Surface slicing and toolpath planning forin-situbioprinting of skin implants.

Bioprinting has emerged as a successful method for fabricating engineered tissue implants, offering great potential for wound healing applications. This study focuses on an advanced surface-based slicing approach aimed at designing a skin implant specifically forin-situbioprinting. The slicing step plays a crucial role in determining the layering arrangement of the tissue during printing. By utilizing surface slicing, a significant shift from planar fabrication methods is achieved. The developed methodology involves the utilization of a customized robotic printer to deliver biomaterials. A multilayer slicing and toolpath generation procedure is presented, enabling the fabrication of skin implants that incorporate the epidermal, dermal, and hypodermal layers. One notable advantage of using the approximate representation of the native wound site surface as the slicing surface is the avoidance of planar printing effects such as staircasing. This surface slicing method allows for the design of non-planar and ultra-thin skin implants, ensuring a higher degree of geometric match between the implant and the wound interface. Furthermore, the proposed methodology demonstrates superior surface quality of thein-situbio-printed implant on a hand model, validating its ability to create toolpaths on implants with complex surfaces.

Simulating and Predicting the Mechanical Behavior of Electrospun Scaffolds for Cardiac Patches Fabrication.

Fabricating helical scaffolds using electrospinning is a common approach for cardiac implantation, aiming to achieve properties similar to native tissue. However, this process requires multiple experimental attempts to select suitable electrospun properties and validate resulting mechanical responses. To overcome the time and cost constraints associated with this iterative procedure, Finite Element Analysis (FEA) can be applied using stable hyperelastic and viscoelastic models that describe the response of electrospun scaffolds under different conditions. In this study, we aim to create accurate simulations of the viscoelastic behavior of electrospun helical scaffolds. We fabricated helical fibers from Poly (3-caprolactone) (PCL) using the electrospinning process to achieve this. The electrospun samples were subjected to uniaxial deformation, and their response was modelled using existing hyperelastic and stress relaxation models. The simulations were built on experimental data for specific deformation speed and maximum strain conditions. The FEM results were evaluated by accounting for the stress-softening phenomenon, which significantly impacted the models. The electrospun scaffolds' predictions were performed in other than the initial experimental conditions to verify our simulations' accuracy and reliability.

Evaluating the effect of environmental conditions on high compressive strain rates in unfilled and filled neoprene rubbers.

Elastomers are known for their strain-rate-dependent properties not only to quasistatic but also to high strain rate deformations, where mechanical behavior is significantly affected by inertia. Concurrently, environmental changes, such as temperature and humidity variations, can impact their stress response to deformation. This study investigates the effects of material layers within neoprene samples on mitigating these environmental changes. While the presence of an intermediate layer proves effective against temperature and humidity influence, it fails to block the impact of increasing high strain rates. Moreover, the different humidity levels at room and elevated temperatures do not significantly alter the mechanical behavior of filled neoprene samples compared to pure neoprene. Notably, in unfilled neoprene, an increase in humidity levels, other than an absolutely dry environment, leads to a notable stress level rise at room temperature, while under elevated temperature conditions, there is a significant stress decrease with increasing humidity. However, neoprene filled with polyester/cotton or nylon displays resilience to diminishing mechanical behavior under various temperature and humidity regulations, indicating that the material layer within these samples effectively "protects" the rubbers from potential stress lapses observed in unfilled neoprene. While a high strain rate compression affects the behavior of the filled variants significantly, increasing humidity and temperature have minimal impact on their stress levels. These findings offer valuable insights into the dynamic responses of elastomers to environmental changes, highlighting the advantages of using filled rubbers in diverse applications.

Multistep deformation of helical fiber electrospun scaffold toward cardiac patches development.

