Dynamic material properties of the pregnant human uterus.

Given that automobile crashes are the largest single cause of death for pregnant females, scientists are developing advanced computer models of pregnant occupants. The purpose of this study is to quantify the dynamic material properties of the human uterus in order to increase the biofidelity of these models. A total of 19 dynamic tension tests were performed on pregnant human uterus tissues taken from six separate donors. The tissues were collected during full term Cesarean style deliveries and tested within 36 h of surgery. The tissues were processed into uniform coupon sections and tested at 1.5 strains/s using linear motors. Local stress and strain were determined from load data and optical markers using high speed video. The experiments resulted in a non-linear stress versus strain curves with an overall average peak failure true strain of 0.32±0.112 and a corresponding peak failure true stress of 656.3±483.9 kPa. These are the first data available for the dynamic response of pregnant human uterus tissues, and it is anticipated they will increase the accuracy of future pregnant female computational models.

Eye injury risk from water stream impact: biomechanically based design parameters for water toy and park design.

PURPOSE: Interactive water displays are becoming increasingly popular and can result in direct eye contact. Therefore, the purpose of this study is to investigate eye injury risk from high speed water stream impacts and to provide biomechanically based design parameters for water toys and water park fountains. METHODS: An experimental matrix of 38 tests was developed to impact eight porcine eyes with water streams using a customized pressure system. Two stream diameters (3.2 mm and 6.4 mm) were tested at water velocities between 3.0 m/s and 8.5 m/s. Intraocular pressure was measured with a small pressure sensor inserted through the optic nerve and used to determine the injury risk for hyphema, lens dislocation, retinal damage, and globe rupture for each impact. RESULTS: Experimental water stream impacts created a range of intraocular pressures between 3156 mmHg and 7006 mmHg (61 psi to 135 psi). Injury risk varied between 4.4%-27.8% for hyphema, 0.0%-3.0% for lens dislocation, and 0.1%-3.3% for retinal damage. All tests resulted in 0.0% injury risk for globe rupture. The two water stream diameters did not result in significantly different water stream velocities (P = 0.32); however, the variation in water stream diameter did result in significantly different intraocular pressures (P = 0.03) with higher pressures for the 6.4 mm stream. CONCLUSIONS: This is the first study to experimentally measure intraocular pressure from high speed water stream impacts and quantify the corresponding eye injury risk. It is recommended that toy water guns and water park fountains use an upper threshold of 8.5 m/s for water stream velocities to minimize the risk of serious acute eye damage from impacts.

Evaluating the risk of eye injuries: intraocular pressure during high speed projectile impacts.

PURPOSE: To evaluate the risk of eye injuries by determining intraocular pressure during high speed projectile impacts. METHODS: A pneumatic cannon was used to impact eyes with a variety of projectiles at multiple velocities. Intraocular pressure was measured with a small pressure sensor inserted through the optic nerve. A total of 36 tests were performed on 12 porcine eyes with a range of velocities between 6.2 m/s and 66.5 m/s. Projectiles selected for the test series included a 6.35 mm diameter metal ball, a 9.25 mm diameter aluminum rod, and an 11.16 mm diameter aluminum rod. Experiments were designed with velocities in the range of projectile consumer products such as toy guns. RESULTS: A range of intraocular pressures ranged between 2017 mmHg to 26,426 mmHg (39 psi-511 psi). Four of the 36 impacts resulted in globe rupture. CONCLUSIONS: Intraocular pressures dramatically above normal physiological pressure were observed for high speed projectile impacts. These pressure data provide critical insight to chronic ocular injuries and long-term complications such as glaucoma and cataracts.

Effect of strain rate on the tensile material properties of human placenta.

