Numerical Analysis Of LCS Flow Field Under Microgravity Gravity Conditions

The continuous loss of bone minerals due to experiencing an environment of microgravity long-term is among the most serious physiological consequences faced by humans in space flight,which was an urgent medical problem to be solved.We hypothesized that the decrease of tissue fluid transport under microgravity would lead to osteoporosis.The osteons are cylindrical structures located between the inner and outer annular bone plates and are the main structural units of the long bone diaphysis.In this paper,a 3D axisymmetric fluid-solid coupling finite element model of an osteon with a two-stage pore structure(Haversian canals and lacunar-canalicular network)and osteocytes was established by using COMSOL Multiphysics software,and the osteon model was established taking pulsating pressure of arterioles in Haversian canal into consideration,in which the range of human pulsating pressure was 33-45 mm Hg.Certain small molecules,such as nutrients and signal molecules,that are necessary for osteocyte metabolism as solutes in tissue fluid were simplified to micro-particles by using the particle tracing function of the software.A multiscale numerical simulation was used to compare environments that had microgravity(0g),Earth's gravity(1g)and higher than Earth's gravity(2-8g),the transport behavior of tissue fluid in the lacunar-canalicular system under static load was studied,and the mass transfer behavior in the lacunar-canalicular system under the cyclic load(0.2hz-2hz)was compared between the microgravity field and the earth gravity field.This paper mainly studies from the following aspects:By applying static load to the 3D axisymmetric fluid-solid coupling finite element model of osteon containing osteocytes,the average velocity of fluid transport in lacunae in microgravity was 2-3 orders of magnitude lower than the average velocity of fluid transport in Earth's gravitational field.the liquid was moved under pulsating pressure into the LCS from the upper end of the Haversian canal,flowing through narrow channels prior to returning to the Haversian canal lower down.The mean liquid flow velocity in osseous lacunae in Earth's gravitational field was greater than that in microgravity.In addition,the maximum liquid flow velocity occurred at the junction of the lacunae and canaliculi.In general,an increase in gravitational force did not change law of liquid flow direction in the LCS,but the flow velocity increased linearly with increased gravity(1g-8g).By applying cyclic load of different frequencies(0.1 Hz,0.2 Hz,0.5 Hz)to the2 D axisymmetric fluid-solid coupling finite element model of osteon containing osteocytes to simulate human motion,and a comparative study of fluid pressure,velocity and stress in osteocyte was made.The results showed that the significantliquid pressure gradient in the osteocytes within the earth gravity field,compared with the 0 g gravity field,and the fluid pressure,the stress and flow rate of the osteocyte increased with the increase of the loading frequencies,but the pressure gradient was not observed in the micro-hydraulic stress field in the lacunae in a 0g gravitational field.Based on fluid-solid coupling finite element model of osteon,the effects of varying gravitational fields and load frequencies on solute transport behaviors in the LCS were firstly studied using the particle tracking function of finite element software.The results showed that the transport velocity of solute in microgravity was 3 orders of magnitude slower than in Earth's gravitational field,and the microgravity field was easy to cause particles to accumulate in the lacunar.Solute transport quantity along the radial direction to the deep lacuna increased monotonously with increasing load frequency in the range of 0.2Hz-2.5Hz in the Earth ' s gravitational field.Solute transport quantity along the radial direction to the deep lacuna did not increase monotonously with increasing load frequency in the range of 0.2Hz-2.5Hz within microgravity,such as the solute transport quantity increase with increasing load frequency in the range of 0.2Hz-0.5Hz,and decrease with increasing load frequency in the range of 0.5Hz-0.8Hz,and then increase with increasing load frequency in the range of 0.8Hz-2Hz,which phenomenon would be related to the vibration characteristics of bone tissue itself.When the astronaut in spaceship at high speed in orbit,the gravity who receives was used to generate the centripetal acceleration of the human body moving around the earth.Therefore,the effect of gravity was greatly decreased to make the human body in microgravity environment(0g).At this time,no matter what posture the human body was in,the body was stretched out,basically eliminating sedimentation,buoyancy convection and static pressure gradient in the body.The results showed that a significant liquid pressure gradient was observed in the lacunae in an environment with gravity compared to one in microgravity,and pressure stress concentration phenomena reduced as gravity increased,indicating that osteocytes in microgravity would not experience the pressure difference observed within gravity,thus inhibiting the activity of osteocytes.These reasons possibly causing a decrease in mechanical conduction and biochemical function,even cell death,leading to increased osteoclast activity,eventually causing the loss of a large quantity of bone.it can also be concluded that the appropriate frequency of exercise load can effectively avoid the apoptosis of deep osteocytes due to the lack of nutrients.This study is expected to explain how the decrease in liquid transmission and mechanical response on osteocytes within microgravity can be a cause of osteoporosis in astronauts from the perspective of biomechanics using a fundamental research approach.