Browsing by Subject "Biomechanics"

More than 400,000 anterior cruciate ligament (ACL) injuries occur annually in the United States, 70% of which are non-contact. A severe consequence of ACL injury is the increased risk of early-onset of osteoarthritis (OA). Importantly, the increased risk of OA persists even if the ACL is surgically reconstructed. Thus, due to the long term physical consequences and high financial burden of treatment, injury prevention and improved reconstruction techniques are critical. However, the causes of non-contact ACL injuries remain unclear, which has hindered efforts to develop effective training programs targeted at preventing these injuries. Improved understanding of the knee motions that increase the risk of ACL injury can inform more effective injury prevention strategies. Furthermore, there is presently limited in vivo data to describe the function of ACL under dynamic loading conditions. Understanding how the ACL functions to stabilize the knee joint under physiologic loading conditions can inform design criteria for grafts used in ACL reconstruction. Grafts that more accurately mimic the native function of the ACL may help prevent these severe long term degenerative changes in the knee joint after injury.

To this end, measurements of in vivo ACL function during knee motion are critical to understanding how non-contact ACL injuries occur and the function of the ACL in stabilizing the joint during activities of daily living. Specifically, identifying the knee motions that increase ACL length and strain can elucidate the mechanisms of non-contact ACL injury, as a taut ligament is more likely to fail. Furthermore, measuring ACL elongation patterns during dynamic activity can inform the design criteria for grafts used in reconstructive surgery. To obtain measurements, 3D imaging techniques that can be used to measure dynamic in vivo ACL elongation and strain at high temporal and spatial resolution are needed.

Thus, in this dissertation a method of measuring knee motion and ACL function during dynamic activity in vivo using high-speed biplanar radiography in combination with magnetic resonance (MR) imaging was developed. In this technique, 3D surface models of the knee joint are created from MR images and registered to high-speed biplanar radiographs of knee motion. The use of MR imaging to model the joint allows for visualization of bone and soft tissue anatomy, in particular the attachment site footprints of the ligaments. By registering the bone models to biplanar radiographs using software developed in this dissertation, the relative positions of the bones and associated ligament attachment site footprints at the time of radiographic imaging can be reproduced. Thus, measurements of knee kinematics and ligament function during dynamic activity can be obtained at high spatial and temporal resolution.

We have applied the techniques developed in this dissertation to obtain novel dynamic in vivo measurements of the mechanical function of the knee joint. Specifically, the physiologic elongation and strain behaviors of the ACL during gait and single-legged jumping were measured. Additionally, the dynamic function of the patellar tendon during single legged jumping was measured. The findings of this dissertation have helped to elucidate the knee kinematics that increase ACL injury vulnerability by identifying the dynamic motions that result in elongation and strain in the ACL. Furthermore, the findings of this dissertation have provided critical data to inform design criteria for grafts used in reconstructive surgery such that reconstructive techniques better mimic the physiologic function of the ACL.

The methodologies described in this dissertation can be applied to study the mechanical behavior of other joints such as the spine, and other soft tissues, such as articular cartilage, under various loading conditions. Therefore, these methods may have a significant impact on the field of biomechanics as a whole, and may have applicability to a number of musculoskeletal applications.

For primates, and other arboreal mammals, adopting suspensory locomotion represents one of the strategies an animal can use to prevent toppling off a thin support during arboreal movement and foraging. While numerous studies have reported the incidence of suspensory locomotion in a broad phylogenetic sample of mammals, little research has explored what mechanical transitions must occur in order for an animal to successfully adopt suspensory locomotion. Additionally, many primate species are capable of adopting a highly specialized form of suspensory locomotion referred to as arm-swinging, but few scenarios have been posited to explain how arm-swinging initially evolved. This study takes a comparative experimental approach to explore the mechanics of below branch quadrupedal locomotion in primates and other mammals to determine whether above and below branch quadrupedal locomotion represent neuromuscular mirrors of each other, and whether the patterns below branch quadrupedal locomotion are similar across taxa. Also, this study explores whether the nature of the flexible coupling between the forelimb and hindlimb observed in primates is a uniquely primate feature, and investigates the possibility that this mechanism could be responsible for the evolution of arm-swinging.

To address these research goals, kinetic, kinematic, and spatiotemporal gait variables were collected from five species of primate (Cebus capucinus, Daubentonia madagascariensis, Lemur catta, Propithecus coquereli, and Varecia variegata) walking quadrupedally above and below branches. Data from these primate species were compared to data collected from three species of non-primate mammals (Choloepus didactylus, Pteropus vampyrus, and Desmodus rotundus) and to three species of arm-swinging primate (Hylobates moloch, Ateles fusciceps, and Pygathrix nemaeus) to determine how varying forms of suspensory locomotion relate to each other and across taxa.

From the data collected in this study it is evident the specialized gait characteristics present during above branch quadrupedal locomotion in primates are not observed when walking below branches. Instead, gait mechanics closely replicate the characteristic walking patterns of non-primate mammals, with the exception that primates demonstrate an altered limb loading pattern during below branch quadrupedal locomotion, in which the forelimb becomes the primary propulsive and weight-bearing limb; a pattern similar to what is observed during arm-swinging. It is likely that below branch quadrupedal locomotion represents a “mechanical release” from the challenges of moving on top of thin arboreal supports. Additionally, it is possible, that arm-swinging could have evolved from an anatomically-generalized arboreal primate that began to forage and locomote below branches. During these suspensory bouts, weight would have been shifted away from the hindlimbs towards forelimbs, and as the frequency of these boats increased the reliance of the forelimb as the sole form of weight support would have also increased. This form of functional decoupling may have released the hindlimbs from their weight-bearing role during suspensory locomotion, and eventually arm-swinging would have replaced below branch quadrupedal locomotion as the primary mode of suspensory locomotion observed in some primate species. This study provides the first experimental evidence supporting the hypothetical link between below branch quadrupedal locomotion and arm-swinging in primates.

It is currently an exciting time to be doing research at the intersection of sports and engineering. Advances in wearable sensor technology now enable large quantities of physiological and biomechanical data to be collected from athletes with minimal obstruction and cost. These technological advances, combined with an increased public awareness of the relationship between exercise, fitness, and health, has created an environment where engineering principles can be integrated with biomechanics, exercise physiology, and sports science to dramatically improve methods for physiological assessment, injury prevention, and athletic performance.

