Creative Outcomes Using Resin Systems
February 2014 Issue
As technicians, we're expected to be able to conquer uncommon fabrication challenges. Creating a lightweight, thin structure that will withstand all the forces applied throughout the device is the minimum requirement. In addition, we are often tasked with achieving other characteristics, such as a rigid finish for durability, or a flexible finish to assist in donning a device or to improve comfort during extended use.
Whether you are fabricating an orthosis or prosthesis, you have many ways to accomplish these creative outcomes. However, few options rival the flexibility of resin systems for use in fabricating custom products.
A Controlled Process
To have repeated success when transforming materials, it's important to control the process. Every factor that influences the result-time, temperature, pressure, and technique-can be controlled. When you are using new techniques, the best way to control these variables is to record them. Precisely following instructions and recording each step ensures that you can replicate the process. This will allow you to create consistent results.
All resins must be maintained under elevated vacuum for the resin to cure. You create a controlled environment for the resin with a vacuum system sealed within an inner and outer PVA bag. Depending on the technique, the best vacuum pressure may range from 5 to 25 inches of mercury (inHg).
When you mix pigments into the resin to achieve a specific color, do not exceed 3 percent of the resin's volume. For example, for 100 grams of resin, the maximum amount of pigment you should use is three grams. Exceeding the manufacturer's recommended maximum may cause the resin to overheat and allow gas bubbles to form. These bubbles result in pores throughout the lamination.
A single-shot lamination technique will achieve a continuous laminated structure that is either rigid or flexible (Figure 1). At Ottobock, we often use this technique for lower-limb orthoses and prostheses.
For a carbon composite, especially when reinforcements are required, we prefer Ottobock C-Orthocryl® (617H55), a rigid, acrylic-based resin system that maintains strength and shape with thin, laminated walls and is skin-friendly. Designed especially for use with carbon fibers, it is thinner to better saturate the carbon fiber and minimize the resin-to-carbon ratio.
Some resins can be combined to modify the structure. For example, C-Orthocryl can be mixed with any of the other Orthocryl resins to increase its softness and flexibility.
Start by applying the inner PVA bag over the mold. Once you have the Perlon and any carbon fiber set in the desired layout, apply the outer PVA bag. If you combine resins, mix them. Add pigment, if required, and mix again. Add promoting agent and mix to activate the curing reaction. Pour the prepared resin into the PVA bag, and seal the end. Once you have finished that step, apply vacuum at about 20 inHg for the best results. C-Orthocryl has a longer cure time-40 to 50 minutes-than traditional acrylic resin, giving you time to saturate a large surface area when working with orthoses. Once the resin cures, you can complete the device with your usual finishing process.
When a fabrication project requires a combination of flexible and rigid structures, you can create two separate layers, an inner layer made of flexible thermoplastic or flexible lamination attached to an outer, rigid frame. We often combine two types of resin in one structure. For an upper-limb prosthesis, for instance, we may be asked to create a rigid structure to support the forearm and wrist unit plus flexible brims for easier donning and more comfortable use. Similarly, we may create a flexible flap for the opening of an otherwise rigid prosthetic hip.
For the first lamination, we choose a blend of 40 percent Orthocryl® Sealing Resin (617H21), a rigid resin, and 60 percent Orthocryl Lamination Resin 80:20 (617H19), a premixture of rigid and soft resins designed to bond with a second, flexible lamination.
Once you apply the outer PVA bag over your layup, mix the resins. Add pigment and mix again. Then add the promoting agent (no more than 3 percent of the resin). Pour the mixed resin into the bag and tie off the bag.
Set the vacuum to 5 to 7 inHg. This is a level that will prevent the pressure from pulling resin into areas reserved for flexible resin. Carefully begin to saturate the materials. Stop just before the desired flexible locations (Figure 2). Allow the resin to cure for eight to ten minutes. Once the resin has cured, remove the outer PVA bag, and use sandpaper to roughen the surface of the initial laminated socket lightly. Clean the surface with isopropanol, being careful not to get any debris into the remaining, unsaturated Perlon, and then apply a new PVA bag.
For the flexible lamination, switch to Orthocryl Flexible, which is designed to bond to Orthocryl 80:20. When mixing this resin, be sure to use the same percentage of pigment to match the initial lamination. After mixing the pigment, add hardening powder equal to 2 percent of the resin. Be careful not to add too much promoter, which can cause the resin to overheat and create small gas pockets throughout the lamination. Pour the resin into the PVA bag and seal the bag. Increase the vacuum pressure to 10 inHg, which should assist in getting resin throughout the system more quickly. Saturate the remainder of the Perlon and any carbon materials (Figure 3). Allow 30 to 40 minutes for curing. Then remove the PVA bag, and use your standard finishing process.
This technique creates a seamless transition from rigid to flexible while minimizing process time, weight, and thickness, especially compared to fabrication of a separate inner and outer socket.
Ottobock developed a third process that allows us to combine rigid and flexible structures without having to laminate twice. Any resin combination can be used, but at Ottobock we use Orthocryl Lamination Resin 80:20 for rigid areas and Orthocryl Flexible to create flexible additions. This technique uses preformed Ottobock Polyethylene Filling Sleeves (616S2) to help saturate a large surface area and distribute soft resin to specific locations to create flexible flaps and openings. We typically apply this technique to fabrication of a hip prosthesis with a flexible brim. It often requires the assistance of another technician, both for placement of filling sleeves and for saturation of a large area with additional resin.
Once you complete the layup, study the contours of the mold to identify locations that you want to be flexible or large surface areas that need additional resin to saturate fully before the resin cures. Position the filling sleeves just before applying the final PVA bag. It may help to have someone maintain the position of the filling sleeves while the PVA bag is donned (Figure 4).
Mix the flexible resin and pigment. In a separate container, mix the rigid resin and pigment. Measure promoting agent for each resin. After mixing promoting agent in the flexible resin, fill the appropriate tubes. Working quickly, squeeze from the end of the PVA bag to push the resin through the filling sleeves into the materials (Figure 5). When the proper amount of resin has been delivered, pull out the filling sleeves.
Immediately mix promoting agent into the rigid resin. If extra resin is needed in a large area, fill the appropriate sleeves. Push the resin through the filling sleeves to the materials that need extra resin. Then pull out the filling sleeves. Tie off the bag, then apply vacuum at 15 to 20 inHg. Saturate the rest of the materials in your mold using the remaining rigid resin, and allow 40 to 50 minutes for the resin to cure. Then finish the project with your standard process.
Understanding how your materials work gives you the ability to create unique outcomes-from rigid, laminated structures to soft, flexible laminations, to a combination of both. Using this knowledge creatively with resins allows you to consistently produce a lighter, thinner, and stronger device that will delight both the patient and practitioner.
Justin Eitel is the technical orthopedics lead for Ottobock US. He oversees all orthotic and prosthetic fabrication at the Ottobock technical center, Minneapolis, Minnesota.