After a traffic-packed morning on I-40, I made it back to Chesterfield for another day of fun and adventure! Today, I worked with Brian, a PHD student working to create orthopedic applications. The issue Brian is trying to tackle involves patients that have deteriorated knee joints that require fillers to help their mobility. Often times, the surgery required to install these fillers will lead to bacteria-caused infections, complicating the process and potentially requiring extra-procedures that might damage the patient’s health even more. Once more, there are not many effective methods to apply antibiotics to these infections once the surgeries are complete. Thus, Brian is developing a specially designed reservoir containing antibiotics that is to be put within the bone implants to combat potential infections. Antibiotics are either stored in a paste or a hydrogel inside of the reservoir. The goal of the project is to create a reservoir which will be easily implemented into the fillers, but also to have an extended amount of extrusion time. After a surgery, the reservoir is intended to constantly extrude antibiotics from the paste to the patient’s blood for a period of over 90 days. In order to achieve an extrusion process for such a long time, the reservoir must be carefully designed to let out small amounts of antibiotics over a large period of time. Brian is studying the numerous amounts of designs up for consideration.
Essentially, the design is a half moon shape since that will most easily fit into the knee filler’s open space. Current knee fillers are made of a bone cement material that is not particularly strong, so Brian is making his filler prototypes with a stronger material, RPU 60, a polyurethane. Some of the reservoir prototypes are also being made in the form of cubes for easier testing. As depicted in the images, each of these reservoirs has a holes, or channels, in the walls so that antibiotic paste can disperse into surrounding liquid overtime. The diameter, length, and orientation of these channels are the main factors being tested. As you can see in the image with the half moon samples, there are tiny channels on the walls where the antibiotics can escape. These holes may also connect to tubes that extend into the reservoir. The length of these tubes is a strong factor that affects the time it takes for all the antibiotic paste to disperse into surrounding liquid. The longer the time, the better. Brian has already made hundreds of different reservoir samples using a carbon printer in the lab, and has tested reservoir samples for antibiotic concentration, obtaining massive collections of useful data. One of his designs has exceeded the expected 90 day requirement and reached a 105-day span until all the antibiotics were extruded into surrounding liquid!
Brian creates all of the reservoir samples using a carbon printer: a sophisticated machine that prints using resin and a UV light. This machine even has a foot-motion activated door! First, the desired compound is ejected using a gun into a beaker. The RPU 60 material that Brian uses contains two components that need to be mixed thoroughly before printing. The gun’s tip helps to mix them well. As depicted in the image below, the gun’s nozzle has a long spiral design that receives two separate substances but quickly mixes them thoroughly. After the required amount of resin is extruded from the bottle (77 mL in this print), it is poured into a clear bed in the printer. The door closes, and a platform lowers into the bed to touch it. The technology behind the print lies in the UV light. As UV light shines underneath the bed, any resin exposed to the UV light will solidify. The plate above helps to create a mask above any resin that is not to be solidified. Layer by layer, the plate raises by only a few millimeters per hour to create the print. Each a layer receives a unique mask that will cover parts that don’t need to be solidified and expose parts that do. After about 2 hours, our print was successfully made to an extremely high precision quality. Today, we printed two more unique reservoir designs, a funnel used to aid in pouring the powder into vials to make the antibiotic paste, and a stirring rod. Brian designs all his prints using AutoCad, a highly sophisticated program that allows for extremely precise creations that can take hours upon hours to design.
The finished products were quite a sight! They were stuck to the top plate, as usual, but the carbon printer has some quirky characteristics, one being that a lot of the original resin is stuck to the products. The resin is highly viscous and is a slight pain to clean up. Nevertheless, there are certain cleanup procedures that make the process a little easier. First, the items are pried off of the top plate and placed in a tub of isopropyl alcohol (2-proponal) and shaken until the viscous resin gets off of the solid products. At the same time, the clear tub and top plate are both cleaned out with isopropyl alcohol and acetone is used to wipe down the clear tub’s glass. Though messy, the cleanup process is crucial to ensuring that the finished products set properly and that the printer’s parts can be reused for future prints. To clean the hollow cubes thoroughly, a syringe is used to pump isopropyl alcohol in and out of the small channels to ensure a resin-free product. After everything is sparkling clean, the products are placed in an oven set at 120 *C for 4 hours to allow the material to achieve its highest potential in terms of its structural properties.
In the afternoon, Brian and I designed a Cary Academy charger horse keychain on AutoCad, and we plan to print it out tomorrow using the carbon printer! After much troubleshooting, the design turned out fairly pleasing. Although today was a short day, I learned substantial knowledge about the different medical applications 3D printers can truly bring forth. With all the advanced technology Chesterfield Lab has, there are an endless amount of possibilities for the devices one can create. The sky is truly the limit on this one!