The potential lubrication mechanisms for articular cartilage remain as complex as the material itself. Many studies have examined systematically the effects of sliding velocity on the frictional response of cartilage, but a systematic analysis of probe curvature and contact area remains largely uncharted. Here, we explore this potential effect in three ways: a constant force experiment, a constant pressure experiment, and a nonconstant pressure and force experiment. Hard, impermeable spheres are used as probes and with sizes varied between 1.6 mm and 25.4 mm, allowing a sweep of physiologically relevant pressures. The aim is to effectively isolate the influence of probe curvature on the friction measurements. Articular cartilage samples were collected from horses at necropsy from the left and right distal and medial aspects of the radius. Each test was run for 5 min at a reciprocating sliding speed of 1 mm/s. Smaller probe diameters (and curvature) consistently resulted in high measured coefficients of friction of cartilage. Two lubrication regimes are observed during testing: an area-dependent regime at lower areas and an area-independent regime at higher areas. A power law (-1/3 to -1/2) relationship between friction coefficient and probe diameter is observed. Therefore, measurements made using different probe sizes will most likely result in different measurements of the friction behavior, and great care should be given when comparing these results.

Additive manufacturing (AM) has revolutionized rapid prototyping and manufacturing. However, limited research has been done on the effect of build orientation and surface roughness on AM parts' frictional and wear characteristics. This study examines how different print and system parameters influence the surface topography and tribological behavior of 3D printed PLA. The samples were printed in three orientations and tested under varying normal loads (50-100 N). The vertically printed samples resulted in the best wear performance compared to the other two build orientations (similar to 26.75% and 18.47%, respectively, at 100 N normal load). The coefficient of friction also showed dependency on the orientation of the print. The effect of surface topography parameters on tribological properties was also investigated. Skewness, Ssk, and maximum valley depth, S, exhibited a strong positive correlation with the coefficient of friction, indicating that tribological behaviors are more sensitive to extreme surface topography features than average surface roughness (Sa). A data-driven approach was employed to predict wear rate and coefficient of friction using four machine learning models: Support Vector Regression (SVR), Artificial Neural Network (ANN), Random Forest (RF), and Extreme Gradient Boosting (XGBoost), where decision tree-based models outperformed others. The RF model achieved an R2 value of 0.98 in predicting the wear rate and the coefficient of friction, where surface roughness parameters and operational parameters (normal loads, sliding distance) played critical roles.

Caprock formations are a crucial part of subsurface-engineered systems. Composed largely of shale, caprocks act as natural barriers that prevent the upward migration of fluids, thereby ensuring the containment of stored substances in subsurface formations. Fractures in these formations are potential leakage pathways for stored fluids. Mineral precipitation reactions in these fractures, particularly calcite, can significantly restrict the fluid permeability, reducing leakage potential. However, predictive capabilities of mineral precipitation in fractures and associated permeability evolution are limited due to a lack of fundamental understanding of such reactions in natural samples, complicated by mineral heterogeneity and the complexity of the fracture structure. In this study, 3D-printed fracture samples are used to understand the impact of carbonate mineralization on fracture aperture and permeability evolution. Samples were printed using a digital light processing (DLP) 3D printer and commercial liquid resin. Calcite precipitation was first tested on printed 2D films before conducting plug flow column experiments aimed to understand fracture permeability changes due to mineral precipitation. Contact angle measurement and Fourier transform infrared (FTIR) spectroscopy on printed 2D films show evidence of a substantial amount of surface energy for calcite nucleation and precipitation. Surface topography analysis of printed fractured surfaces reveals comparable values, highlighting the high replicability of the printed samples. During the column experiments, the permeability reduces exponentially due to a decrease in fracture aperture. Reductions in fracture aperture estimated from effluent concentration and 3D X-ray computed tomography (CT) show comparable results. Moreover, 3D X-ray CT images suggest the impact of local flow velocities on precipitation. The insights gained from this research contribute to a deeper understanding of the permeability evolution due to carbonate mineralization in caprock formations.

Surface roughness of additively manufactured metal parts has significant impacts on the part’s performance (e.g., fatigue resistance) and poses a significant roadblock to the wider adoption of metal additive manufacturing. This work aims to show that, in some cases, it is possible to estimate the depth of the deepest valley of an additively manufactured part with a relatively high accuracy using either only line measurements from a contact profilometer (and limited areal scans used for calibration) or areal scans from optical areal scanners of a considerably smaller area of the same specimen or even another specimen produced in the same batch under the same conditions. Both problems are approached by employing the block-maxima method from extreme value theory, whereby the underlying distribution of the depth of individual valleys is modeled with a Gumbel distribution. The experimental results from additively manufactured Ti-6Al-4V specimens demonstrate that the proposed methods can produce estimates that significantly outperform more straightforward benchmarks (e.g., simply using linear parameters for areal ones), and the proposed methods achieve a relatively accurate estimation with mean errors of 5–15