The first part of this dissertation develops a new method for analyzing heart rate (HR) and oxygen uptake (VO2) dynamics. A dynamical system model was derived based on the equilibria and stability of the HR and VO2 responses. The model accounts for nonlinear phenomena and person-specific physiological characteristics. A heuristic parameter estimation algorithm was developed to determine model parameters from experimental data. An artificial neural network (ANN) was developed to predict VO2 from HR and exercise intensity data. A series of experiments was performed to validate: 1) the ability of the dynamical system model to make accurate time series predictions for HR and VO2; 2) the ability of the dynamical system model to make accurate submaximal predictions for maximum heart rate (HRmax) and maximal oxygen uptake (VO2max); 3) the ability of the ANN to predict VO2 from HR and exercise intensity data; and 4) the ability of a system comprising an ANN, dynamical system model, and heuristic parameter estimation algorithm to make submaximal predictions for VO2max without requiring VO2 data collection. The dynamical system model was successfully validated through comparisons with experimental data. The model produced accurate time series predictions for HR and VO2 and, more importantly, the model was able to accurately predict HRmax and VO2max using data collected during submaximal exercise. The ANN was successfully able to predict VO2 responses using HR and exercise intensity as system inputs. The system comprising an ANN, dynamical system model, and heuristic parameter estimation algorithm was able to make accurate submaximal predictions for VO2max without requiring VO2 data collection.

The second part of this dissertation applies a support vector machine (SVM) to classify lower extremity movement patterns that are associated with increased lower extremity injury risk. Participants for this study each performed a jump-landing task, and experimental data was collected using two video cameras, two force plates and a chest-mounted single-axis accelerometer. The video data was evaluated to classify the lower extremity movement patterns of the participants as either excellent or poor using the Landing Error Scoring System (LESS) assessment method. Two separate linear SVM classifiers were trained using the accelerometer data and the force plate data, respectively, with the LESS assessment providing the classification labels during training and evaluation. The same participants from this study also performed several bouts of treadmill running, and an additional set of linear SVM classifiers were trained using accelerometer data and gyroscope data to classify movement patterns, with the LESS assessment again providing the classification labels during training and evaluation. Both sets of SVM's performed with a high level of accuracy, and the objective and autonomous nature of the SVM screening methodology eliminates the subjective limitations associated with many current clinical assessment tools.

Chronic kidney disease (CKD) is a degenerative disorder that affects millions of people worldwide and there are no targeted therapeutics. Given the global burden and increasing prevalence of CKD, the kidneys represent an attractive target for regenerative medicine. The most severe forms of CKD involve irreversible damage to kidney glomerular podocytes - the specialized epithelial cells that encase glomerular capillaries and regulate the removal of toxins and waste from blood. Therefore, the goal of this research proposal was to develop a novel strategy to protect or promote repair of injured human kidney tissues with an initial focus on glomerular podocytes. To achieve this goal, we leveraged advances in the directed differentiation of stem cells and in vitro disease modeling techniques to develop translationally relevant human models of podocyte injury. We used these models to identify potential biomarkers of early onset podocyte dysfunction, endogenous therapeutic targets, and reno-protective drug candidates, with a particular emphasis on studying pathways implicated in biomechanical signaling. Our studies revealed that the mechanosensitive proteins YAP, CTGF, and Cyr61 may be viable endogenous therapeutic targets, while CTGF and Cyr61 expression could serve as biomarkers of podocyte mechanical integrity and cell health. Additionally, our preliminary high-throughput drug screens have identified promising podocyte-protective drug candidates, which will be the subject of future studies.

Chronic and acute back injuries are widespread, affecting people in environments where they are exposed to vibration and repeated shock. These issues have been widely reported among personnel on aircraft and small watercraft; operators of heavy industrial or construction equipment may also experience morbidity associated with cyclic loading. To prevent these types of injuries, an improved understanding is needed of the spine’s tolerance to fatigue injury and of the factors that affect fatigue tolerance.

These types of vibration and shock exposures are addressed by international standards that propose limitations on the length and severity of the accelerations to which an individual is subjected. However, the current standard, ISO 2631-5:2004, provides an imprecise health hazard assessment. In this dissertation, a detailed technical critique is presented to examine the assumptions on which ISO 2631-5:2004 is based. An original analysis of existing data yields an age-based regression of the ultimate strength of lumbar spinal units and demonstrates sources of error in the strength regression in the standard. This dissertation also demonstrates that, contradicting earlier assumptions, the ultimate strength of the spine does not lie on a power-law S-N curve, and fatigue tolerance cannot be extrapolated from ultimate strength tests.

An alternative approach is presented for estimating the injury risk due to repeated loading. Drawing from existing data in the literature, a large dataset of in vitro fatigue tests of lumbar spinal segments was assembled. Using this fatigue data, a survival analysis approach was used to estimate the risk of failure based on several factors. Number of cycles, load amplitude, sex, and age all were significant predictors of bony failure in the spinal column. The parameter described by ISO 2631-5:2004 to quantify repeated loading exposure was modified, and an injury risk model was developed based on this modified parameter which relates risk of vertebral failure to repeated compressive loading. Notably, the effect of sex on fatigue tolerance persisted after normalizing by area, emphasizing the need for men and women to be addressed separately in the creation of injury risk predictions and occupational guidelines.

Posture has also been implicated in altering the injury mechanisms and tolerance to fatigue loading. However, few previous investigations in cyclic loading have addressed non-neutral postures. To assess the influence of dynamic flexion on the fatigue tolerance of the lumbar spine, a series of tests were conducted which combined a cyclic compressive force with a dynamic flexing motion. A study of 17 spinal segments from six young male cadavers was conducted, with tests ranging from 1000 to 500 000 cycles. Of the 17 specimens, 7 failed during testing. These failures were analyzed using a Cox Proportional Hazards model. As in compressive fatigue behavior, significant factors were the magnitude of the applied load and the age of the specimen. However, when the dynamically flexed specimens in these tests were compared to the specimens in the axial fatigue dataset, the flexion condition did not have a detectable effect on fatigue tolerance.

The Hybrid III dummy is a critical tool the assessment of such loading. Although the Hybrid III was originally designed for automotive frontal impact testing, these dummies have since been used to measure exposures and estimate injury risks of a wide variety of scenarios. These scenarios often involve using the dummy under non-standard temperatures or with little recovery interval between tests. Series of tests were conducted on the Hybrid III neck and lumbar components to assess the effects of rest duration intervals and a range of temperatures. Variations in rest duration intervals had little effect on the response of either component. However, both components were extremely sensitive to changes in temperature. For the 50th percentile male HIII neck, the stiffness fell by 18% between 25°C and 37.5°C; at 0°C, the stiffness more than doubled, increasing by 115%. Temperature variation had an even more pronounced effect on the HIII lumbar. Compared to room temperature, the lumbar stiffness at 37.5°C fell by 40%, and at 12.5°C, the stiffness more than doubled, increasing by 115%.

This dissertation has advanced the state of knowledge about the fatigue characteristics of the spine. An injury risk function has been developed that can serve as a tool for health hazard assessment in occupational standards. It has also contributed a fatigue dataset with dynamic flexion. This work will improve the scientific community’s ability to prevent repeated loading injuries. This dissertation has also demonstrated the immense sensitivity to temperature of the Hybrid III spinal components. This finding has major implications for the interpretation of previously published work using the Hybrid III, for the conduct of future research, and for future dummy design.