Automobile crashes are the largest cause of injury death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. Computational models, useful tools to evaluate the risk of fetal loss in motor vehicle crashes, are based on a limited number of quasistatic material tests of the placenta. This study presents a total of 64 uniaxial tensile tests on coupon specimens from six human placentas at three strain rates. Material properties of the placental tissue were evaluated at strain rates of 0.07/s, 0.70/s, and 7.00/s. The test data have average failure strains of 0.34, 0.36, and 0.37, respectively. Failure stresses of 10.8 kPa, 11.4 kPa, and 18.6 kPa correspond to an increase in strain rate from 0.07/s to 7.0/s. The results indicate rate dependence only when comparing the highest strain rate of 7.0/s to either of the lower rates. There is no significant rate dependence between 0.07/s and 0.70/s. When compared with previous testing of placental tissue, the current study addresses the material response to more strain rates as well as provides a much larger set of available data. In summary, tensile material properties for the placenta have been determined for use in computational modeling of pregnant occupant kinematics in events ranging from low impact activities to severe impacts such as in motor vehicle crashes.

Dynamic material properties of the human sclera.

As a result of trauma, approximately 30,000 people become blind in one eye every year in the United States. A common injury prediction tool is computational modeling, which requires accurate material properties to produce reliable results. Therefore, the purpose of this study was to determine the dynamic material properties of the human sclera. A high-rate pressurization system was used to create dynamic pressure to the point of rupture in 12 human eyes. Measurements were obtained for the internal pressure, the diameter of the globe, the thickness of the sclera, and the changing coordinates of the optical markers using high-rate video. A relationship between true stress and true strain was determined for the sclera tissue in two directions. It was found that the average maximum true stress was 13.89+/-4.81 MPa for both the equatorial and meridional directions, the average maximum true strain along the equator was 0.041+/-0.014, and the average maximum true strain along the meridian was 0.058+/-0.018. Results show a significant difference in the maximum strain in the equatorial and meridional directions (p=0.02). In comparing these data with previous studies, it is concluded that the human sclera is both anisotropic and viscoelastic. The dynamic material properties presented in this study can be used for advanced models of the human eye to help prevent eye injuries in the future.

Evaluation of eye injury risk from projectile shooting toys using the focus headform - biomed 2009.

Half of eye injuries in the United States are caused by a blunt impact and more specifically, eye injuries effecting children often result from projectile shooting toys. The purpose of this study is to evaluate the risk of eye injuries of currently available projectile shooting toys. In order to assess the risk of each toy, a Facial and Ocular Countermeasure Safety (FOCUS) headform was used to measure the force applied to the eye during each hit for a total of 18 tests. The selected toys included a dart gun, a foam launcher, and a ball launcher. The force ranged from 4-93 N and was analyzed using the injury risk function for globe rupture for the FOCUS headform. Projectile characteristics were also examined using normalized energy to determine risk of corneal abrasion, hyphema, lens dislocation, retinal damage and globe rupture. It was found that the three toys tested produced peak loads corresponding with risk of globe rupture between 0% and 17.3%. The normalized energy results show no risk of hyphema, lens dislocation, retinal damage or globe rupture and a maximum risk of corneal abrasion of 5.9%. This study concludes that although there are many eye injuries caused by projectiles, the selected toys show a very low risk of eye injury.

The effect of stress and strain formulations on the representation of biological tissue mechanical properties - biomed 2009.

With the increasing use of computational models there is a growing need to establish and analyze the material properties of biological tissues. The purpose of this study is to analyze and compare the various methods used for reporting stress and strain for biological materials. For this study, three biological materials (bone, sclera, and placenta) were used to compare the effects of reporting stress and strain relationships using various formulations. For each material the engineering, 2nd Piola Kirchhoff and true stresses were calculated along with the engineering, Green-Lagrangian, and true strains. The energetic equivalents were then plotted to investigate the various formulations. As expected, it was found that for strain values less than 5% the various formulations were not significantly different but as the strain increased (30%) the non-linear terms began to increase the difference between the formulations to a maximum of 36%. Also, to represent a material correctly, the energy conjugates must be used together. Measuring the true stress and true strain relationship is advantageous given that most computational models use this formulation.

