RESUMO
Many colonic surgeries include colorectal anastomoses whose leaks may be life-threatening, affecting thousands of patients annually. Various studies propose that mechanical interaction between the staples and neighboring tissues may play an important role in anastomotic leakage. Therefore, understanding the mechanical behavior of colorectal tissue is essential to characterizing the reasons for this type of failure. So far, experimental data characterizing the mechanical properties of colorectal tissue have been few and inconsistent, which has significantly limited understanding their behavior. This research proposes an approach to developing an anisotropic hyperelastic material model for colorectal tissues based on uniaxial testing of freshly harvested porcine specimens, which were collected from several age- and weight-matched pigs. The specimens were extracted from the same colon tract of each pig along their circumferential and longitudinal orientations. We propose a constitutive model combining Yeoh isotropic hyperelastic material with fibers oriented in two directions to account for the hyperelastic and anisotropic nature of colorectal tissues. Experimental data were used to accurately determine the model's coefficients (circumferential, R2 = 0.9968; longitudinal, R2 = 0.9675). The results show that the proposed model can be incorporated into a finite element model that can simulate procedures such as colorectal anastomoses reliably.
RESUMO
The advancement of 3D printing has led to its widespread use. NinjaFlex®, a thermoplastic polyurethane (TPU) filament, is a highly durable and flexible material that has been used to create flexible parts. While this material has been available for nearly two decades, the mechanical properties of 3D printed NinjaFlex® parts are not well-understood, especially in bending. The focus of this research was predicting the behavior of small 3D printed NinjaFlex® components. Three-dimensionally printed rectangular specimens of varying lengths and aspect ratios were loaded as cantilevers. The deflection of these specimens was measured using a computer. The experimental results were compared to a modified form of the Euler-Bernoulli Beam Theorem (MEB), which was developed to account for nonlinearities associated with large deflection. Additionally, experimental results were compared to the finite element analysis (FEA). The results showed that both modeling approaches were overall accurate, with the average difference between experimental deflection data and MEB predictions ranging from 0.6% to 3.0%, while the FEA predictions ranged from 0.4% to 2.4%. In the case of the most flexible specimens, MEB underestimated the experimental results, while FEA led to higher retraction.
RESUMO
Sub-nanoliter volumes of the Belousov-Zhabotinsky (BZ) reaction are sealed in microfluidic devices made from polydimethylsiloxane (PDMS). Bromine, which is a BZ reaction intermediate that participates in the inhibitory pathway of the reaction, is known to permeate into PDMS, and it has been suggested that PDMS and bromine can react ( J. Phys. Chem. A. 108, 2004, 1325-1332). We characterize the extent to which PDMS affects BZ oscillations by varying the volume of the PDMS surrounding the BZ reactors. We measure how the oscillation period varies with PDMS volume and compare with a theoretical reaction-diffusion model, concluding that bromine reacts with PDMS. We demonstrate that minimizing the amount of PDMS by making the samples as thin as possible maximizes the number of oscillations before the BZ reaction reaches equilibrium and ceases to oscillate. We also demonstrate that the deleterious effects of the PDMS-BZ interactions are somewhat mitigated by imposing constant chemical boundary conditions through using a light-sensitive catalyst, ruthenium, in combination with patterned illumination. Furthermore, we show that light can modulate the frequency and phase of the BZ oscillators contained in a PDMS matrix by 20-30%.