Carbon Fiber: The More You Know, the More You Can Do

By Justin Eitel

Once a clinician sees a patient, the creative process of designing and fabricating a suitable prosthetic or orthotic device begins. Ideally, the clinician and the technician work together, using the technician's expertise in materials and technical aptitude to achieve desired characteristics.

As the technician, you need to understand the physical properties of the materials available to you. Because fabrication of all devices involves transforming materials, you will be more successful using known traits to your advantage rather than trying to force materials to do what their chemical or mechanical structure will not allow.

Using the right material for the right application holds especially true for a high-cost, high-tech material like carbon fiber because mistakes are expensive. The unusual properties of carbon fiber create real potential for errors. However, it also has distinct advantages. If the clinician wants the device to be lightweight and strong, think carbon fiber. Carbon fiber can greatly reduce the weight and thickness of components while boosting structural strength, stiffness, and stability—if it's used properly.

Carbon Fiber Basics

Carbon is used in orthoses and prostheses as a durable, fiber-reinforced composite, similar to traditional laminations made with Perlon or Nyglass stockinettes. Carbon fabric looks flimsy until it physically and chemically bonds with a resin system to create carbon fiber reinforced plastic (CFRP).

With carbon fiber, Ottobock uses a thinner acrylic resin to minimize the resin-to-carbon ratio and lengthen cure time, an important advantage when laminating larger devices.

The resin fixes the carbon fibers in a geometric arrangement, transmits force to the fibers, and stabilizes the fibers as pressure builds. The embedded fibers strengthen and stiffen the composite and absorb forces throughout the length of the fibers. The chemical bond created by carbon atoms in the resin matrix produces strength superior to most metals and other fiber-reinforced composites, giving you the ability to fabricate dynamic, energy-returning structures in passive devices.

Carbon fiber comes in two weaves:

  1. Unidirectional (all fibers are parallel). In a 0-degree orientation, aligned with the fibers, unidirectional (UD) carbon fiber provides high bending strength against the progression of forces, which makes it perfect for the carbon-fiber prosthetic blades used by Paralympic sprinters. In a 90-degree orientation, perpendicular to the fibers, it flexes. In either orientation, it has low torsional strength. In all applications, another layer of fabric, such as Perlon, must be laid under and over UD carbon-fiber weave.
  2. Bidirectional (fibers cross at a 90-degree angle). In a 0-degree or a 90-degree orientation, bidirectional carbon fiber features medium bending strength and medium torsional strength. At a 45-degree orientation, it is more flexible and has high torsional strength, which are just the right properties for a transfemoral socket, for example.

You can even tailor the composite's properties with fiber length, the type of weave, fiber orientation, the number of layers, and the resin system to modify the component's capability to bear mechanical loads.

Guidelines to Using Carbon Fiber

To make carbon fiber perform as expected follow these guidelines:

Handle with care. Carbon fiber may provide the lightweight strength of futuristic aircraft, but carbon fabric is surprisingly delicate. Folding it or scratching it with a sharp tool can damage filaments, which could produce a weak spot in the finished component. Under force, broken fibers become a natural breaking point. Carbon fabric stretches little in length or width, but be alert to its tendency to stretch at an angle.

Identify areas requiring flexibility. Because bidirectional carbon-fiber weave aligned 45 degrees to the line of progression is the most flexible, incorporate it where a component needs low resistance. We use it in the orthosis footplate that fits inside a shoe to provide the flexibility that allows the patient's forefoot to roll over smoothly for a more natural gait. (Editor's note: to learn more, read "Making an Orthotic Carbon-Fiber Footplate with a Flexible Forefoot," The O&P EDGE, September 2012.)

Elastic weft threads in unidirectional stockinette maintain the orientation of carbon fibers for axial reinforcement against bending forces.

CFRP has a limited range of flexibility, and it must be on an even plane to allow the carbon fiber to flex. Do what you can to prevent additional forces that may lead to premature failure, such as incorporating heel height and toe pitch into the cast or plaster modifications for a footplate.

Identify areas requiring torsional resistance. Bidirectional carbon weave at 45 degrees also contours well under applied pressure, like weight load. Torsional strength qualifies it as the best choice for a prosthetic socket. Woven carbon-fiber stockinette simplifies socket fabrication, reinforcement frames, and struts. Choose the stockinette width that positions fibers in both directions at roughly 45 degrees to the line of progression because any other angle will lower torsional resistance. An additional layer of bidirectional cloth also can add torsional resistance near the brim, thigh cuffs, and calf cuffs.

Identify areas requiring reinforcement. Where you need strength, go with UD carbon fiber in a 0-degree orientation. Horizontal UD fabric provides rigidity in the thigh and calf shells of a KAFO. For axial reinforcement against bending forces, carbon-fiber UD stockinette makes jobs easier because elastic weft threads allow carbon fibers to maintain their orientation and form to the model's contours.

PVC profile bars as core material inserted into a narrow width of carbon bidirectional stockinette fortifies CFRP enough to replace part or all of the metal sections in an AFO. In addition, a heavier patient might need another layer of carbon fiber or a PVC profile bar in carbon stockinette as reinforcement in areas prone to pressure.

Align precisely. In a fiber-reinforced composite, UD carbon fiber offers maximum strength when the carbon fibers align with the direction of force. However, according to Ottobock's research, turning the orientation of the carbon fibers 10 degrees from the direction of force reduces the composite's absorption of force by 62 percent. That's a huge difference. So it's vital to ensure correct orientation.

For proper orientation of bidirectional fabric, the arrow pattern (running at a 45-degree angle) in the carbon weave must follow the line of progression.

Consider the resin. CFRP is a duroplastic-it is strong, light, and able to retain shape even with thin walls. Duroplastics are produced by the cross-linking chemical reaction between the resin and hardener. The resulting chemical bond is irreversible, so you can't reshape the finished component. Applying heat will only cause it to melt.

What About Prepreg?

Pre-impregnated carbon fiber, better known as prepreg carbon, is a high-performance reinforced material that is used in racecar bodies and aircraft fuselages-plus prosthetic and orthotic devices.

It is manufactured in sheets or rolls already containing an optimized balance of epoxy resin and carbon fiber. Prepreg carbon minimizes weight and maximizes responsiveness. The low resin-to-carbon ratio accentuates the ability of the carbon fibers to respond dynamically. The energy return nearly equals the energy applied by the patient, which makes ambulation more efficient.

Available in both unidirectional and bidirectional carbonfiber weave, prepreg is cut to fit, so there's no trimming of excess material during the fabrication process.

CFRP components, with an average carbon content of about 30 percent, usually are made with one of two types of resin systems:

  • Epoxy. Epoxy produces a strong, stiff structure with good adhesion to the carbon fibers. Because of its long setting time, some technicians prefer epoxy for long devices, such as KAFOs.
  • Acrylic. Acrylic produces a colorless, transparent structure with skinfriendly, antibacterial properties. The resin matrix has a short setting time and surrounds, but does not actually adhere to, the carbon fibers.

Ottobock, which uses acrylic resin for most fiber-reinforced composite in orthoses and prostheses, developed C-Orthocryl® especially for use with carbon fiber. It's thinner to minimize the resin-to-carbon ratio, and it has a longer cure time to ease fabrication of larger devices. It also can be used to make rigid structures or soft, flexible laminates that will not stiffen over time.

Now that you know the properties of carbon fiber, you can fabricate better devices that will perform better for patients.

Justin Eitel is the technical orthopedics lead for Ottobock US. He oversees all orthotic and prosthetic fabrication at the Ottobock technical center, Minneapolis, Minnesota.