Mechanical Mechanism Of Multifunctionality Of Human Heel Pad And Its Bionic Research

The human heel pad is a highly specialised biological tissue located under the calcaneus bone,which is the part of the human body that initially contacts the ground when walking,and it is also the biological tissue in the human body that suffers from complex mechanical effects.People walk up to 10,000 steps per day,and the heel pad is subjected to cyclic loading up to 10,000 times;and the impact force at the heel when walking is about 1.2 times the body weight,while the impact force at the heel when running is up to 2-3 times the body weight.In order to adapt to these complex mechanical conditions,heel pads need to be equipped with excellent biomechanical functions such as cushioning,vibration damping,anti-fatigue,and ground stabilisation.However,the reasons for heel pads to combine so many biomechanical functions are still unclear,so this study focuses on revealing the multifunctional mechanical mechanism of heel pads and its biomimetic research.This study is of great scientific significance and potential application value for human foot health protection and the development of multifunctional biomimetic flexible composites.In this paper,the compartmental structure of the human heel pad is carefully characterized,and the biomechanical functions of the in vitro heel pad,such as cushioning,vibration damping,anti-fatigue,and mechanical anisotropy,are systematically analyzed.Based on the established heel pad structure and material model,the design and preparation of the bionic heel pad were carried out.On this basis,by adjusting the structural and material parameters of the bionic heel pad,the effects of the compartment structure and liquid content on the multifunctional characteristics of the human heel pad,such as cushioning,vibration damping,anti-fatigue,and mechanical anisotropy,were investigated,and the mechanic mechanism of the multifunctionality of the human heel pad was finally revealed.The main research contents and conclusions of this paper are as follows:（1）The structural properties of the compartment of the human heel pad and the material properties of the heel pad are analyzed.Based on MRI,the structural model of the first-level fiber membrane of the in vivo heel pad was established in this paper.The results showed that the septum was approximately symmetrically convex in the sagittal and coronal planes,and the density of the compartmental units was concentrated near2.0 pcs·cm^-2.Based on Micro-CT,the fiber membrane structure model of the in vitro heel pad was established in this paper.The analysis found that the fiber membrane accounted for 37%of the volume of the whole heel pad,and the fat in liquid state accounted for 63%of the volume of the whole heel pad,i.e.,the liquid content of the human heel pad was 63%.Combined with the above analysis,this paper finds that the human heel pad is a biological tissue with solid-liquid composite material properties reinforced by a fibrous membrane.（2）The biomechanical functions of the in vitro heel pad,such as cushioning,vibration damping,anti-fatigue,and mechanical anisotropy,were elucidated.Under the dynamic loading at three different frequencies of 1 Hz,2 Hz and 4 Hz,the storage modulus of the heel pad was 0.44 MPa,0.69 MPa and 0.72 MPa,the loss modulus was0.08 MPa,0.126 MPa and 0.14 MPa,and the loss factor Tanδwas 0.186,0.18 and 0.19,respectively.The cushioning test showed that the impact velocity is 2m/s,the peak impact load is 2144 N;the impact velocity is 1.3m/s,the peak impact load is 870 N.The vibration damping test shows that at resonance,the vibration transfer ratio of the human heel pad is 2.0.The anti-fatigue test shows that the initial fatigue cyclic loading causes a serious degradation of the stiffness of the heel pad,and the stiffness degradation is no longer obvious after more than 10000 times of fatigue cycling.After10,000 fatigue cycles,the stiffness degradation of the heel pad was no longer obvious.The mechanical anisotropy test showed that the heel pads exhibited obvious mechanical anisotropy under different compression loads.（3）Based on the established structural model of the heel pad,the design and preparation of the bionic heel pad and the characterization of the basic mechanical properties were completed.The results show that under static compression load,the stiffness of the bionic heel pad decreases with the increase of liquid content,and the stiffness of the bionic heel pad gradually increases with the increase of unit density.Under dynamic cyclic loading,the storage modulus decreases with increasing liquid content and increases with increasing unit density.The loss modulus tends to increase and then decrease with the increase of liquid content,and reaches the peak at 60%and70%of liquid content.The loss modulus tends to increase and then decrease with increasing unit density,reaching a peak at a unit density of 2 pcs·cm^-2.（4）Elucidated the influence mechanism of unit density and liquid content on the cushioning and vibration damping performance of the bionic heel pad.Based on the cushioning test platform,the effects of liquid content and unit density on the cushioning performance of the bionic heel pad were analyzed.The results showed that the peak impact force of the bionic heel pad was lowest at the fat content closest to that of the human heel pad.It has a lower value at a unit density of 2 pcs·cm^-2,and the peak impact load tends to increase again after the unit density exceeds2 pcs·cm^-2.This indicates that the human heel pad optimizes its cushioning performance by adjusting the fat content inside the compartment and the unit density of the compartment,and it has good cushioning performance at a liquid content of 60%and a unit density of about 2 pcs·cm^-2.Based on the vibration-damping test platform,the effects of liquid content and unit density on the vibration-damping performance of the bionic heel pad were analyzed.The results show that the bionic heel pad with 60%liquid content has good vibration damping performance,and the bionic heel pad with unit density of 2 pcs·cm^-2 has the lowest vibration transfer ratio.Therefore,the human heel pad has good vibration damping performance when the liquid content is 60%and the unit density is around 2pcs·cm^-2.（5）Revealed the influence mechanism of unit density and liquid content on the anti-fatigue performance of bionic heel pads.Based on the TA Electro Force 3100 fatigue testing machine,the effects of liquid content and unit density on the anti-fatigue performance of bionic heel pads were analyzed.The results showed that the increase of liquid content caused the decrease of fatigue resistance of the bionic heel pads;when the liquid content was 60%,the bionic heel pads maintained good fatigue resistance under 50,000 and 100,000 fatigue cycles,and the fatigue resistance of the bionic heel pads was optimal when the unit density was 2 pcs·cm^-2.In other words,with 60%liquid content and a unit density of about 2 pcs·cm^-2,the structural material properties of the heel pads provide good fatigue resistance,which enables them to adapt to the fatigue load brought about by the human body walking every day.（6）Analyzing the effects of unit density and liquid content on the mechanical anisotropy of the bionic heel pad,the mechanical anisotropy of the bionic heel pad is analyzed.Based on the constructed mechanical anisotropy test platform,the mechanical anisotropy test of the bionic heel pad under different liquid contents and different unit densities was completed.The results show that the stiffness ratio was lowest at 60%liquid content and unit density of 2 pcs·cm^-2 for different compression loads.Therefore,the mechanical anisotropy of the human heel pad is adjusted by adjusting the unit density and fat content of the compartment,and the mechanical anisotropy is optimized when the unit density is 2 pcs·cm^-2 and the liquid content is about 60%,which effectively ensures the stability of the human body when walking and touching the ground.（7）Synthesis reveals the mechanical mechanism of human heel pad multifunctionality.The material properties of fibre membrane reinforced solid-liquid composites combined to regulate the mechanical properties of the heel pads to an excellent condition.The human heel pad was able to optimise the biomechanical properties by adjusting the viscosity,stiffness and deformation by adjusting the fat content inside the compartment and the unit density of the compartment.The mechanical properties of the heel pads were optimised when the fat content was about60%and the unit density was 2 pcs·cm^-2,thus adapting to the harsh mechanical environment of the foot.