The scaffolds used for cardiac patches must mimic the viscoelastic behavior of the native tissue, which expands up to high deformation levels of its sedentary size during the systole segment of pumping blood. In our study, we exposed fabricated electrospun samples to repeated multistep tension by applying and removing deformation to mimic the mechanical behavior of helical fibered cardiac scaffolds. Since the fiber-based specimens exhibit viscoelastic behavior, the transient responses to constant deformation caused stress relaxation and stress recovery. However, these transient viscoelastic operations performed at high strain enable unpredictable phenomena, usually hidden behind stress softening and folding (plasticity) phenomena; the material significantly reduces the required stress, and remaining deformation occurs. Thus, by regulating the fabrication (electrospinning parameters) process and preconditioning before setting, the actual viscoelastic behavior of the electrospun scaffolds will be evident, as well as their limitations towards their application to cardiac patches development.

Characterization of Mode I and Mode II Interlaminar Fracture Toughness in CNT-Enhanced CFRP under Various Temperature and Loading Rates.

This study investigates the influence of temperature and loading rate on the Mode I and Mode II interlaminar fracture behavior of carbon-nanotubes-enhanced carbon-fiber-reinforced polymer (CNT-CFRP). CNT-induced toughening of the epoxy matrix is characterized by producing CFRP with varying loading of CNT areal density. CNT-CFRP samples were subjected to varying loading rates and testing temperatures. Fracture surfaces of CNT-CFRP were analyzed using scanning electron microscopy (SEM) imaging. Mode I and Mode II interlaminar fracture toughness increased with increasing amount of CNT to an optimum value of 1 g/m2, then decreased at higher CNT amounts. Moreover, it was found that CNT-CFRP fracture toughness increased linearly with the loading rate in Mode I and Mode II. On the other hand, different responses to changing temperature were observed; Mode I fracture toughness increased when elevating the temperature, while Mode II fracture toughness increased with increasing up to room temperature and decreased at higher temperatures.

Identification of Hyperelastic Material Parameters of Elastomers by Reverse Engineering Approach.

Simulating the mechanical behavior of rubbers is widely performed with hyperelastic material models by determining their parameters. Traditionally, several loading modes, namely uniaxial tensile, planar equibiaxial, and volumetric, are considered to identify hyperelastic material models. This procedure is mainly used to determine hyperelastic material parameters accurately. On the contrary, using reverse engineering approaches, iterative finite element analyses, artificial neural networks, and virtual field methods to identify hyperelastic material parameters can provide accurate results that require no coupon material testing. In the current study, hyperelastic material parameters of selected rubbers (neoprene, silicone, and natural rubbers) were determined using an artificial neural network (ANN) model. Finite element analyses of O-ring tension and O-ring compression were simulated to create a data set to train the ANN model. Then, the ANN model was employed to identify the hyperelastic material parameters of the selected rubbers. Our study demonstrated that hyperelastic material parameters of any rubbers could be obtained directly from component experimental data without performing coupon tests.

High-Strain-Rate Compression of Elastomers Subjected to Temperature and Humidity Conditions.

Elastomers exhibit a complex response to high-strain-rate deformation due to their viscoelastic behaviour. Environmental conditions highly impact this behaviour, especially when both temperature and humidity change. In several applications where elastomers are used, the quantity of real humidity might vary, especially when the temperature is elevated. In the current research, elastomeric materials were subjected to high-strain-rate compression in various elevated and lowered (cold) temperatures. Different humidity levels were applied at room and elevated temperatures to analyze the behaviour of rubbers in dry and moist conditions. Results showed that the mechanical behaviour of rubbers is highly affected by any environmental change. In particular, the impact caused by humidity variations is relative to their ability to absorb or repel water on their surface.

The effect of the printing temperature on 4D DLP printed pNIPAM hydrogels.