It is well understood that loss of motion following spinal fusion increases strain in the adjacent motion segments. However, it is unclear if to date, studies on cervical spine biomechanics can be affected by the role of coupled motions in the lumbar spine. Accordingly, we investigated the biomechanics of the cervical spine following cervical fusion and lumbar fusion during simulated whiplash.

A validated whole-human finite element model was used to investigate whiplash injury. The cervical spine before and after spinal fusion was subjected to simulated whiplash exposure in accordance with Euro NCAP testing guidelines, and the strains in the anterior longitudinal ligaments of the adjacent motion segments were computed.

In the models of cervical arthrodesis, peak ALL strains were higher in the motion segments adjacent to the level of fusion, and strains directly increased with longer fusions. The mean strain increase in the motion segment immediately adjacent to the site of fusion from C2-C3 through C5-C6 was 26.1% and 50.8% following single- and two-level cervical fusion (p=0.03). On average, peak strains experienced in a lumbar-fused spine were 1.0% less than those seen in a healthy spine (p=0.61). The C3-C4 motion segment had disproportionately high increases in strain following cervical fusion. The C6-C7 motion segment experienced high absolute strain under all tested conditions but the increase in strain following fusion was very small. This study provides support for both the hypothesis that adjacent segment disease is associated with post-arthrodesis biomechanical influences and the hypothesis that adjacent segment disease is a result of natural history, and inherent structures at risk.

Elastic energy plays important roles in biology across scales, from the molecular to organismal level, and across the tree of life. The ubiquity of elastic systems in biology is partly due to the variety of useful functions they permit such as the simplification of motor control in running cockroaches and the efficient recycling of kinetic energy in hopping kangaroos. Elastic energy is also responsible for ultrafast movements; the fastest movements in animals are not powered directly by muscle, but instead by elastic energy stored in a spring. By demonstrating that the power required to generate ultrafast movements exceeds the limits of muscle, many studies conclude that energy storage is necessary; but, what these studies do not explain is how the properties of a biological structure affect its capacity for energy storage. In this dissertation, I test the general principles of energy storage by investigating elastic systems at three hierarchical levels of organization: a single structure, multiple connected structures, and a spring system connected to muscle. By using a multi-level approach, my aim is to demonstrate, at each of the mentioned levels, how properties of the spring system affect where or how much energy is stored in the system as well as how these conclusions can be combined to inform our understanding of the biomechanics of hierarchical elastic systems.

When considering spring systems at the level of a single structure, morphology is one major structural aspect that affects mechanics. Continuous changes in morphology are capable of dividing a structure into regions that are responsible for the two contradicting functions that are essential for spring function: energy storage (via deformation) and structural support (via resistance to deformation). Using high quality micro computed tomography scans, I quantify the morphology of the mantis shrimp (Stomatopoda) merus, a single structure of the raptorial appendage hypothesized to store the elastic energy that drives ultrafast strikes. Comparing the morphology among the species, I find that the merus in smashers, species that depend heavily on elastic energy storage, have relatively thicker ventral regions and more eccentric cross-sections than spearers, species that strike relatively slower. I also conclude that differential thickening of a region can provide structural support for resisting spring compression as well as facilitate structural deformation by inducing bending. This multi-level morphological analysis offers a foundation for understanding the evolution and mechanics of monolithic systems in biology.

When two or more structures are connected, their relative physical properties determine whether the structures store energy, provide structural support, or some combination of both. Although the majority of elastic energy is stored via large deformations of the merus in smashers, some spearer species show relatively little meral deformation, and it is unclear whether elastic energy is stored in these systems. To determine whether the apodeme (arthropod tendon) provides energy storage in species that exhibit low meral deformation, I measure the physical properties of the lateral extensor apodeme and the merus to which it is connected. Comparisons of these properties show that in the spearer species I tested, the merus has a relatively higher spring constant than the apodeme, which results in the merus providing structural support and the apodeme storing the majority of elastic energy. Comparing the material properties of the apodemes with those of other structures reveals that apodemes and other biological spring systems share similar material characteristics. This study demonstrates that in order to understand the biomechanics of spring systems comprised of connected structures, it is necessary to compare their relative mechanical properties.

Finally, because muscles are responsible for loading spring systems with potential energy, muscle dynamics can affect elastic energy storage in a spring system. Although spring systems can circumvent the limits imposed by muscle via power amplification, they are not entirely independent from muscle dynamics. For example, if an organism has relatively low time to prepare and stretch the spring prior to the onset of movement, the limits of muscle power can dominate energy storage. To test the effects of muscle dynamics on spring loading, I implement a mathematical model that connects a Hookean spring model to a Hill-type muscle model, representing the muscle-tendon complex of the hindlimbs of American bullfrogs, in which the muscle dynamics are well understood and the duration of spring loading is low. I find that the measured spring constants of the tendons nearly maximize energy storage within the duration of in vivo spring loading. Additionally, the measured spring constants are lower than those predicted to produce maximal energy storage when infinite time is available for spring loading. Together, these results suggest that the spring constants of the tendons of American bullfrogs are tuned to maximize elastic energy for small durations of spring loading. This study highlights the importance of assessing muscle dynamics and their effect on energy storage when assessing the functional significance of spring constants.

Traumatic brain injuries (TBIs) are a major public health concern and socioeconomic burden worldwide. In recent years, brain injuries in US service personnel have focused attention on TBI affecting the military population (Bass et al., 2012). Blast injuries have become the most common cause of mortality and morbidity in soldiers returning from Iraq and Afghanistan (Owens et al., 2008, Warden, 2006). The frequency of blast-related sequelae found in allied forces has led some to call it the ‘signature wound’ of the wars abroad.

The growing incidence of TBI has spurred an increase in research efforts within the neurotrauma community to define TBI etiology. Identification of the critical injury mechanisms underlying TBI is an area of greatest need. Our understanding of TBI etiology, physical damaging mechanisms, and pathophysiology remains inadequate. The ability to design specific countermeasures and targeted prevention strategies is restricted by an incomplete understanding of the underlying damaging mechanisms.

Cavitation, the formation of vapor filled cavities in a liquid medium, has been proposed as a damaging mechanism of TBI in both blunt impacts (Ward et al., 1948, Gross, 1958) and blast-induced neurotrauma (Moore et al., 2008, Panzer et al., 2012c). The cavitation hypothesis of TBI centers on observation that high energy events such as high-explosive blast impingement onto the head generate large pressure transients in and around the brain. Localized areas of low pressure may surpass the tensile limits of the cerebrospinal fluid vaporizing the fluid and forming cavitation bubbles. These voids grow, potentially displacing surrounding tissue. When the bubbles collapse, perhaps violently, jets of liquid with potentially large localized pressures and temperatures may be created, damaging surrounding tissue.

