in the fermenter MPXV7002DP differential pressure sensor ADS1115 16-bit analog to digital converter Generic, chrome plated brass, 2.5 mm hose barb to M3 adapters and 2.5 mm pneumatic hose Generic 12V aquarium diaphragm pump (model no longer listed on eBay) Plumbing The first step was to install new dip tubes into the fermenter. These are just two tubes in the tank that terminate with their openings directed downward. They are connected to cam-lock adapters on the outside of the tank for attachment to hoses. I also wanted the portions of pipe inside the fermenter to have as few recesses as possible to limit contamination by undesirable microbes. I soldered the NPT stainless fittings using acidic flux (I used Stay-Clean). The 1/4″ stainless tubing connects through a compression fitting. These leaves less room for leaks and microbes. Now that we have dip tubes between which to measure the pressure difference, the first step in design is to calculate the expected difference in the pressure at the tip of each tube. The difference in their heights is approximately 15 cm. The conversion between Pascals and centimeters of water is , so the differential pressure between the two tubes due to a column of water is: The original gravity for a generic pale ale is approximately 1.05, so the differential pressure at the beginning of fermentation would be: The final gravity for a generic pale is about 1.011, so the differential pressure at the end of fermentation would be And finally, the change in the differential pressure is . The dip tubes need to be connected to a differential pressure sensor. The pressure sensors in the price range for this application connect via 2.5 mm pneumatic hose, but there are no adapters between pipe fittings (what I use in my brewery) and this diameter hose. So, I made adapters by taking a short length of 1/2″ NPT copper pipe and soldering a brass hose barb into it. The plating on the brass interfered with the soldering, so grinding down the threads on the barb before soldering was necessary. Diaphragm Pumps Lastly, the dip tubes will have a tendency to fill with fluid. Measuring the differential pressure will require a means to push the fluid out and fill the tube completely with gas. That’s accomplished with two diaphragm pumps. These simply take gas from the top of the fermentor and push it through the tubing leading to the dip tubes. MPXV7002DP This device is a differential pressure sensor. It operates at 5V and outputs an analog voltage between 0.5V and 4.5V that is proportional to the pressure detected by the device. Specifications dictate that this device detects a -2.0 kPa to 2.0 kPa range at 2.5% average and 6.25% maximum error. The first question is whether this is a significant error. At a range of 4000 Pa, the average error is 100

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Continuous Measurement of Fluid Density, Part 1: Concept and Design

Continuous Measurement of Fluid Density, Part 1: Concept and Design

Introduction One of the chores every brewer performs is measurement of the density of a fluid. This measurement gives two critical data from the brewing process. First, a measurement right before fermentation tells how much sugar is in the wort. This is called original gravity and is one of the key parameters of any beer recipe. A large portion of the work that goes into a brew day is to control the quantity and types of sugars that are in the wort. Second, since the density of the beer decreases as the yeast convert sugars to ethanol and other byproducts, measurement of density allows the brewer to monitor the progress of the fermentation. When fermentation is complete, the last density measurement is called the final gravity. The difference between the original and final gravities can be used to calculate the alcohol content of the beer. There are two methods available to brewers to measure density. The simplest device is the hydrometer, a glass bob that is submerged in a cylinder of liquid. The denser the liquid, the higher the hydrometer floats. The other device is a refractometer, which measures how much the fluid refracts light. Sugar and alcohol in the beer change its refractive index, so a few calculations can yield the concentration of sugar and alcohol in the fluid. Considering how important a task this is, it’s embarrassing to admit that I absolutely hate doing it. Each of the above methods requires collection of a sample of the fluid. I don’t mind this on brew day, but collecting a sample from the fermentor using sanitary technique is a hassle. I like to be hands off during fermentation; that’s why I went through the trouble of building an automated temperature controller. It’d be great to add density measurement to my existing system. I’ve been slowly tinkering with a solution, but the idea of automatic specific gravity measurement on a homebrewer’s scale isn’t new. The homebrew forums have been kicking ideas around for years, and have beaten it to death. Then, the Beer Bug hit the scene with what is probably the best solution. If I’d known about it sooner, I probably would have bought one. However, I’ve put a fair amount of work into my prototype, and I’d like to see it through. P&ID Legend There are a few diagrams to follow, and I’ll use symbols that are mostly self-explanatory. Two that may not be well known require some explanation.This is a manual valve. It sits at the bottom of my fermentor. I open it when I want to collect yeast or transfer the beer to a keg.This is a check valve, or a one way valve. It allows fluid to flow in the direction of the arrow, but not the other direction. Concept One of the simplest ways to find the density of a fluid is to measure the pressure caused by a height of the fluid. The diagram above shows the fermentor and two ports at different heights connected to a sensor. The

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Heat Exchange Snobbery

The recipe for beer is pretty simple. One important step is boiling the wort, the sweet mixture that yeast will ferment into beer. Boiling sanitizes the beer so that yeast can work its magic, but not before the wort is cooled back down or the yeast meets the same quick, steamy end as its potential competitors. There are many ways to cool wort, but I recently upgraded my cooling technique to the optimum method: countercurrent heat exchange. I previously used this immersion cooler. This is a good method in which a length of coiled tubing is placed into the wort. Cool water is pumped into one side and it comes out hot on the other, all the while cooling the wort. Despite the ease of execution of an immersion chiller, it’s limited. Initially, there is a large temperature difference between the wort and the cold water running through the chiller, and it works well. As the wort cools, the temperature difference between the wort and the water decreases, and the cooling is less effective. It takes more and more water to accomplish the same heat transfer. Countercurrent exchange doesn’t suffer this limitation, and this is why it’s a superior heat transfer method. How it avoids this limitation can be demonstrated by comparing the countercurrent configuration to the parallel configuration, which is somewhat similar to the immersion chiller method. Here’s a diagram showing both flow configurations: Both involve a tube within a tube. The inner tube is usually made of thin metal so that the fluids can easily exchange heat across the metal. In the parallel configuration, hot wort is introduced at the same side as cold water. As the wort and water flow through the tube and exchange heat, they approach the same temperature you would get if you mixed the two in a bucket (ignoring the thermodynamic effects of mixing). In the countercurrent configuration, hot wort is introduced at one end and cold water is introduced on the other. This results in a very different temperature profile for each fluid along its path. Check out this diagram from Engineering Toolbox: The result is that the wort is continuously cooled throughout its path while the cold water is continuously heated throughout its path. To phrase this more practically, you more fully utilize the cooling ability of all the water you use. Unlike the immersion chiller that only heats the first amount of water to near the starting wort temperature, all the water used in a countercurrent heat exchanger will be heated near the starting wort temperature. While countercurrent heat exchange is limited by the surface area and the flow rates, so are other methods. For any exchange method, too small a surface area or too high a flow rate and the fluids don’t have a chance to exchange enough energy. Countercurrent exchange is not equilibrium limited, and this is the difference. Countercurrent exchange isn’t just an engineering technique. It evolved biologically and is evident in many homeostasic mechanisms. My favorite is the kidney, an analogous example of countercurrent mass transfer. Remember those tubules and the sodium gradient? That’s countercurrent

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