Linxia Gu

Bioink properties before, during and after 3D bioprinting

Bioprinting is a process based on additive manufacturing from materials containing living cells. These materials, often referred to as bioink, are based on cytocompatible hydrogel precursor formulations, which gel in a manner compatible with different bioprinting approaches. The bioink properties before, during and after gelation are essential for its printability, comprising such features as achievable structural resolution, shape fidelity and cell survival. However, it is the final properties of the matured bioprinted tissue construct that are crucial for the end application. During tissue formation these properties are influenced by the amount of cells present in the construct, their proliferation, migration and interaction with the material. A calibrated computational framework is able to predict the tissue development and maturation and to optimize the bioprinting input parameters such as the starting material, the initial cell loading and the construct geometry. In this contribution relevant bioink properties are reviewed and discussed on the example of most popular bioprinting approaches. The effect of cells on hydrogel processing and vice versa is highlighted. Furthermore, numerical approaches were reviewed and implemented for depicting the cellular mechanics within the hydrogel as well as for prediction of mechanical properties to achieve the desired hydrogel construct considering cell density, distribution and material-cell interaction.

Role of helmet in the mechanics of shock wave propagation under blast loading conditions

The effectiveness of helmets in extenuating the primary shock waves generated by the explosions of improvised explosive devices is not clearly understood. In this work, the role of helmet on the overpressurisation and impulse experienced by the head were examined. The shock wave–head interactions were studied under three different cases: (i) unprotected head, (ii) head with helmet but with varying head–helmet gaps and (iii) head covered with helmet and tightly fitting foam pads. The intensification effect was discussed by examining the shock wave flow pattern and verified with experiments. A helmet with a better protection against shock wave is suggested.

The Relation Between the Arterial Stress and Restenosis Rate After Coronary Stenting

Two commercially available stents (the Palmaz-Schatz (PS) and S670 stents) with reported high and low restenosis rates, respectively, have been investigated in this paper. Finite element models simulating the stent, plaque, and artery interactions in 3 mm stenosed right coronary arteries were developed. These models were used to determine the stress field in artery walls after stent implantation. The material properties of porcine arteries were measured and implemented in the numerical models. The stress concentration induced in the artery by the PS stent was found to be more than double that of the S670 stent. It demonstrated a good correlation with the reported restenosis rate. The effects of stent structures, compliance mismatch, plaque geometry, and level of stenosis were studied. Results suggested that stent designs and tissue properties cause alterations in vascular anatomy that adversely affect arterial stress distributions within the wall, which impact vessel responses such as restenosis. Appropriate modeling of stent, plaque, and artery interactions provided insights for evaluating alterations to the arterial mechanical environment, as well as biomechanical factors leading to restenosis.

Influence of Crosslink Density and Stiffness on Mechanical Properties of Type I Collagen Gel

The mechanical properties of type I collagen gel vary due to different polymerization parameters. In this work, the role of crosslinks in terms of density and stiffness on the macroscopic behavior of collagen gel were investigated through computational modeling. The collagen fiber network was developed in a representative volume element, which used the inter-fiber spacing to regulate the crosslink density. The obtained tensile behavior of collagen gel was validated against published experimental data. Results suggest that the cross-linked fiber alignment dominated the strain stiffening effect of the collagen gel. In addition, the gel stiffness was enhanced approximately 40 times as the crosslink density doubled. The non-affine deformation was reduced with the increased crosslink density. A positive bilinear correlation between the crosslink density and gel stiffness was obtained. On the other hand, the crosslink stiffness had much less impact on the gel stiffness. This work could enhance our understanding of collagen gel mechanics and shed lights on designing future clinical relevant biomaterials with better control of polymerization parameters.

Experimental and Numerical Investigation of the Mechanism of Blast Wave Transmission Through a Surrogate Head

This work is to develop an experiment-validated numerical model to elucidate the wave transmission mechanisms through a surrogate head under blast loading. Repeated shock tube tests were conducted on a surrogate head, i.e., water-filled polycarbonate shell. Surface strain on the skull simulant and pressure inside the brain simulant were recorded at multiple locations. A numerical model was developed to capture the shock wave propagation within the shock tube and the fluid-structure interaction between the shock wave and the surrogate head. The obtained numerical results were compared with the experimental measurements. The experiment-validated numerical model was then used to further understand the wave transmission mechanisms from the blast to the surrogate head, including the flow field around the head, structural response of the skull simulant, and pressure distributions inside the brain simulant. Results demonstrated that intracranial pressure in the anterior part of the brain simulant was dominated by the direct blast wave propagation, while in the posterior part it was attributed to both direct blast wave propagation and skull flexure, which took effect at a later time. This study served as an exploration of the physics of blast-surrogate interaction and a precursor to a realistic head model. [DOI: 10.1115/1.4026156]