The main objective of this dissertation was to develop an experimental foundation and provide empirical evidence for cavitation as a damaging mechanism of blast-induced TBI. This dissertation uses biofidelic surrogate head models of blast and in vivo animal models of blast injury to address the unanswered questions surrounding cavitation and blast neurotrauma. Foremost, cavitation response was observed in the surrogate head form exposed to blast conditions associated with injury. The 50% risk of cavitation occurs at a blast level of 262 kPa incident overpressure and 1.96 ms duration. This blast dosage represents a 62% chance of mild intracranial bleeding from scaled ferret experiments (Rafaels et al., 2012). Cavitation onsert, growth, and collapse were confirmed through high-speed imaging of the fluid layers of the contrecoup, while strong acoustic emission signatures associated with cavity collapse were captured and time matched with the video. Near-harmonic frequencies at 64 kHz, 126 kHz, and 267 kHz were associated with the energetic collapse of the bubbles. Our results provide compelling evidence that primary blast alone may induce cavitation that leads to TBI.

Evidence of cavitation was recorded in live porcine specimen exposed to blast. Acoustic sensors mounted to the skull of each specimen recorded acoustic emissions during blast exposure. Scaled spectral analysis revealed acoustic energy in higher frequencies bands with peaks at 64 kHz, 139 kHz, and 251 kHz, closely matching the spectral peaks associated with void collapse in surrogate experiments. To our knowledge, this study is the first to present evidence of blast-induced cavitation in a live animal model in the field of cavitation TBI research.

The results presented in this dissertation also greatly improve our understanding of how mechanical loads are imparted onto the head during a blast exposure and how this loading leads to cavitation onset. Strain analysis of the surrogate head indicates wall compliance from skull deformation and shear wave propagation through the skull as significant physical factors driving the tensile fluid responses in the head. Future design considerations for preventative measures should account for these physical mechanisms.

This dissertation also makes important contributions to blast injury research by presenting a clinically relevant murine model of blast TBI. Murine blast lethality risk and functional behavior outcomes before and after blast injury are presented. We provide guidelines for small animal blast testing, along with methodological recommendations for benchtop shock tube design and specimen placement in relation to the shock tube.

The contributions of this dissertation further serve as an important methodological guide to the neurotrauma and biomechanics community studying blast-related TBI and cavitation as a damaging mechanism. The developed surrogate head system and cavitation detection techniques provide a research template and are a springboard to future research efforts elucidating the damaging effects of cavitation during TBI.

Juvenile nucleus pulposus (NP) cells of the intervertebral disc (IVD) are large, vacuolated cells that form cell clusters with numerous cell-cell interactions. With maturation and aging, NP cells lose their ability to form these cell clusters, with associated changes in NP cell phenotype, morphology and proteoglycan synthesis that may contribute to IVD degeneration. Studies demonstrate healthy, juvenile NP cells exhibit potential for preservation of multi-cell clusters and NP cell phenotype when cultured upon soft, laminin-containing substrates; however, the mechanisms that regulate metabolism and phenotype of these NP cells are not understood. N-cadherin is a cell adhesion molecule that is present in juvenile NP cells, but disappears with age. The goal of this dissertation was to reveal the role of N-cadherin for NP cells in multi-cell clusters that contribute to the maintenance of the juvenile NP cell morphology and phenotype in vitro, and to evaluate the potential for laminin- functionalized poly(ethylene glycol) (PEG-LM) hydrogels to promote human NP cells towards a juvenile NP cell phenotype.

In this dissertation, juvenile porcine IVD cells were promoted to form cell clusters in vitro, and analyzed for preservation of the juvenile NP phenotype on soft, laminin-rich hydrogels. In the first part of this dissertation, preservation of the porcine juvenile NP cell phenotype and presence of N-cadherin was analyzed by culturing porcine NP cells on soft, laminin-rich or PEG-LM hydrogels. Secondly, cadherin-blocking experiments were performed to prevent cluster formation in order to study the importance of cluster formation in NP cell signaling. Finally, human IVD cells were cultured on PEG-LM hydrogels to investigate the potential to revert degenerate, human NP cells toward a juvenile NP cell phenotype and morphology.

Findings reveal soft (<500 Pa), laminin-rich substrates promote NP cell clustering, a key feature of the juvenile NP cell that is associated with N-cadherin positive expression. Additionally, N-cadherin-mediated cell-clustering regulates NP cell matrix production and gene expression of NP-specific and NP-matrix related markers. Inhibition of N-cadherin-mediated contacts resulted in decreased expression of juvenile NP cell features. Finally, juvenile human NP cells are also able to form N-cadherin positive cell clusters on soft, PEG-LM hydrogels with higher expression of juvenile NP cell features compared to culturing on stiff PEG-LM hydrogels. Some degenerate, human NP cells are also able to form N-cadherin positive cell clusters with some features of the juvenile NP cell.

The studies presented in this dissertation support the proposed hypothesis and establish the importance of soft, laminin-rich substrates in promoting NP cell clustering behaviors with associated features of a juvenile cell phenotype and morphology. Additionally, these studies establish a regulatory role for N-cadherin in juvenile NP cells and suggest that preservation of N-cadherin-mediated cell-cell contacts is important for preserving the juvenile NP cell phenotype and morphology. Furthermore, findings from this dissertation reveal the ability to promote degenerate, mature human NP cells towards a juvenile NP cell phenotype, demonstrating the potential to use PEG-LM hydrogels as a means for autologous cell delivery for the restoration of healthy IVD.

Traumatic Brain Injury (TBI) has become a marquee injury of this generation, prevalent in both military and civilian populations (Meaney 2014). Blunt impacts to the head are the known cause of approximately 1.7 million of TBI hospitalizations per year (Meaney 2014), and while mild TBI has the highest incidence (approximately 75% of TBIs) the injuries range from mild concussions to life threating severe bleeding within the brain (Meaney 2014).Due to wide spread prominence, blunt impact TBI has garnered a wealth of academic research interest focusing on the full spectrum of the biological scale, from subcellular and cellular response, to global human body modeling. The foundational theory of current blunt impact TBI research is neurological tissue damage by simple shear strain caused by motion of the skull (Cullen 2016, Alshareef 2020). While this likely contributes to tissue damage, its global perspective does not provide a satisfactory solution to the focal symptomology of TBI etiology. This is most likely because there are less appreciated mechanisms of injury contributing to TBI such as shear shock formation or cerebrospinal fluid (CSF) cavitation. The focus of this dissertation is to unpack the role of CSF cavitation in blunt impact TBI and contribute an important piece missing from the mechanistic understanding of TBI. This work develops an acoustic biomarker that indicates transient cavitation collapse, uses this biomarker to investigate cavitation mechanisms, observes cavitation in fresh, non-frozen, full body pig cadaver blunt impact testing, and provides clinical implications for transient cavitation through a reanalysis of live subhuman primate seminal data. It takes advantage of the large magnitude wideband acoustic emission of transient cavitation collapse, advanced acoustic sensor technology, and novel acoustic analysis methods to uncover a piece of the mechanistic mystery surrounding blunt impact TBI. There are five major conclusions reached in this dissertation. 1: The blunt impact head kinematics that induce cavitation are not significantly influenced by neck strength or cervical muscle activation. 2: Broadband acoustic emissions can be used as an acoustic biomarker to detect the incidence of transient cavitation collapse through the skull. 3: Compliance of the vessel containing a cavitating medium significantly influences the levels at which cavitation occurs during a blunt impact. 4: Blunt impact CSF cavitation occurs in a fresh, non-frozen, uncompromised pig cadaver head at impact levels below catastrophic injury thresholds. 5: Brain contusions are a potential clinical implication of transient cavitation collapse. Due to a lack of tools and technology, previous work on blunt impact cavitation was restricted to experimentation with limitations prohibiting the direct study of intracranial transient CSF cavitation. This innovative work provides direct observation of blunt impact CSF cavitation that benefits tools, injury risk functions, safety device design, and detection methodologies.

