A constitutive model for ballistic gelatin at surgical strain rates

Increased deformations are dispensable for cell mechanoresponse in engineered bone analogs mimicking aging bone marrow

Aged individuals and astronauts experience bone loss despite rigorous physical activity. Bone mechanoresponse is in-part regulated by mesenchymal stem cells (MSCs) that respond to mechanical stimuli. Direct delivery of low intensity vibration (LIV) recovers MSC proliferation in senescence and simulated microgravity models, indicating that age-related reductions in mechanical signal delivery within bone marrow may contribute to declining bone mechanoresponse. To answer this question, we developed a 3D bone marrow analog that controls trabecular geometry, marrow mechanics and external stimuli. Validated finite element (FE) models were developed to quantify strain environment within hydrogels during LIV. Bone marrow analogs with gyroid-based trabeculae of bone volume fractions (BV/TV) corresponding to adult (25%) and aged (13%) mice were printed using polylactic acid (PLA). MSCs encapsulated in migration-permissive hydrogels within printed trabeculae showed robust cell populations on both PLA surface and hydrogel within a week. Following 14 days of LIV treatment (1g, 100 Hz, 1 hour/day), type-I collagen and F-actin were quantified for the cells in the hydrogel fraction. While LIV increased all measured outcomes, FE models predicted higher von Mises strains for the 13% BV/TV groups (0.2%) when compared to the 25% BV/TV group (0.1%). Despite increased strains, collagen-I and F-actin measures remained lower in the 13% BV/TV groups when compared to 25% BV/TV counterparts, indicating that cell response to LIV does not depend on hydrogel strains and that bone volume fraction (i.e. available bone surface) directly affects cell behavior in the hydrogel phase independent of the external stimuli. Overall, bone marrow analogs offer a robust and repeatable platform to study bone mechanobiology.

Micro-mechanism study on tissue removal behavior under medical waterjet impact using coupled SPH-FEM

To fully grasp the numerical characteristics of the interaction process between medical waterjet and soft tissue, the smoothed particle hydrodynamics (SPH)-finite element method (FEM) was used in the simulation of this complex process to avoid the unstable error caused by indirect measurement in experiments. The SPH was applied to the numerical simulation of medical waterjet, and a three-dimensional model of gelatin sample was proposed with the FEM. The impact process between two extremely deformed materials was reproduced, and the established model was verified by comparison with experimental data; the comparison showed relatively consistent results. The separation effect under three operating modes was deduced with the stress and strain range. For the vertical impact condition, the higher the waterjet impact pressure is, the higher the biological tissue deformation bulge height is. For oblique intrusion, the longitudinal separation rate decreases and the kerf width increases with the increase of the incident angle. For the moving impact condition, with the increase of the waterjet moving speed, the longitudinal high-stress distribution range of the impact object decreases slightly.

Computational modelling of the mechanical behaviour of protein-based hydrogels

Protein-based hydrogels have been extensively studied in the field of biomaterials given their ability to mimic living tissues and their special resemblance to the extracellular matrix. Despite this, the methods used for the control of mechanical properties of hydrogels are very limited, focusing mainly on their elasticity, with an often unrealistic characterization of mechanical properties such as extensibility, stiffness and viscoelasticity. Being able to control these properties is essential for the development of new biomaterials, since it has been demonstrated that mechanical properties affect cell behaviour and biological processes. To better understand the mechanical behaviour of these biopolymers, a computational model is here developed to characterize the mechanical behaviour of two different protein-based hydrogels. Strain-stress tests and stress-relaxation tests are evaluated computationally and compared to the results obtained experimentally in a previous work. To achieve this goal the Finite Element Method is used, combining hyperelastic and viscoelastic models. Different hyperelastic constitutive models (Mooney-Rivlin, Neo-Hookean, first and third order Ogden, and Yeoh) are proposed to estimate the mechanical properties of the protein-based hydrogels by least-square fitting of the in-vitro uniaxial test results. Among these models, the first order Ogden model with a viscoelastic model defined in Prony parameters better reproduces the strain-stress response and the change of stiffness with strain observed in the in-vitro tests.

Wound contraction under negative pressure therapy measured with digital image correlation and finite-element analysis in tissue phantoms and wound models

The purpose of this study is to demonstrate the capabilities of finite-element (FE) models to predict contraction of wounds managed with negative pressure wound therapy (NPWT). The features of wounds and surrounding tissues were analysed to gain insights into the mechanical effects of NPWT on them. 3D digital image correlation (DIC) measurement of soft tissue phantoms was used to investigate the effect of wound thickness, size, and shape, which were further compared with results of FE simulations. It was noticed that with an increased NP level the difference between DIC and FE in wound contraction became more pronounced, particularly for the thick wounds. In addition, the locations of the wounds were evaluated to predict their contraction characteristics, based on surrounding tissue structures, in 3D using the developed FE models. It was demonstrated that features and location of wounds influenced their deformations differently for the same pressure levels. Overall, this study, involving a combined experimental and computational approach, allowed the important insights into mechanical effects of NPWT.

Study on the Similarity of Biomechanical Behavior between Gelatin and Porcine Liver

As a natural polymer, gelatin is increasingly being used as a substitute for animals or humans for the simulation and testing of surgical procedures. In the current study, the similarity verification was neglected and a 10 wt.% or 20 wt.% gelatin sample was used directly. To compare the mechanical similarities between gelatin and biological tissues, different concentrations of gelatin samples were subjected to tensile, compression, and indentation tests and compared with porcine liver tissue. The loading rate in the three tests fully considered the surgical application conditions; notably, a loading speed up to 12 mm/s was applied in the indentation testing, the tensile test was performed at a speed of 1 mm/s until fracture, and the compression tests were compressed at a rate of 0.16 mm/s and 1 mm/s. A comparison of the results shows that the mechanical behaviors of low-concentration gelatin samples involved in the study are similar to the mechanical behavior of porcine liver tissue. The results of the gelatin material were mathematically expressed by the Mooney-Rivlin model and the Prony series. The results show that the material properties of gelatin can mimic the range of mechanical characteristics of porcine liver, and gelatin can be used as a matrix to further improve the similarity between substitute materials and biological tissues.

Extending the Capability of Using a Waterjet in Surgical Interventions by the Use of Robotics

In waterjet surgery, a thin high-pressure jet is used for dissections and surface abrasion of soft tissue. This selective preparation method preserves nerves and vessels, whereas the surrounding soft tissue is washed away.

OBJECTIVE

The aim of this study is to enhance the application field of this technique by resolving technological limitations.

METHODS

A technical task definition of handling a hand-guided waterjet applicator is derived from the literature. All reported procedures require to follow a trajectory superimposed with an oscillating movement. By introducing a robotic system and a specialized kinematic approach, the limited dexterity of the waterjet applicator is increased. Additionally, the system provides assistance by automatically performing parts of the task.

RESULTS

The method is applied to two different procedures: a minimally invasive dissection and a surface abrasion for open medical treatments. On the basis of experiments with gelatine phantoms, the performance of the method is shown for both procedures.

CONCLUSION

In the minimally invasive use case, the reachability limited by the conventional manual tools is extended by the capabilities of the robotic system. Simultaneously, the handling is simplified by automation of the superimposed oscillation. In the surface abrasion case, a dense coverage of the treated area is achievable. The risk of cross infections could be reduced by spatial separation of patient and staff.

SIGNIFICANCE

Thus, the waterjet technology can be fully integrated into robotic surgery systems and benefit from their inherent abilities.