The elastic moduli of tissues are connected to their states of health and function. The epithelial monolayer is a simple, minimal, tissue model that is often used to gain understanding of mechanical behavior at the cellular or multi-cellular scale. Here we investigate how the elastic modulus of Madin Darby Canine Kidney (MDCK) cells depends on their packing density. Rather than measuring elasticity at the sub-cellular scale with local probes, we characterize the monolayer at the multi-cellular scale, as one would a thin slab of elastic material. We use a micro-indentation system to apply gentle forces to the apical side of MDCK monolayers, applying a normal force to approximately 100 cells in each experiment. In low-density confluent monolayers, we find that the elastic modulus decreases with increasing cell density. At high densities, the modulus appears to plateau. This finding will help guide our understanding of known collective behaviors in epithelial monolayers and other tissues where variations in cell packing density are correlated with cell motion.

Simple synthetic and natural hydrogels can be formulated to have elastic moduli that match biological tissues, leading to their widespread application as model systems for tissue engineering, medical device development, and drug delivery vehicles. However, two different hydrogels having the same elastic modulus but differing in microstructure or nanostructure can exhibit drastically different mechanical responses, including their poroelasticity, lubricity, and load bearing capabilities. Here, we investigate the mechanical response of collagen-1 networks to local and bulk compressive loads. We compare these results to the behavior of polyacrylamide, a fundamentally different class of hydrogel network consisting of flexible polymer chains. We find that the high bending rigidity of collagen fibers, which suppresses entropic bending fluctuations and osmotic pressure, facilitates the bulk compression of collagen networks under infinitesimal applied stress. These results are fundamentally different from the behavior of flexible polymer networks in which the entropic thermal fluctuations of the polymer chains result in an osmotic pressure that must first be overcome before bulk compression can occur. Furthermore, we observe minimal transverse strain during the axial loading of collagen networks, a behavior reminiscent of open-celled cellular solids. Inspired by these results, we applied mechanical models of cellular solids to predict the elastic moduli of the collagen networks and found agreement with the moduli values measured through contact indentation. Collectively, these results suggest that unlike flexible polymer networks that are often considered incompressible, collagen hydrogels behave like rigid porous solids that volumetrically compress and expel water rather than spreading laterally under applied normal loads.

Transitions between solid-like and fluid-like states in living tissues have been found in steps of embryonic development and in stages of disease progression. Our current understanding of these transitions has been guided by experimental and theoretical investigations focused on how motion becomes arrested with increased mechanical coupling between cells, typically as a function of packing density or cell cohesiveness. However, cells actively respond to externally applied forces by contracting after a time delay, so it is possible that at some packing densities or levels of cell cohesiveness, mechanical coupling stimulates cell motion instead of suppressing it. Here we report our findings that at low densities and within multiple ranges of cell cohesiveness, cell migration speeds increase with these measures of mechanical coupling. Our observations run counter to our intuition that cell motion will be suppressed by increasingly packing or sticking cells together and may provide new insight into biological processes involving motion in dense cell populations.

In this study, irradiation-assisted stress corrosion cracking (IASCC) resistance of as-built AM 316L stain-less steel (SS) made by direct energy deposition (DED) was evaluated in a simulated boiling water reactor (BWR) environment. Different from the intergranular cracking typically seen from 316L SS, unique intra-granular cracking away from grain boundaries (GB) was observed. The presence of high-density micron-sized delta ferrite near the melt pool boundaries promoted the in-grain strain localization and crack nu-cleation. Delta ferrite particles can efficiently retard dislocations during deformation, altering the strain localization from the GB to its surrounding area and thus reducing the chance of GB cracking. The phe-nomenon was further confirmed by the higher nanoindentation hardness measured from the correspond-ing area, and by characterizing the evolution of microstructure, cracking morphology, and dislocation channels.(c) 2023 Elsevier B.V. All rights reserved.

Additive manufacturing is rapidly growing in popularity for manufacturing parts with tunable mechanical properties. Recent studies show that mechanical properties can be achieved by controlling the layer orientation and build structure. In this work the effect of print orientation on tribological properties of 3D printed PLA and ABS are investigated. PLA and ABS samples are printed using fused deposition modeling (FDM) with three different print orientations. Tribological results show that variation in build direction relative to the sliding direction causes anisotropy in wear properties. The best wear properties are achieved with the samples printed where the layers remain orthogonal to the sliding direction. The coefficient of friction remains mostly unaffected by print orientation. PLA samples demonstrate significantly better tribological properties compared to ABS. Varying the sliding speed between the interacting surfaces also affects the wear properties of both PLA and ABS. The results suggest that optimizing the build orientation can improve the wear performance of additively manufactured thermoplastics. This enables an additional paradigm when designing for functionally graded materials.