A current area of interest in lumbar injury is vertical impact loading. This includes effects of underbody blast (UBB), high speed planing boat impacts, and helicopter crashes. Lumbar spine fractures occur in 18% of all wounded in action injuries and 26% of soldiers killed in action exposed to UBB. Further, the US military has moving towards including women in all combat roles, including as the Special Forces beginning in January 2016. Volunteer and cadaveric data exist which suggest that male and female injury risk is not the same for equal stresses or loads. Dynamic injury mechanisms and thresholds have been extensively studied in the cervical spine, but not for the lumbar spine. While many lumbar models are available, no previously developed model is appropriate for high-rate vertical impact loading with intra-abdominal pressure and active musculature. So, the primary objective of this dissertation was to create a biofidelic hybrid multibody/finite element model to compare male and female response and assess the importance of active musculature and intra-abdominal pressure during single accelerative loading impacts (3-15g).

A 50th percentile male model was created using data from literature, experimental data, and medical imaging data. Scaling relationships for the 50th and 85th percentile female were derived, and an 85th percentile female model developed. The 85th percentile female model is mass-matched to the 50th percentile male model. An 11% increase in ischium breadth in the 85th female changes the line of action for muscles inserting on the pelvis. These changes resulted in a female model having increased axial loads over a male model when matched for mass.

The 50th percentile male osteoligamentous model was validated against developed strain energy-force corridors and T12/L1 injury risk functions. A 50th percentile risk of spinal fracture of 5237 N was reported. During failure loading (as seen from experimental tests), the osteoligamentous spine model predicts a 41% risk of failure. While the model slightly underpredicts the risk of injury, the peak compressive load in T12/L1 lies within the 95th percentile confidence intervals for the 50th percentile risk of injury.

In this dissertation, it was hypothesized that men and women do not have the same risk of injury on an effective stress basis. This hypothesis was supported by comparing mass-matched male and female hybrid multibody/finite element models during an underbody blast loading condition. Based on the comparison between the 85th percentile female model to the 50th percentile male model predicts higher axial loads due to changes in musculature. It was also hypothesized that the use of active musculature decreases injury tolerance in compressive loading. This hypothesis was supported by comparing model intervertebral axial loads to both experimental (underbody blast) and epidemiological (electroconvulsive therapy) loading conditions. This research demonstrates higher muscle activations increase risk of lumbar spine injury in vertical impact loading. A tensed activation state contributes 48 percent of the compressive load estimated to fracture the lumbar spine during underbody blast. While this corresponds to a low risk of injury (<10%), this could exacerbate risk of injury during additional compressive loading.

By better understanding the female lumbar spine response, new safety measures can be developed. This work could inform the design of new protective personal equipment, or guide permissible exposure limits and risk of injury.

This study is focused on designing and characterizing protein-based building blocks in order to construct self-assembled nano-structured biomaterials. In detail, this research aims to: (1) investigate a new class of proteins that possess nanospring behaviors at a single-molecule level, and utilize these proteins along with currently characterized elastomeric proteins as building blocks for nano-structured biomaterials; (2) develop a new method to accurately measure intermolecular interactions of self-assembling two or more arbitrary (poly)peptides, and select some of them which have appropriate tensile strength for crosslinking the proteins to construct elastomeric biomaterials; (3) construct well-defined protein building blocks which are composed of elastomeric proteins terminated with self-oligomerizing crosslinkers, and characterize self-assembled structures created by the building blocks to determine whether the elasticity of proteins at single-molecule level can be maintained.

Primary experimental methods of this research are (1) atomic force microscope (AFM) based single-molecule force spectroscopy (SMFS) that allows us to manipulate single molecules and to obtain their mechanical properties such as elasticity, unfolding and refolding properties, and force-induced conformational changes, (2) AFM imaging that permits us to identify topology of single molecules and supramolecular structures, and (3) protein engineering that allows us to genetically connect elastomeric proteins and self-assembling linkers together to construct well-defined protein building blocks.

Nanospring behavior of á-helical repeat proteins: We revealed that á-helical repeat proteins, composed of tightly packed á-helical repeats that form spiral-shaped protein structures, unfold and refold in near equilibrium, while they are stretched and relaxed during AFM based SMFS measurements. In addition to minimal energy dissipation by the equilibrium process, we also found that these proteins can yield high stretch ratios (>10 times) due to their packed initial forms. Therefore, we, for the first time, recognized a new class of polypeptides with nanospring behaviors.

Protein-based force probes for gauging molecular interactions: We developed protein-based force probes for simple, robust and general AFM assays to accurately measure intermolecular forces between self-oligomerization of two or more arbitrary polypeptides that potentially can serve as molecular crosslinkers. For demonstration, we genetically connected the force probe to the Strep-tag II and mixed it with its molecular self-assembling partner, the Strep-Tactin. Clearly characterized force fingerprints by the force probe allowed identification of molecular interactions of the single Strep-tag II and Strep-Tactin complex when the complex is stretched by AFM. We found a single energy barrier exists between Strep-tag II and Strep-Tactin in our given loading rates. Based upon our demonstration, the use of the force probe can be expanded to investigate the strength of interactions within many protein complexes composed of homo- and hetero-dimers, and even higher oligomeric forms. Obtained information can be used to choose potential self-assembling crosslinkers which can connect elastomeric proteins with appropriate strength in higher-order structures.

