3-D Printed Human Surrogates for Injury RDT&E

Musculoskeletal injury is a significant problem in the U.S. Military and civilian populations. As such, there is a critical need to develop methods and systems to accurately assess the risk of injury during military operational activities, vehicular accidents, and other potentially injurious environments in order to develop countermeasures and systems to mitigate this risk. However, advances in developing injury countermeasures have been severely restricted by the limitations of available tools to determine the biomedical basis of human injury. Current methods of investigating musculoskeletal injuries include in vivo (both human and animal), cadaver, and surrogate testing (anthropomorphic test device (ATD)), as well as computational modeling. Each of these approaches has limitations that limit its applicability to developing injury criteria and injury countermeasures, personal protection equipment (PPE), and safety systems. Human in vivo investigations are limited by the inability to investigate injurious loading conditions and to directly measure tissue responses to loading. Cadaver testing is limited by the availability of cadavers representing target populations, the large number of specimens and significant costs required to obtain statistically significant results, and the extensive safety protocols and methods required when handling and testing human tissue. Human surrogates (ATDs) do not faithfully represent human anatomy or injury, only represent a limited slice of the population (e.g., 50th percentile male, 5th percentile female), cannot be used to investigate population variability, are only valid when subjected to specific loading conditions (e.g., frontal accelerations), and only measure gross responses that then must be correlated to expected human tissue injuries. Finally, computational modeling requires extensive model verification and validation along with significant training and expertise for users to obtain reliable and credible results.

The objective of this project was to investigate the use of 3-D printing to develop a highly biofidelic, low-cost, biomechanically verified and validated, physical surrogate of the human cervical spine for human injury and injury countermeasure research, development, testing, and evaluation (RDT&E). To achieve this overall goal, an existing benchtop 3-D-fused deposition modeling printer was used to construct soft tissue surrogates that represent the major soft tissue components of the human cervical spine: anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, interspinous ligament, joint capsule, and the intervertebral disc. The project team developed custom printable architectures and material composites that resulted in "3-D printable" surrogate materials that closely mimic the biomechanical behavior of the target human soft biological tissues. Using techniques derived from our experience in hierarchical, probabilistic computational model development, verification, and validation, procedures were applied to evaluate the biomechanical behavior of the surrogate system in comparison to human biomechanical data.

In Phase I, the resulting 3-D printable surrogate materials closely matched the target cervical spine soft biological tissue material elastic modulus. All developed surrogate materials matched their target human soft tissue elastic modulus to within 10 percent with a minimum 90 percent probability. Phase II focused on developing 3-D printed cervical spine motion segments (two vertebra and all associated soft tissues) that are both anatomically and biomechanically biofidelic. While the tension- compression behavior of the vertebral body-intervertebral disc-vertebral body construct closely matched the behavior of cadaver specimens reported in the literature, when tested in flexion-extension bending, the 3-D printed surrogate motion segments were significantly stiffer than equivalent cadaver specimens. This behavior was a result of the printed surrogate ligaments exhibiting compressive behavior significantly stiffer than natural ligaments. In conclusion, we have shown that it is feasible to custom design printable architectures and material composites using a "metamaterial" approach that resulted in "3-D printable" surrogate materials that closely mimic the biomechanical behavior of the target human soft biological tissues.