Aaron M Scroggin -- Purdue Graduate Student
 Development of Pressure Sensitive Paint
Mr. Scroggin's graduate research project is sponsored by Boeing. It is an applications oriented project. In the field of aeronautics there is a strong desire to obtain massive amounts of data concerning localized pressures on aeordynamic surfaces. Typically such data is collected in wind tunnels using "pressure taps" (sensors which are individually mounted in carefully prepared arrays of holes). This is a very costly process and limits data to the exact location of each "tap".
The thrust of Mr. Scroggin's work is to use luminophores in an applied layer on such aeronautical structures to enable the collection of data from infinitely resolvable points, and at the same time dramatically reduce costs of test preparation.
Luminophores are materials that absorb radiation to go to a higher energy state. Typically such material will return to its base state with the emission of a photon, at a wavelength longer than the original radiation. If a luminophore is in contact with oxygen, it excites the oxygen which can take on the energy without alowing the photon emission. This characteristic is referred to as oxygen quenching. Since the "oxygen quenching" effect is proportionally related to the partial pressure of oxygen, the brightness (emission of photons) of the luminophore is inversely related to pressure. Thus luminophores are potentially useful for monitoring pressure on a materials surface.
Prior work has put luminophores into polymers to create a "paint". Unfortunately, the polymers drastically diminish the response time for photon emission. However, if luminophores can be integrated into ceramic coatings the response time will be nearly instantaneous. This is the thrust of this project.
In the experimental stages, alumina particles were coated with a very active luminophore. These were then "tape cast" into a flexible and easy to handle film down to thicknesses as low as 0.01 mm. Strips of this film were applied to the wing and other aerodynamic surfaces of Purdue University's executive plane. During a flight test the tapes were excited by a scanning laser light system and photon emissions were captured to a computer for analysis.
The actual data collected was less than desirable. One of the tapes peeled off of the wing. Another problem is the fact the the photon emission is strongly related to temperature as well as to pressure. Some way of compensating for or mitigating this effect must be found.
Next steps for the project will be to develop tapes for use at cryogenic temperatures (where aeronautical types can use small models to represent large structures), and to quantify photon emission with respect to binder content in the tapes.
Marc Yap -- Purdue Graduate Student
Titanium Powder Metallurgy for Spinal Implants
In the human spine there are 23 articulating vertebrae and 9 fused. When a disc requires repair the current practice is to encourage interbody fusion between the adjacent vertebrae. The design criteria for such repair is replacement of the damaged disc with new bone growth, and to restore the interspace.
Early fusion methods called packing relocated bone between the vertebrae. Complications of this method included pain, collapse, and settling. This was followed by using rigid interbody spacers and posterior rods. These, however, frequently failed from fatigue loading. Next came the bone dowel (which was more rigid than the packed bone) and then the concept of the threaded diffusion cage (a porous metal dowel packed with bone).
The emphasis of this research is material development for a new surgical mesh (in a trapezoidal [wedge] shape) which will be an improvement on the threaded cage. The porous wedge holds position better than the dowel but is much more expensive and difficult to manufacture.
Titanium and specifically powder formed titanium alloys offer great potential for the trapezoidal surgical wedge because of their low elastic modulus, biocompatability, and ease of manufacturing to near net shape. A potential improvement to the technology is the development of a "dual density" wedge. This is easily accomplished by multiple sintering operations of the formed powder, and offers the advantage of a fairly dense and strong outer load bearing shell, with a much more porous inner ring. (The added inner porosity can be optimized for ingrowth of new living bone while providing high strength and a low modulus). At this time the demonstration has been restricted to simple geometric shapes, but the dual density titanium P/M technology should easily transfer to the more complex surgical wedge. This work is in need of sponsorship. If interested, please contact Dr. Matthew Krane, School of Materials Engineering, Purdue University.