Self-assembled nano-structured biomaterials with well-defined protein-based building blocks: We constructed well-defined protein building blocks with tailored mechanical properties for self-assembled nano-structured materials. We engineered protein constructs composed of tandem repeats of either a I27-SNase dimer or a I27 domain alone and terminated them with a monomeric streptavidin which is known to form extremely stable tetramers naturally. By using molecular biology and AFM imaging techniques, we found that these protein building blocks transformed into stable tetrameric complexes. By using AFM based SMFS, we measured, to our knowledge for the first time, the mechanical strength of the streptavidin tetramer at a single-molecule level and captured its mechanical anisotropy. Using streptavidin tetramers as crosslinkers offers a unique opportunity to create well-defined protein based self-assembled materials that preserve the molecular properties of their building blocks.

The largest obstacle in nanoscale microscopy is the diffraction limit. Although several means of achieving sub-diffraction resolution exist, they all have shortcomings such as cost, complexity, and processing time, which make them impractical for widespread use. Additionally, these technologies struggle to find a balance between a high resolution and a large field of view. In this introduction of dissertation, we provide an overview of various microsphere based super resolution techniques that address the shortcomings of existing platforms and consistently achieve sub-diffraction resolutions. Initially, the theoretical basis of photonic nanojets, which make microsphere based super resolution imaging possible, are discussed. In the following sections, different type of acoustofluidic scanning techniques and intelligent nanoscope are explored. The introduction concludes with an emphasis on the limitless potential of this technology, and the wide range of possible biomedical applications.First, we have documented the development of an acoutofluidic scanning nanoscope that can achieve both high resolution and large field of view at the same time, which alleviates a long-existing shortcoming of conventional microscopes. The acoutofluidic scanning nanoscope developed here can serve as either an add-on component to expand the capability of a conventional microscope, or could be paired with low-cost imaging platforms to develop a stand-alone microscope for portable imaging. The acoutofluidic scanning nanoscope achieves high-resolution imaging without the need for conventional high-cost and bulky objectives with high numerical apertures. The field of view of the acoutofluidic scanning nanoscope is much larger than that from a conventional high numerical aperture objective lens, and it is able to achieve the same resolving power. The acoutofluidic scanning nanoscope automatically focuses and maintains a constant working distance during the scanning process thanks to the interaction of the microparticles with the liquid domain. The resolving power of the acoutofluidic scanning nanoscope can easily be adjusted by using microparticles of different sizes and refractive indices. Additionally, it may be possible to further improve the performance of this platform by exploring additional microparticle sizes and materials, in combination with various objectives. Altogether, we believe this acoutofluidic scanning nanoscope has potential to be integrated into a wide range of applications from portable nano-detection to biomedicine and microfluidics. Next, we developed a dual-camera acoustofluidic nanoscope with a seamless image merging algorithm (alpha blending process). This design allows us to precisely image both the sample and the microspheres simultaneously and accurately track the particle path and location. Therefore, the number of images required to capture the entire field of view (200 × 200 μm) by using our acoustofluidic scanning nanoscope is reduced by 55-fold compared with previous designs. Moreover, the image quality is also greatly improved by applying an alpha blending imaging technique, which is critical for accurately depicting and identifying nanoscale objects or processes. This dual-camera acoustofluidic nanoscope paves the way for enhanced nanoimaging with high resolution and a large field of view. Next, we developed an acoustofluidic scanning nanoscope via fluorescence amplification technique. Nanoscale fluorescence signal amplification is a significant feature for many biomedical and cell biology research area. Different types of fluorescence amplification techniques were studied; however, those technologies still need a complex process and rely on an elaborate optical system. To conquer these limitations, we developed an acoustofluidic scanning nanoscope via fluorescence amplification with hard PDMS membrane technique. The microsphere magnification by photonic nanojets effect with the hard PDMS could deliver certain focal distance to maximize the amplification. Moreover, a bidirectional acoustofluidic scanning device with an image processing also developed to perform 2D scanning of large field of view area. In the image processing procedure, we applied a correction of lens distortion to provide a restored distortion image. This fluorescence amplification via acoustofluidic nanoscope allow us to afford a nanoscale fluorescence imaging. Next, we developed an intelligent nanoscope that combines machine learning and microsphere array-based imaging to: (1) surpass the diffraction limit of the microscope objective with microsphere imaging to provide high-resolution images; (2) provide large field-of-view imaging without the sacrifice of resolution by utilizing a microsphere array; and (3) rapidly classify nanomaterials using a deep convolution neural network. The intelligent nanoscope delivers more than 46 magnified images from a single image frame so that we collected more than 1,000 images within 2 seconds. Moreover, the intelligent nanoscope achieves a 95% nanomaterial classification accuracy using 1,000 images of training sets, which is 45% more accurate than without the microsphere array. The intelligent nanoscope also achieves a 92% bacteria classification accuracy using 50,000 images of training sets, which is 35% more accurate than without the microsphere array. This platform accomplished rapid, accurate detection and classification of nanomaterials with miniscule size differences. The capabilities of this device wield the potential to further detect and classify smaller biological nanomaterial, such as viruses or extracellular vesicles. Lastly, this chapter serves a conclusion. Here, I discuss current issues regarding the acoustofluidic scanning nanoscope across review the current limitations of the technology and give suggestions for different direction of microsphere imaging. Moreover, I provide my perspective on the future development of acoustofluidic scanning nanoscope and potential new applications. I discuss how the technologies developed in this dissertation can be improved and applied to new applications in nanoimaging.

Lumbar intervertebral discs (IVD) play a critical role in facilitating the mobility and load-bearing functionality of the spine. Consequently, degeneration of the IVDs has been linked to the development of low back pain (LBP), a leading cause of disability in the world. While the pathomechanisms leading to the development of IVD degeneration and LBP are heterogeneous and often difficult to discern, it is believed that the changes in IVD function (i.e., mechanics, composition, tissue structure) may be closely related to the development of discogenic LBP. Specifically, because the IVD has a limited capacity to repair itself, disruptions to IVD tissue structures and biochemical composition may enable nervous tissue innervation into the IVD, potentiating the development of discogenic LBP. However, because our ability to study these changes in vivo remains limited, it remains unknown whether or not we can leverage the study of IVD function to identify risk factors associated with the development of LBP prior to their transition to a painful state. Accordingly, the overarching goal of this work is to develop non-invasive imaging techniques which may be used to perform spatiotemporal analyses of IVD kinematics and composition in vivo. Building upon prior work in our lab, Specific Aim 1 of this proposed research first seeks to develop a controlled methods to investigate the links between IVD function, composition and LBP by examining the in vivo response of IVDs to controlled dynamic loading in asymptomatic individuals. Using data generated in the prior aim, Specific Aim 2 then seeks to first develop and validate an image-segmentation method which enables precise kinematic analysis of the IVD to be carried out in an automated fashion, in vivo. Subsequently, Specific Aim 2 then seeks expand our current ability to characterize IVD function in response to dynamic activity by developing and validating a novel methodology for evaluating three-dimensional (3D) internal spatiotemporal changes in IVD kinematics using a novel deep-learning-based deformable image registration network. This dissertation is organized as a collection of original research articles which were conducted during my time as a PhD student in the Musculoskeletal Bioengineering Laboratory. The first of these (Chapter 3 - Increasing BMI Increases Lumbar Intervertebral Disc Deformation Following A Treadmill Walking Stress Test) was published in the Journal of Biomechanics (Coppock et al., 2021) in May 2021. The second of these (Chapter 4 - In vivo Intervertebral Disc Mechanical Deformation Following a Treadmill Walking “Stress Test” is Inversely Related to T1rho Relaxation Time) was published in the Osteoarthritis and Cartilage (Coppock et. al, 2022). The third, and fourth manuscripts are currently under review (Chapter 5 - Automated Segmentation and Prediction of Intervertebral Disc Morphology and Uniaxial Deformations from MRI; Chapter 6 - In Vivo Analysis of Intervertebral Disc Mechanics Using a Diffeomorphic Deep-Learning Approach. Chapter 7 - The Effects of a 6-month Weight Loss Intervention on Physical Function and Serum Biomarkers in Older Adults with and without Osteoarthritis - is published in Osteoarthritis and Cartilage, Open.

