Lasers are an integral part of the modern battlefield, used for applications as diverse as point-to-point communications and ballistic missile defense. Their widespread use increases the warfighter's likelihood of being exposed to laser hazards, and damage to an individual's eyes and skin can be serious. AFRL has served as a leading authority on laser-induced damage thresholds for many years.
Scientists assigned to AFRL's Advanced Laser Bioeffects Team, under the direction of Dr. Benjamin A. Rockwell, recently demonstrated a tissue substitute suitable for use in laser damage experiments. The goal of the team's project, dubbed "LaserMan," was to develop synthetic materials that could effectively mimic the properties of human skin, bone, muscle, and fat. Scientists have traditionally relied upon medical subjects and excised tissue samples in conducting laser damage experiments. While tissue samples are helpful, however, they lack the ability to adequately mimic healthy human tissue, can be quite costly, and require a great deal of preparation and a strictly controlled environment. These limitations ultimately prompted the search for an optical material phantom: a synthetic replacement for human tissue in laser damage experiments.
Researchers have used phantom tissue for many years, primarily in the capacity of structural models used for medical applications dealing with energy. Phantoms are also popular mechanisms among military and civilian groups that train medical clinicians on the use of ultrasound techniques and diagnoses. While these phantoms are useful for highly specific medical purposes, they do not exhibit the parameters necessary for optical studies and are therefore not suitable as tissue substitutes for laser testing.
In order to synthesize a material that would match the desired properties, the AFRL researchers acquired over a dozen possible constituent materials. They characterized each sample according to its thermal, optical, and physical properties and then compared this data to values obtained from the available literature for human tissue. The team's main interest was the materials' refractive index, as well as their light absorption and scattering characteristics. The scientists then examined the results of laser tests incorporating the different materials and compared these outcomes with previously collected data assessing measured laser interactions with human tissue. By combining several different materials in just the right proportions, the team created a promising substitute material for replacing human tissue in studies of laser-induced damage. The LaserMan prototype material is a combination of agar, ballistic media, India ink, and Intralipid (see Figure 1). The properties of the agar and ballistic media closely resemble the optical characteristics of human muscle and fat, whereas the added proportions of India ink and Intralipid correct differences in optical scattering and absorption rates (see Figure 2).
Researchers are already in the process of planning more realistic phantoms. For instance, they plan to synthesize a human arm by combining the LaserMan material with an artificially constructed skeleton of a human arm. This model will then undergo a series of applicable tests to ensure it can withstand both field conditions and experimental constraints.
As the LaserMan phantom evolves, it will impact a variety of applications. Researchers will use small models for close-range testing and also place full-sized models along the perimeter of large-scale field tests to determine stray exposure levels. In future versions of LaserMan, they expect to include more detailed anatomy, incorporating organs, veins, and arteries and possibly using materials capable of simulating the biological effects of laser damage (e.g., protein coagulation). Additionally, researchers hope to incorporate biomechanical properties, such as elasticity and tensile strength, into future material variants.
Scientists will use LaserMan experiments to safely determine laser-induced damage thresholds and limits and also to validate the computer modeling associated with realistic laser hazard evaluations. AFRL researchers intend to design experiments in which their tissue phantom enables the accurate prediction of physiological effects occurring at various exposure levels; they will then apply this knowledge in developing treatments and protective equipment for personnel at risk from laser exposure. The aim of the initial LaserMan material synthesis effort was to evaluate the effects of near-infrared laser wavelengths, but AFRL researchers also plan to create LaserMan phantoms that will emulate the effects of a broad range of laser wavelengths.
Mr. Manuel Figueroa and Ms. Nichole Jindra, of the Air Force Research Laboratory's Human Effectiveness Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document HE-H-05-02.