Joe Cesarano holds a jawbone with a robocast scaffold.
Growing Bones in the Lab
October / November 2004
Volume 2 Number 5
Imagine that a disease or traumatic injury has caused you to lose bone, perhaps in your skull, spine or jaw. Surgeons can replace that missing bone, but only by pirating healthy bone, maybe from your pelvis. Now imagine that your missing bone can be re-grown around and through a synthetic scaffold that is custom-made to fit the gap left by the lost bone. Researchers at Sandia National Laboratories and their university colleagues have developed a process that can do just that and much more. No need to sacrifice part of your pelvis and no need to have invasive surgeries in two distant parts of your body.
Sandia's Joe Cesarano and his team have developed a new way to fabricate ceramics that requires no molds or machining. Known as robocasting, it relies on robotics for computer-controlled deposition of pastes—€”mixtures of ceramic or metallic powders, water and trace amounts of chemical modifiers—€”through a syringe. The material, which flows like toothpaste, is deposited in thin sequential layers onto a heated base and can form second- or third-dimensional structures made from single or multiple materials.
Scaffolds for Bone Growth
To create the artificial bone scaffolding, the Sandia team, with colleagues from the University of Illinois at Urbana-Champaign, programmed the robot to dispense a mixture in cross-laid slivers each about as thick and as far apart as the diameters of 10 human hairs. "Bone cells, blood vessels and collagen love to grow into a structure with pores of that size [200-500 microns]," Cesarano explains. "The material becomes a hardtissue scaffold for promoting new bone growth." The trick in building it, he says, is that "the paste has to be strong enough to span a gap as it's being laid to set in place without underlying supports."
If approved by the FDA for testing, the scaffoldlike structure would substitute for a portion of the skeleton until healthy, newly grown bone and blood vessels could weave their way through it. The ceramic scaffolding would reduce the pain, recovery time and chances of infection of those needing bone replacements. The scaffolds would be built mainly of hydroxyapatite, a material already approved by FDA for body implants, so approval of the new device could be swift. Sandia has applied for a patent and testing has begun.
In the first test of robocasting for bone scaffolding, an elderly woman at the Carle Hospital in Illinois was fitted with a Sandia-created implant for a missing portion of her lower jaw. The implant was created using only a CAT scan of the patient's skull, a sketch from the surgeon, robocasting technology and CAD/CAM methods at Sandia. The purpose of the fitting was to see if an implant could be remotely fabricated and customized to fit precisely into a damaged region of bone without ever seeing the patient. Precise details for size, shape and inclusion of a clearance groove to protect a nerve had to be met. Observers said it fit like a glove, better than traditional implants built by surgeons. As University of Illinois surgeon Michael Goldwasser put it, "What we want is a method by which I can see a patient in Illinois, transmit X-ray information to someone who can make a substitute part that would have the porous properties that would allow bone to grow into it, yet be strong enough for normal function." Unfortunately, after testing the fit, the scaffold had to be removed pending approval of use of such devices in humans. The scaffolding is much less painful and invasive than the standard method of bone replacement, which involved cutting a several-square-inch piece of bone from her pelvis that was then power-sawn and drilled into the correct shape right in the operating room, a process that takes about an hour. "Surgeons and patients would love to eliminate both the retrieval and implant preparation processes," says Cesarano. "This test showed we can make artificial porous implants prior to surgery that will fit perfectly into the damaged region. The reconstructive procedure would then only require attaching the implant and closing the wound." In the future, similar devices may be used in thousands of procedures requiring load-bearing bone scaffolds.
Multiple Materials and Internal Structures
Before the opportunity for building bone-growth scaffolds arose, the original goal of this R&D project was to develop a freeform fabrication technique for ceramic, metal, or bi-material/composite components for weapons components. To satisfy that goal, Cesarano and his team developed pastes from colloidal suspensions, or slurries, with the appropriate rheology (the deformation and flow of matter), density, and drying rate, as well as the software and equipment for laying down the pastes with precise control of layer thickness and feature resolution. The team (including Professor Paul Calvert from the University of Arizona) succeeded in fabricating components by computer-controlled dispensing of pastes through a nozzle and showed that any two-dimensional (2-D) pattern can be written, layer-by-layer, into a three-dimensional (3-D) shape.
Cesarano's team also incorporated laser diagnostics with their robotics to make a metallized ceramic that conforms to a non-planar, or 3-D, surface. In other words, they can build, layer by layer, a 3-D ceramic and metal structure that automatically follows the contours of the structure on which it is built. In this mode, the robocaster can also be used as a precision writing device for depositing commercial inks onto existing components.
Robocasting, as its name implies, relies on robotics for computer-controlled deposition of ceramic pastes through a syringe or nozzle. "Layer by layer the part grows before your eyes." Cesarano describes the sequence, "The robot squeezes the paste out of the syringe, almost like a cake decorator, following a pattern prescribed by computer software." Because robocasting allows a dense ceramic part to be free-formed, dried, and baked in less than 24 hours, it is perfect for rapid prototyping without molds or extensive machining. Engineers can quickly change a design of a part and physically see if it works.
Robocasting has other virtues besides requiring limited machining and no molds, and being able to make complex parts in less than 24 hours. "Unlike the more traditional methods of ceramics fabrication, this has the advantage of being able to apply more than one material at a time," Cesarano explains. "Thus, materials can be graded, for example, going from a ceramic material to a metal within one part without causing structural damage." Another positive feature is that the technique enables the discrete placing of a "fugitive" material that would evaporate or burn away during the baking process, similar to the lost-wax process in jewelry making.
Multiple Applications for Freeform Fabrication
In addition to recognition for its work on bone scaffolding, the Sandia team and researchers at the University of Illinois were also acknowledged by Chemical and Engineering News, the weekly publication of the American Chemical Society, as one of the nine most interesting materials achievements of 2002. Cesarano and Illinois Professor Jennifer Lewis used colloidal gels (inks) and solid freeform fabrication to automatically construct intricate 3-D structures with micrometer-size features and overall dimensions of a few millimeters. The C&E News wrote, "Imagine building a staircase using only one tool: a spray gun that squirts out a stream of concrete that spontaneously assumes the required shape."
Possible uses of the colloidal gel technology include advanced ceramics, photonic materials, catalyst supports, as well as biocompatible tissue scaffolds.
As a possible military application, the casting system could be transported into a battle zone to make replacement parts, instead of having to transport a huge inventory of materiel to a battlefield. Sandia is also exploring the use of robocast lattices for reducing emissions from gas turbine power plants and diesel engines. A drawback to the traditional straight-channel honeycomb catalyst-support structure is the relatively poor contact of the combustion products with the catalyst. Robocasting can produce lattices with alternative 3-D geometries, providing twisting pathways to increase contact, which will reduce the emissions by greatly improving contact between combustion products and catalytic surfaces.
The robocasting project has led to a number of applications both inside and outside the national security complex. Cesarano believes that the technology can be scaled-up and that commercial opportunities, through licensing or collaborative R&D, may exist. He sums up the input from the breakthrough: "We started out trying to develop a new freeform technology for ceramics but we ended up with much more, a technology tool for myriads of applications requiring precision deposition or printing of materials."
Margaret Lovell is a technical writer for Technically Write assigned to Sandia National Laboratories.
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