Mechanical properties have a decisive role in the fundamental functions of biological systems, including migration of cells, cell apoptosis, and proliferation of cells and bacteria. This is also true for cancer metastasis and morphogenetic processes during embryonic development. It isn’t easy, however, to study biological systems due to their complex behavior, such as their activity and nonlinear material properties. Note that while the individual mechanical properties of specific biological systems, such as biopolymers, have been well established, the collective behavior of these elements has a different response, as the comparative studies of the mechanical properties of single cancer cells and cancerous tissue demonstrate. Thus, numerous experimental instruments have been developed over the years to investigate biological systems’ mechanical properties, individually or collectively. These experimental techniques can evaluate mechanical properties at multiple scales. Theycan target individual biological entities, like single cells, or assess the collective mechanical properties of more complex biological systems, such as tissues or organoids. The resolution of these studies ranges from single-cell analyses to those concerning embryonic morphogenesis. Simulating biological systems’ individual or collective behavior using a discretized approach (i.e., Molecular Dynamics) or a continuum approach (i.e., Finite Element) is an adjunct to experimental studies. This thesis explores the collective behavior observed in individual cells and embryonic tissue. This exploration was carried out through the development of experimental protocols and the application of continuum mechanics models. In the initial two chapters of this thesis, we delve into the fundamental mechanical concepts essential for understanding the mechanical properties of cells and tissues. We also discuss prior studies that employed shell mechanics to model cellular and embryonic deformations. In the third chapter, we detail our collaborative work with Dr. Samaneh Rezvani focuses on the role of the actin cortex in the deformation of individual suspended spherical cells. For this purpose, we utilized double-trap optical tweezers in conjunction with a viscoelastic pressurized-thick-shell model. Using our simulation approach, we determined the mechanical properties of the actin cortex from the experimental results. The elastic shear modulus of the actin cortex ranged between 4.5 kPa and 7.5 kPa. In modeling the steady deformation of single cells with the shell model, we observed that cell volume remains conserved during deformation. Instead of reducing volume, cells extend the actin cortex to accommodate the increased surface area. We also introduced a multilayer viscoelastic shell model to examine the time-dependent mechanical behaviors of cells, focusing on hysteresis due to dissipative processes. Our model incorporated a fluid core within a viscoelastic shell, offering a more thorough understanding of cell mechanics. Our findings indicate that the damping response in cells is predominantly influenced by the viscosity of the cytosol rather than that of the actin cortex. The fourth chapter describes the modeling of experiments conducted by Dr. Renata Garces on gram-negative E. coli bacteria uniaxially compressed between parallel plates. We used Finite Element Modeling (FEM) to examine the collective mechanical behavior of the peptidoglycan layer (PG) in the bacterial cell wall, modeled as a thin, pressurized rod-shaped shell. Finally, in chapter five, we investigated, in collaboration with Dr. Chonglin Guan, the cells’ collective behavior in epithelial tissue during dorsal closure (DC) in developing Drosophila melanogaster embryos (DME). Utilizing glass microprobes, we deformed various tissue types, specifically amnioserosa (AS) and lateral epidermis (LE), and subsequently recorded their responses to assess the impact of tissue mechanical properties on embryonic development. We simulated a viscoelastic flat shell, replicating the geometry of individual embryos, using the Finite Element Method (FEM) to model tissue deformations. Through this methodology, we quantified the mechanical characteristics of amnioserosa and lateral epidermis, encompassing both their viscosity and elasticity. Our analyses determined the elasticity of AS to be approximately (110 to 180 kPa) and its viscosity to be (0.86 to 1.05 Pa.s). Additionally, we executed step-function experiments to ascertain tissue mechanical properties and evaluate tissue relaxation time. Our findings are in line with our previous results obtained from hysteresis studies.

High shear stresses and shear rates in left ventricular assist devices (LVADs) make endothelialization of the LVAD difficult and likely contribute to cleavage of large von Willebrand factor multimers and resulting bleeding problems in patients. To better understand shear in a centrifugal LVAD, flow was simulated using finite volume and computational fluid dynamics (CFD) analysis. The k-ω model simulated turbulence and sliding meshes were used to model the movement of the impeller. CFD results showed high-shear backflows in the radial gap between the impeller and the volute wall, but residence times in this region were under 5ms. It is unclear if this is sufficient to cleave VWF, and more study is necessary to determine if other areas in the LVAD have potential for VWF cleavage. Although the walls near the outlet experience low shear stress and may be good candidates for endothelialization, shear stresses above 20-30Pa on all other walls of the pump make the possibility of endothelial cell growth elsewhere in the LVAD unlikely. An LVAD designed specifically to have low shear may be a better candidate for endothelialization.

Cell sheet morphogenesis is essential for metazoan development and homeostasis, contributing to key developmental stages such as neural tube closure as well as tissue maintenance through wound healing. Dorsal closure, a well-characterized stage in Drosophila embryogenesis, has emerged as a model for cell sheet morphogenesis. Closure is a remarkably robust process where coordination of conserved gene expression and signaling cascades regulate cellular movements that drive closure. While well-characterized, new ‘dorsal closure genes’ continue to be discovered due to advances in microscopy and genetics. Here, we use live imaging and a set of large deletions, deficiencies (Dfs), that together remove 98.9% of the genes on 2L in order to identify regions of the genome required for normal closure. We successfully screened 96.1% of the genes on 2L and identified diverse dorsal closure defects in embryos homozygous for 47 Dfs, 26 of which have no known dorsal closure gene located within the Df region. We have already identified pimples, odd-skipped, paired, and sloppy-paired 1 as dorsal closure genes on the 2L affecting lateral epidermal cell shapes, and anticipate we will continue to identify novel ‘dorsal closure genes’ with further analysis. We also investigate the changes in dorsal closure dynamics and forces in the even-skipped (eve) mutant, which has aberrant cell shapes and behaviors as well as reduced actin and myosin at the purse string, but completes closure. We find that loss of wg/wnt-1 signaling in eve causes the observed defects in closure and that crumbs, a regulator of actin and myosin, is mis-expressed. Additionally, laser microsurgery demonstrates that the eve or wg mutant embryos are under a global tension in the anterior-posterior direction. Lastly, we identify a lesion in echinoid that is responsible for the jagged purse string and ectopic zipping dorsal closure phenotype previously thought to be due to a lesion in Zasp52.

