2010

Biotic-Abiotic Interface Between the Body and an Artificial Limb

A modified titanium construct and specially machined surface increase adherence of tissue to a prosthetic limb.

Because of its high mechanical properties, chemical stability, and biocompatibility, titanium is a commonly used material in dental and orthopedic applications. Its excellent biocompatibility allows titanium implants to be directly anchored to bone, or osseo-integrated. The conventional prosthetic replacement in amputees is a stump-socket design, which transfers forces through the prosthetic to an external contact point on the patient. Such a design results in non-uniform distribution of pressure, and can lead to pain, infection, and necrosis of the soft tissues at the point of contact.

It is believed that intra-osseous transcutaneous amputation prostheses (ITAPs) can overcome these issues by directly attaching the implant to the skeleton through transcutaneous abutment. Optimizing the attachment of the skin to the prosthetic will lead to clinically viable ITAPs. Human skin is multifunctional, and consequently, has a complex architecture comprised of multiple layers with some indistinct boundaries. Skin acts as an active protective agent, or barrier, against traumas such as friction, impact, pressure, and shear stress.

In order to develop a clinically viable ITAP, the device must be mechanically strong, provide a tight seal at the bioticabiotic interface, and take into account the complex properties of skin and other native tissues. A surface-modified titanium construct with an etched surface was developed to create a surface that would allow for direct tissue adherence, as well as scaffold adherence. These constructs were analyzed, and adherence of a polycaprolactone (PCL) scaffold was tested both in vitro and in vivo. The use of different antibacterial agents was investigated to reduce bactieral invasion.

altA computer-aided biology (CAB) tool, which was previously known as the BioAssembly Tool or BAT, was developed to produce artificial constructs that would demonstrate properties of native tissue (microenvironment, three-dimensional organization, and intercellular contact). The CAB tool utilizes a computer-aided-design/computer-aided-manufacturing (CAD/CAM) approach to build heterogeneous tissue models. This system is a multi-head, through-nozzle deposition machine developed to conformably deposit biomaterials, cells, and co-factors on various supporting surfaces to create surrogate tissues and tentative platforms for experiments in cell biology and tissue engineering. The device contains an XY coordinate system with a stage; a number of Z-traveling deposition heads (currently up to 3), each of which is supplied with an individual controlling video camera; LED work area illumination; a fiber-optic light source to illuminate the deposition area and cure photopolymers in-line; individual ferroelectric temperature controls for each deposition head; a water jacket temperature control for the stage; stainless steel and anodized work surfaces; and a piezoelectric humidifier.

When PCL was printed on top of smooth titanium buttons, the PCL peeled off of the buttons upon drying. When the titanium buttons were either acid-etched or holes were added to the button surface, the PCL would remain on the titanium button. The different surface modifications were examined to determine which provided the best adhesion between the PCL and the button. Buttons that were acid-etched had an average adhesive strength of 0.13 MPa. When holes were added to the buttons, there was a significant increase in the adhesive strength as compared with acid-etched buttons with no holes. And, while there was a slight increase in the average adhesive strength, there was no significant difference between holed buttons that were acid-etched as compared with holed buttons that were polished. Thus, the addition of holes gave the desired result of a significant increase in adhesive strength.

When an antibacterial agent was added to the PCL or placed on the titanium button, there was no significant decrease in the number of viable bacteria when compared with bacteria in culture. However, when an antibacterial agent was added to type I collagen or hyaluronic acid, there was a decrease.

These experiments made some important observations. First, the study demonstrated the feasibility of these experiments in testing prototypes for optimizing the epithelial prosthesis interface. Secondly, the study showed that the modified titanium material has an advantage over the plain titanium material to maintain a better interface interaction.

Based on the experimental results, by machining the surface of the titanium, the 70% PCL adheres to the surface like a bio-concrete. By adding a natural biopolymer such as HA or collagen and mixing it with an antibacterial agent, the titanium will not only exhibit adhesion, but also prevent bacterial invasion.

This work was done by Dr. Kenneth H. Church of Sciperio, Incorporated for the Air Force Office of Scientific Research. For more information, download the Technical Support Package (free white paper) at www.medicaldesignbriefs.com/briefs. AFRL-0158

This Brief includes a Technical Support Package (TSP).

Biotic-Abiotic Interface Between the Body and the Artificial Limb (reference AFRL-0158) is currently available for download from the TSP library.

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