Abstract Additive manufacturing is rapidly growing in popularity due to its capability to produce parts with customized mechanical properties. Recent studies show that mechanical properties can be altered in a controlled manner by adjusting the relationship between build orientation and the applied loading from the service. In this work, the effect of build orientation on the tribological properties of 3D printed PLA – Polylactic acid and ABS- Acrylonitrile butadiene styrene is investigated. PLA and ABS samples are printed using material extrusion with three different build orientations. Tribological results show that variation in build direction relative to the sliding direction leads to anisotropic wear properties. The best wear properties are achieved when the layers are oriented orthogonally to the sliding direction. The coefficient of friction remains mostly unaffected by the build orientation. PLA samples demonstrate significantly better tribological properties compared to ABS. Varying the sliding speed between the interacting surfaces also affects the wear properties of both PLA and ABS. The results suggest that optimizing the build orientation with respect to service loading can improve the wear performance of additively manufactured thermoplastics. This presents an additional paradigm when designing additively manufactured parts potentially for functionally graded materials.

The increased versatility in the design and production of low-volume components, as well as the shorter time between design and prototype, has intensified interest in the subject of additively printed electronics. The continuous attempts to miniaturize traditional forms in terms of both size and weight are a reason why flexible electronics will emerge as a modern alternative in the technical industry. Direct printing on a range of substrates, whether rigid, flexible, or conformable, offers numerous advantages over traditional electronics production processes. Furthermore, the increasing complexity of flexible electronics demands the creation of multilayered circuits comparable to classic PCBs in order to reduce the volumetric and gravimetric influence of the underlying electronics. The majority of commercial PCBs have many component attachments using solder and multilayer functionality. There is a paucity of information on component attachment for flexible electronics applications that integrate multilayer functional qualities employing electrically conductive adhesive (ECA) and low-temperature solder (LTS). In this paper, we used OrCAD software to design and analyze an LED flashing circuit in order to imitate the applications of the Rigid PCBs. We investigated the influence of photonic curing over multiple passes on the mechanical and electrical properties of printed electronics utilizing a microdispensing unit during the multilayer development process. Capacitors, resistors, inductors, diodes, and Op-Amps are examples of components attached utilizing electrically conductive adhesive (ECA) and low-temperature solder (LTS). We will also examine the circuit performance for the two binding materials in order to make a solid application-based material decision.

Background Subsurface mechanisms can greatly affect the mechanical behavior of biological materials, but observation of these mechanisms has remained elusive primarily due to unfavorable optical characteristics. Researchers attempt to overcome these limitations by performing experiments in biological mimics like hydrogels, but measurements are generally restricted due to the spatio-temporal limitations of current methods. Objective Utilization of contemporary 3D printing techniques into soft, transparent, aqueous yield-stress materials have opened new avenues of approach to overcoming these roadblocks. By incorporating digital image correlation with such 3D printing techniques, a method is shown here that can acquire full-field deformation of a hydrogel subsurface in real-time. Methods Briefly, the method replaces the solvent of a transparent and low polymer concentration yield-stress material with an aqueous hydrogel precursor solution, then a DIC speckle plane is 3D printed into it. This complex is then polymerized using photoinitiation thereby locking the speckle plane in place. Results Full-field deformation measurements are made in real-time as the embedded speckle plane (ESP) responds with the bulk to the applied load. Example results of deformation and strain fields associated with indentation, relaxation, and sliding contact experiments are shown. Conclusions This method has successfully observed the subsurface mechanical response in the bulk of a hydrogel and has the potential to answer fundamental questions regarding biological material mechanical behaviors.

Printed electronics are rapidly increasing and competing against the traditional way of manufacturing electronics. Different aspects of the processes are already being taken over that avoids material wastage, reduces cost and lead times. For example, printing technologies are very popular in depositing reliable and very fine conductive traces as opposed to conventional etching. Previously, authors have developed process studies by utilizing Aerosol Jet Printing technology in realizing multi-layer metallization with successive metal and dielectric layers and additively printing a donut micro-via. However, the merits of printing technologies do not stop here. Recently, assembly of Surface Mount Devices (SMD) on a flexible substrate has been of interest that requires low processing temperatures. Conventionally, this is done by components mounted on a solder paste and reflowing to about 220 °C. This temperature is extremely high and induces many reliability concerns. With the use of printing technologies, this can be avoided. A viable method would be to utilize low temperature solder for which the peak reflow temperature could be much lower than 220 °C. Nevertheless, with new methods, there comes new complications. For example, reliability concerns involving the interconnection joints between SMD and substrate. Since this is to be done additively, another concern comes about the printer’s capability to dispense the solder paste as well as the metal conductive pad simultaneously. Having to additively print conductive pad, and screen print the solder brings about much manual work and just not feasible. In this work, authors have utilized state-of-art micro-dispense additive printer to dispense the metal conductive ink and the Low Temperature Solder (LTS) for SMD attachment. Process studies are done that addresses the physical profile of the low temperature solder for various print parameters. Multiple SMDs such as resistors, capacitors, and Inductors are mounted with semi-automatic pick-and-place machine. The reflow will still be done but at a lower temperature. Process development of additive part of the assembly will be done, along with the characterization of physical, electrical and mechanical properties.