Head and brain injuries are significant public health issues that affect individuals worldwide and encompass a wide range of injury types and severities. Traumatic head and brain injuries are common injuries observed in sports and motor vehicle crashes (MVC). Specifically, within MVCs they are a common cause of death and disability. Despite decades of dedicated research, the fundamental underlying causes of traumatic brain injuries remain poorly understood. Studying head injuries in vivo requires reliable instrumentation systems which can accurately measure head kinematics under potentially injurious conditions. Head kinematics obtained from reliable instrumentation can be used to study brain response in silico and can support the development of improved injury criteria and injury risk functions. Additionally, with enhanced instrumentation our ability to investigate differences in TBI across different populations (e.g. biological sex or age) would also be greatly enhanced.

Head injuries were studied across multiple domains in this study. First, relative risk of fatality and injury for female vehicle occupants were examined. Data from multiple automotive safety and public health datasets (National Highway Traffic Safety Administration (NHTSA) Fatality Analysis Reporting System (FARS); Centers for Disease Control (CDC) Multiple Cause of Death (MCOD)) were linked to examine head injuries and overall differences in fatality rates in motor vehicle crashes. Matched cases were examined to determine the relative risk (R) of fatality or injury using a double pair comparison method. Young females (20s-40s) are at approximately 20% higher risk of dying in car crashes compared with males of the same age in matched scenarios. For example, 25-year-old female occupants in passenger car crashes were at 20% higher risk of fatality (R = 1.201; 95% CI 1.160–1.250) compared to 25-year-old males. This trend persisted across vehicle type, airbag deployment, seatbelt use, and number of vehicles involved in a crash. Similar trends were noted for matched crashes where head injury was the cause of death: 20-25 year old female occupants were 30% more likely to suffer fatal head injuries than males of the same age (R = 1.29; 95% CI 1.17-1.42). This increased risk was pronounced for young females in both fatal crashes and non-fatal crashes with severe injuries.

Second, the biofidelity of in vivo wearable instrumentation systems used in the study of head kinematics was tested. Instrumented mouthguard systems (iMGs) are commonly used to study rigid body head kinematics across a variety of athletic environments. While iMGs rigidly fixed to anthropomorphic test device (ATD) headforms have demonstrated good correlation with reference systems in previous work, only one validation study focused on iMG performance in human cadaver heads. To assess iMG biofidelity, three unembalmed human cadaver heads were fitted with two instrumented boil-and-bite mouthguards (Prevent Biometrics and Diversified Technical Systems (DTS)) per manufacturer instructions, and were fitted with a properly-sized Riddell SpeedFlex American football helmet. Reference sensors were rigidly fixed to each specimen. Specimens were impacted with a rigid impactor at three velocities and locations.

The Prevent iMG performed comparably to the reference system up to approximately 60g in linear acceleration, but overall had poor correlation (CCC = 0.39). The DTS iMG consistently overestimated the reference across all measures, with linear acceleration error ranging from 10-66%, and angular acceleration errors greater than 300%. Overall, neither iMG demonstrated consistent agreement with the reference system. While iMG validation efforts have utilized ATD testing, this study highlighted the need for cadaver testing and proper validation of devices intended for use in vivo, particularly when considering realistic (non-idealized) sensor-skull coupling, when accounting for interactions with the mandible and when subject-specific anatomy may affect device performance.

Third, a population of Muay Thai martial arts athletes were instrumented with an in-ear wearable sensor system (DASHR) to better understand the epidemiology of head impact exposure during active sparring training. Martial arts athletes sustain many head impacts in a short period of time, and many martial arts forms focus competition victories on knock outs – the loss of consciousness or observable deficits in cognition and motor function due to repetitive impacts to the head. In this study, Muay Thai athletes were found to sustain 2.3 ± 1.8 head impacts per minute while sparring. Impact magnitudes were significantly higher during sessions when athletes wore headgear than during sessions that they did not, with linear accelerations 5.6g higher with headgear (Hodges-Lehman estimate of difference between medians; Mann-Whitney U-test p < 0.0001), angular velocities 1.6 rad/sec higher (p < 0.0001), and angular accelerations 710 rad/s^2 higher (p < 0.0001). While the use of headgear in martial arts has been shown to reduce accelerations during ATD drop testing, and to reduce the incidence of facial lacerations and contusions in competition settings, there is no consensus on the degree to which they protect against concussions and brain injuries. This study showed that during low magnitude impacts in a training setting, head kinematics are higher when headgear is worn than when it is not, potentially suggesting behavioral differences which offset any impact attenuation benefits the headgear offers. A large percentage of the impacts observed in this study (28%) fell below established recording thresholds used by most field-deployable wearables, suggesting a need for triggerless, continuous field recording systems.

Finally, the efficacy of existing injury metrics and risk functions were tested assessed using 731 non-injurious impacts recorded in the Muay Thai population. Head impacts were assessed with 14 injury metrics and 30 injury risk functions. Injury risks were summed across all impacts for each metric to determine the number of expected injury events. Risk functions developed from reconstructed impact events overpredicted injury rates more than risk functions developed from field data. For example, Head Injury Criterion risk functions developed from reconstructed American football events predicted 45-126 concussions, compared to 0 concussions predicted by risk functions developed from collegiate football field data. While cumulative impact exposure may be an important factor in individual risk tolerances, only one measure (RWE) was related to injury outcomes, and predicted concussions for seven of nine athletes in this study where no concussions were observed.

Overall, these studies highlight areas where injury biomechanics research has attempted to move forward without a reliable foundation to build upon. A lack of equitable cadaver testing has led to a gap in the biomechanical understanding of potentially fundamental underlying differences between males and females, reflected in sex differences in injury and fatality rates in motor vehicle crashes. Wearable head instrumentation systems which appear to perform well in controlled laboratory conditions are deployed without validation against biofidelic boundary conditions, significantly limiting their trustworthiness and usability. A large proportion of recorded head acceleration events using an in-ear sensing system fell below established trigger thresholds, suggesting a need for expanded data collection and continuous recording. The lack of cumulative exposure measures correlating to injury outcomes may, in part, be due to a lack of information in this low-level regime. Finally, established injury risk functions incorrectly classified many non-injurious head acceleration events, suggesting a need for new approaches to injury risk assessment using accurate in vivo kinematics as their basis.