High-rate internal pressurization of human eyes to predict globe rupture.

OBJECTIVE: To determine the dynamic rupture pressure of the human eye by using an in vitro high-rate pressurization system to investigate blunt-impact eye injuries. METHODS: Internal pressure was dynamically induced in the eye by means of a drop-tower pressurization system. The internal eye pressure was measured with a small pressure sensor inserted into the eye through the optic nerve. A total of 20 human eye tests were performed to determine rupture pressure and characterize rupture patterns. RESULTS: The high-rate pressurization resulted in a mean (SD) rupture pressure of 0.97 (0.29) MPa (7275.60 [2175.18] mm Hg). A total of 16 eyes ruptured in the equatorial direction, whereas 4 ruptured in the meridional direction. There was no significant difference in the rupture pressure between the equatorial and meridional directions (P= .16). CONCLUSION: As the loading rate increases, the rupture pressure of the human eye increases. CLINICAL RELEVANCE: Eye injuries are expensive to treat, given that the estimated annual cost associated with adult vision problems in the United States is $51.4 billion. Determining globe rupture properties will establish injury criteria for the human eye to prevent these common yet devastating injuries.

Dynamic tensile properties of human placenta.

Automobile crashes are the largest cause of injury death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. Computational models, useful tools to evaluate the risk of fetal loss in motor vehicle crashes, are based on a limited number of quasi-static material tests of the placenta. This study presents a total of 20 dynamic uniaxial tensile tests on the maternal side of the placenta and 10 dynamic uniaxial tensile tests on the chorion layer of the placenta. These tests were completed from 6 human placentas to determine material properties at a strain rate of 7.0 strains/s. The results show that the average peak strain at failure for both the maternal portion and the chorion layer of the placenta are similar with a value of 0.56 and 0.61, respectively. However, the average failure stress for the chorion layer, 167.8 kPa, is much higher than the average failure stress for the placenta with the chorionic plate removed, 18.6 kPa. This is due to differences in the structure and function of these layers in the placenta. In summary, dynamic loading data for the placenta have been determined for use in computational modeling of pregnant occupant kinematics in motor vehicle crashes. Moreover the computational model should utilize the material properties for the placenta without the chorion layer.

High rate internal pressurization of the human eye to determine dynamic rupture pressure.

Over 1.9 million people suffer from eye injuries in the United States, occurring from automobile accidents, sports related impacts, and military combat. The purpose of the current study is to analyze the rupture pressure of human eyes using a high rate pressurization system. Internal pressure was dynamically induced into the eye with a drop tower pressurization system. The rupture pressure was measured with a small pressure sensor inserted into the optic nerve. A total of 10 human eye dynamic pressure tests were performed to determine rupture pressure and to compare the results with previous data. It was found that the average high rate rupture pressure of human eyes is 0.89+/- 0.25 MPa. In comparing these data with previous studies, it is concluded that as the loading rate increases the rupture pressure also increases.

Dynamic material property measurements of human eyes.

As a result of trauma, approximately 30,000 people become blind in one eye every year in the United States. A common injury prediction tool used for eye injuries is computational modeling, which requires accurate material properties to produce reliable results. The purpose of this study is to determine the dynamic material properties of the human sclera. A high rate pressurization system was used to create a dynamic pressure event to the point of rupture in 5 human eyes. Measurements were obtained for the internal pressure, the diameter of the globe, and the changing coordinates of the optical markers. A relationship between true stress and Green-Lagrangian strain was determined for each test specimen in the x and y direction to show directional effects. It was found that the average maximum true stress was 10.45 +/- 2.28 MPa, the average maximum Green-Lagrangian strain in the x-direction was 0.041 +/- 0.012, and the average maximum Green-Lagrangian strain in the y-direction was 0.073 +/- 0.015. In comparing these data with previous studies, it is concluded that the human eye is both anisotropic and viscoelastic. This study presents dynamic material properties that can be used for establishing injury criteria to help prevent eye injuries in the future.