Additive manufacturing (AM), in its little more than 40 years of existence, has already established itself as a technology with enormous potential for several fields, especially the ones that require complex, high resolution, small structures, such as tissue engineering. This field has been especially attracted to the most recent AM evolution, 4D printing, due to its ability to create structures responsive to external stimuli. Among the range of materials that are simultaneously suitable for 4D printing and biological uses, poly(N-isopropylacrylamide) (pNIPAM) stands out. pNIPAM presents exceptional characteristics such as a low critical solution temperature (LCST) close to the human physiological temperature and biocompatibility with several cell types. However, these characteristics are greatly affected by processing parameters. In this work, pNIPAM hydrogels were manufactured by AM using digital light processing; the printing temperature was varied between 5, 10 and 15 °C to analyze how it affects the hydrogels' final properties. The impact on hydrogels was analyzed by differential scanning calorimetry (DSC), swelling, deswelling and reswelling analyses, scanning electron microscopy (SEM) images, and compression tests. Based on our results increasing the production temperature of the hydrogels by 10 °C led to a decrease of more than 50% in the maximum swelling capacity, approximately 10% increase in water retention, and 6.5 °C variation in the LCST. The justification for such behaviour lies in the increase of the crosslinking rate and thickening of the external layer of hydrogels, which prevents the free movement of water from its interior.

Transient Effects of Applying and Removing Strain on the Mechanical Behavior of Rubber.

For viscoelastic materials, the relationship between stress and strain depends on time, where the applied strain (or stress) can be expressed as a step function of time. In the present work, we investigated two temporary effects in the response of viscoelastic materials when a given strain is applied and then removed. The application of strain causes a stress response over time, also known as relaxation. By contrast, recovery is the response that occurs following the removal of an applied stress or strain. Both stress and relaxation constitute transient stages of a viscoelastic material exposed to a permanent force. In the current work, we performed several experimental tests to record the recovery in response to the total or partial removal of the strain. By observing and analyzing the mechanical response of the material to strain, we deduced that recovery is a procedure not only related to creep but also to relaxation. Hence, we created a model that simulates the behavior of viscoelastic materials, contributing to the prediction of relevant results concerning different conditions.

Evaluating FDM Process Parameter Sensitive Mechanical Performance of Elastomers at Various Strain Rates of Loading.

To optimize the mechanical performance of fused deposition modelling (FDM) fabricated parts, it is necessary to evaluate the influence of process parameters on the resulting mechanical performance. The main focus of the study was to characterize the influence of the initial process parameters on the mechanical performance of thermoplastic polyurethane under a quasi-static and high strain rate (~2500 s-1). The effects of infill percentage, layer height, and raster orientation on the mechanical properties of an FDM-fabricated part were evaluated. At a quasi-static rate of loading, layer height was found to be the most significant factor (36.5% enhancement in tensile strength). As the layer height of the sample increased from 0.1 to 0.4 mm, the resulting tensile strength sample was decreased by 36.5%. At a high-strain rate of loading, infill percentage was found to be the most critical factor influencing the mechanical strength of the sample (12.4% enhancement of compressive strength at 100% as compared to 80% infill). Furthermore, statistical analysis revealed the presence of significant interactions between the input parameters. Finally, using an artificial neural networking approach, we evaluated a regression model that related the process parameters (input factors) to the resulting strength of the samples.

FE Analysis of Critical Testing Parameters in Kolsky Bar Experiments for Elastomers at High Strain Rate.

The main aim of this research is to present complete methodological guidelines for dynamic characterization of elastomers when subjected to strain rates of 100/s-10,000/s. We consider the following three aspects: (i) the design of high strain rate testing apparatus, (ii) finite element analysis for the optimization of the experimental setup, and (iii) experimental parameters and validation for the response of an elastomeric specimen. To test low impedance soft materials, design of a modified Kolsky bar is discussed. Based on this design, the testing apparatus was constructed, validated, and optimized numerically using finite element methods. Furthermore, investigations on traditional pulse shaping techniques and a new design for pulse shaper are described. The effect of specimen geometry on the homogeneous deformation has been thoroughly accounted for. Using the optimized specimen geometry and pulse shaping technique, nitrile butadiene rubber was tested at different strain rates, and the experimental findings were compared to numerical predictions.