The Impact of Wire Stent Fabrication Technique on the Performance of Stent Placement

Braided wire stents demonstrate distinct characteristics compared to welded ones. In this study, both braided and welded wire stents with the same nominal dimensions were crimped inside a sheath and then deployed into a stenosed artery using finite element analysis. The braided wire stent was generated by overlapping wires to form crisscross shape. A welded wire stent was created by welding the intersection points of wires to avoid sliding between wires. The effect of fabrication technique on mechanical behavior of Nitinol wire stents was evaluated. The results showed that relative sliding between wires reduced the deformation of the braided stent, which led to less radial strength than the welded one; therefore, the deployed braided stent was more conformed to the anatomic shape of the lesion and much less efficient for restoring the patency of the stenotic artery. Post balloon-dilation was commonly used to improve its performance in terms of lumen gain and deployed shape of the stent. On the contrary, the welded wire stent exhibited a high capacity for pushing the occlusion outward. It reached an approximately uniform shape after deployment. The welded joints caused larger deformation and high strain on the stent struts, which indicate a potential earlier failure for the welded stent. In addition, higher contact pressure at the stent-lesion interface and higher arterial stresses were observed in the artery supported by the welded stent. The peak stress concentration may increase the occurrence of neointimal hyperplasia.

Dynamic response of brain subjected to blast loadings: Influence of frequency ranges

Blast wave induced a frequency spectrum and large deformation of the brain tissue. In this study, new material parameters for the brain material are determined from the experimental data pertaining to these large strain amplitudes and wide frequencies ranging (from 0.01 Hz to 10 MHz) using genetic algorithms. Both hyperelastic and viscoelastic behavior of the brain are implemented into 2D finite element models and the dynamic responses of brain are evaluated. The head, composed of triple layers of the skull, including two cortical layers and a middle dipole sponge-like layer, the dura, cerebrospinal fluid (CSF), the pia mater and the brain, is utilized to assess the effects of material model. The results elucidated that frequency ranges of the material play an important role in the dynamic response of the brain under blast loading conditions. An appropriate material model of the brain is essential to predict the blast-induced brain injury.

Finite Element Analysis of the Implantation of a Self-Expanding Stent: Impact of Lesion Calcification

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THE EFFECT OF SHOCK WAVE ON A HUMAN HEAD

When a pressure wave of finite amplitude is generated in air by a rapid release of energy, such as high-pressure gas storage vessel or the blast from dynamite, there may be undetected brain injuries even though protective armors prevent the penetration of the projectile. To study brain tissue injury and design a better personnel head armor under blast wave, computational models of human head have been developed. Models with and without helmet are built to quantify the intracranial pressure and shear stresses of head subjected to blast wave. All the models are compared against injury thresholds for intracranial pressure and shear stresses. Overall pressure and shear stress level is highest in model without helmet and lowest in model with helmet having foam layer on inner side of helmet. The results show that helmet reduces the pressure and shear stresses generated in the brain. However this reduction in pressure and shear stresses might not be sufficient to mitigate early time, blast induced, traumatic brain injury. The validated results will provide better understanding of the energy transfer characteristics of blast wave through helmet and the injury mechanism of human head.Copyright

Performance of Self-Expanding Nitinol Stent in a Curved Artery: Impact of Stent Length and Deployment Orientation

The primary aim of this work was to investigate the performance of self-expanding Nitinol stents in a curved artery through finite element analysis. The interaction between a PROTÉGÉ™ GPS™ self-expanding Nitinol stent and a stenosed artery, as well as a sheath, was characterized in terms of acute lumen gain, stent underexpansion, incomplete stent apposition, and tissue prolapse. The clinical implications of these parameters were discussed. The impact of stent deployment orientation and the stent length on the arterial wall stress distribution were evaluated. It was found that the maximum principal stress increased by 17.46%, when the deployment orientation of stent was varied at a 5 deg angle. A longer stent led to an increased contact pressure between stent and underlying tissue, which might alleviate the stent migration. However, it also caused a severe hinge effect and arterial stress concentration correspondingly, which might aggravate neointimal hyperplasia. The fundamental understanding of the behavior of a self-expanding stent and its clinical implications will facilitate a better device design.