P. Schneider, B. Bosinski, A. Trimper, K.W. Oh
University at Buffalo's Sensors & MicroActuators Learning Lab (SMALL),
Keywords: acoustic impedance, biometrics, capillary network, microfluidics, subdermal imaging, test phantom, ultrasonic imaging, ultrasonic material characterization
Summary:The field of fingerprint biometrics is migrating from traditional static 2D image analysis to that of subdermal 3D feature measurement thus providing a higher level of security in terms of “Liveness Detection” and “Anti-Spoofing methods” . Leveraging the Sensors & MicroActuators Learning Lab’s (SMALL at University at Buffalo) expertise in both microfluidics and test phantoms we have created a physiologically close representation model of the human finger. This “finger test phantom” will include features such as ridge/valley structures (fingerprints), digital arteries, blood, bone, fat, heart rate, muscle, skin and a 3 dimensional (3D) capillary network. Figure 1. Illustrates the design of the finger phantom and the components. Many works have been done creating simple 2D microfluidic capillary network to study the movement of particles and drug delivery  along with many static test phantoms created for anatomical teaching . While methods have been able to accurately recreate the feature channel size of 10-20μm, the creation of an acoustically equivalent 3D capillary network has yet to be realized due to its high complexity. The initial material property characterization and study of the human physiology has been established by means of acoustic characterization. Taking the known acoustic properties (attenuation, longitudinal velocity, density, acoustic impedance) of the biological material, we can correlate them to a synthetic material that have similar acoustic properties as shown in Table 1. A prototype with skin fat muscles bone and an artery (Figure 2) was imaged under ultrasound and results were shown that the imaging is, while less complex, comparable to that of a human finger (Figure 3). The Qualcomm Snapdragon Sense™ ID fingerprint technology has already been implemented into the Xiomi Mi 5s cell phone, making it the first commercial smartphone with ultrasonic biometric technology. The test phantom finger will help further this technology as a new piece of metrology to help prevent hacking “spoof” attacks on the system. Presentation attacks will be validated through the subdermal imaging vascular patterns, oxygen saturation levels, and capillary blanching of the human finger, all of which can be simulated through the finger test phantom created. By means of advanced microfluidic capabilities (i.e., soft lithography, microfluidic network optimization/analysis, micro/nanofabrication) SMALL is working to create a 3D microfluidic capillary network that mimics that of a capillary mesh. Current fabrication methods consist of traditional microfluidics, 3D printing, and sacrificial sugar structures. Sacrificial sugars have been proven to give a random mesh network on the micro scale . Through our experimentation shown in Figures 4, the creation low cost prototypes of random 3D fluidic networks with channels below 30μm in diameter. Being able to have a controlled test phantom (i.e., blood flow, heart rate, bone structure, fat and muscle thickness, as well as a known capillary design) will allow for advanced sensor/algorithm testing, validation, and calibration. This project provides a valuable training model for future subdermal imaging studies and will aid in the realization of subdermal imaging as a viable biometric.