Amazing research is being done studying biological implants.  These implants have the potential to do everything from restore sight and muscle operation to automatically release pressure to the brain.  They are often tiny microchip sensors that detect and send information to the doctor or the body itself.  One chip, for example, would sit on the back of the retina and send electrical pulses to the brain.  It is important to research the effects of a sensor placed in tissue for a long period of time.  Obviously, the sensor must be inert and not react with any biological tissue.  For this reason, sapphire, Al₂O₃, which has good optical qualities as well as being hard and inert, is often used.  However, these retinal implants tend to quit working with time.  One supposition is that aluminum is leaching out of the sensor into the tissue.  This could cause the mechanism to fail and also deposit harmful metals into the tissue. 

I am studying the application of LIBS on a tissue-like target seeded with metal.  I have researched suitable materials to imitate tissue and various methods of metal placement.  Now, I am finding an appropriate experimental setup to maximize the clarity of the data.  The ability of LIBS to detect the metal and the limits of that detection will soon be determined.  And finally, I will study methods to make the technique less complicated and more portable. 

Members of the wireless implant group of the Smart Sensors and Integrated Microsystems (SSIM) Program are working on creating retinal implants to restore sight to the blind.  The project is huge, requiring expertise from many departments at Wayne State University.  The general gist of the sensor is as follows:  an extra small camera is attached to a tiny processor.  Then some coils turn the image into an electronic signal.  The signal is sent to tiny microbumps on a plate.  Neurons are grown using guides to attach the microbumps to the optic nerve.  The whole system is on a sapphire substrate, chosen for its property of being biologically inert.  Of course, the problem is that this "inert" sapphire might be leaching aluminum into tissue.  Not only is trace aluminum useless for cells, but aluminum is also known to help cause Altzeimer's Disease. 

Both gelatin and agarose showed elements of carbon, calcium, and sodium, though according to London South Bank University [7], calcium and sodium are not in either gel and carbon is not in agarose.   By looking at (C) from the Figure 1, we see that the two spectrum are relatively alike.  The average view brings out the wider peaks like H, the O triplet, and the molecules, while the maximum view shows the narrow, tall peaks well.  The increase in sodium for the gelatin could likely be due to the height of the LIBS spark in the sample;  deeper sparking seems to bring out more sodium.   After seeing the similarities of these spectra, and after the gelatin sample molded, the decision was made to use agarose from then on. 

There was some difficulty in adding aluminum to the samples.  At first, aluminum particles of an average diameter of 20 microns were mixed with the agarose and water before heating.  The sample was then stirred with a magnetic stir rod while the sample was heated and the agarose dissolved.  The aluminum would remain well mixed until the sample was allowed to sit and cool, and then the aluminum would settle out.  An attempt was made to stir the sample while it cooled until it thickened enough to hold the aluminum in suspension, but this did not work.  To fix the problem we considered buying aluminum already in solution.  Since most aluminum molecule solutions were caustic, harmful, or let off poisonous gases when mixed with water, we determined to use a nanoparticle aluminum oxide water solution.  The 50 nanometer or less sized particles remain in suspension without stirring, giving the solution a milky color due to the scattering of light reflecting off it.  Three samples were made by adding the aluminum oxide water solution to the agarose and water before heating.  The concentrations of these samples were 1070 ppm,  203 ppm, and 104 ppm.  LIBS has not yet been done on these samples. 

Some LIBS was done on the micron sized aluminum doped agarose samples, though because some aluminum settled out, the concentration is unknown.  The following spectrum shows the plain agarose (2% concentration) overlaid with an aluminum doped 2% agarose sample.  The aluminum is this picture is especially clear. 

Current work is on testing the new aluminum doped agarose samples and determining the limit of detection for aluminum concentration.  One difficulty is making smaller concentrations than 100 ppm, because such little amounts of Al solution are needed.  Others doing LIBS of trace elements in water have seen concentrations down to 20 ppm [insert reference], so these samples will probably be needed. 

We are also interested in utilizing a cylindrical lens to ablate our samples.  This lens creates a line of ablation instead of a point and ablates more of the surface in less time.  This would be good to help average out the heterogeneity of our samples.  The down side is that less energy is allocated to each point along the line spark, and, as a result, the plasma is not as hot.  After some preliminary tests using the cylindrical lens we found that aluminum dominated the spectrum, but that any other element was difficult to see.  We did find some sodium and carbon after averaging on a particular sample at a particular sample height.  When determining aluminum concentrations the other elements are needed to act as a control.  While the aluminum line intensity should change with concentration, the other elements should remain the same.  Therefore, a ratio of aluminum to another control element will hopefully remove fluctuations from sample height, laser mode, and plasma temperature from the data.   While sodium is notoriously unreliable, the carbon line seen has some potential as a comparison line.  Unluckily the non-doped agarose samples had almost no sparking with this method, so only aluminum samples can be analyzed.  The next step will be to compare the cylindrical lens data's reproducibility and detection limits with that of the pointlike spark. 

The goal of the project is to use samples with known concentrations of aluminum to create calibration curves.  This will be done once we know which lens to use, how to best prepare the aluminum/agarose samples, and how to make the most reproducible measurements.  Hopefully, this will not be too far in the project's future.