Contact mechanics models are widely used to analyze electrical contact behavior. Greenwood and Williamson (GW model) first developed a rough surface contact model to solve the problem of electrical contact. The original GW model used the Hertz single asperity model and a Gaussian distribution of the surface roughness. However, in many of the electrical contact cases, contact area surpasses the Hertz small contact region. For medium to complete contact cases, asperity interactions become very important and the Hertz model cannot predict this behavior. Besides asperity interaction, the probability distribution function (PDF) of the asperities of the rough surface is very important as not all the surfaces are Gaussian in nature. This work has shown the effect of asperity models to predict the asperity interaction behavior. Then the asperity models are applied with different PDF of the asperities of the rough surface in the framework of the statistical model. For the elastic case, the newly proposed rough surface models are compared with a Boundary Element Method (BEM) simulation result and the Persson model. For the elastic-plastic case, electrical contact resistance has been measured between two rough surfaces using the four-wire resistance method and then the newly proposed rough surface models are compared with the experimental results. Comparisons suggest that sinusoidal asperity model and the proper choice of the PDF of the asperity of the rough surface can effectively model the contact resistance behavior.

In this paper, process recipes for interconnection of passive components have been developed using aerosol-jet printing and direct-write printing. Assembly of components on substrates is conventionally done using solder joints. However, additive print methods in many formulations on low temperature substrates may not be compatible with traditional surface-mount processes. In this paper, interconnection process recipes for surface-mount passives including resistor, capacitors and inductors have been explored using a combination of direct-print interconnects and electrically conductive adhesives. Process recipes and the correlation of process parameters with the expected electrical performance, mechanical performance and reliability have been studied. Aerosol Jet Printing technique with silver nanoparticle ink and polyimide dielectric ink has been used to accomplish component attachment. The conductive traces are printed using ultrasonic atomizer system and the dielectric is printed using the pneumatic system. The LC filter circuit printed is compared with the test results of commercial rigid LC filter to showcase the AJ printing capabilities.

Printed electronics is a fastest growing and emerging technology that have shown much potential in several industries including automotive, wearables, healthcare, and aerospace. Its applications can be found not only in flexible but also in large area electronics. The technology provides an effective and convenient method to additively deposit conductive and insulating materials on any type of substrate. Comparing with traditional manufacturing processes, which involves chemical etching, this technology also comes to be relatively environmental friendly. Despite its status, it is not without its challenges. Starting from the material being compatible in the printer equipment to the point of achieving fine resolutions, and with excellent properties are some of the challenges that printed electronics face. Among the myriad of printing technologies such as Aerosol Jet, micro-dispensing, gravure printing, screen printing, Inkjet printing, Inkjet has gained much attention due to its low-cost, low material consumption, and roll-to-roll capability for mass manufacturing. The technology has been widely used in home and office, but recently gained interest in printed electronics in a research and development setting. Conductive materials used in Inkjet printing generally comprises of metal Nanoparticles that need to be thermally sintered for it to be conductive. The preferred metal of choice has been mostly silver due to its excellent electrical properties and ease in sintering. However, silver comes to be expensive than its counterpart copper. Since copper is prone to oxidation, much focus has been given towards photonic sintering that involves sudden burst of pulsed light at certain energy to sinter the copper Nanoparticles. With this technique, only the printed material gets sintered in a matter of seconds without having a great impact on its substrate, due to which it is also preferred in low temperature applications. With all the knowledge, there is still a large gap in the process side with copper where it is important to look how the print process affects the resolution of the print along with the effect of post-print processes on electrical and mechanical properties. In this paper, a copper Inkjet ink is utilized for understanding the effect of Inkjet print parameters on the ejected droplet and its resolution. Post-print process is also quantified using a photonic sintering equipment for excellent electrical and mechanical properties. To demonstrate the complete process, commercial-off-the-shelf components will also be mounted on the additively printed pads via Inkjet. Statistically, control charting technique will be utilized to understand the capability of the Inkjet process.

Filters are used in a variety of signal processing applications in commercial and defense electronics. The use of additively printed electronics in high frequency applications requires an understanding of the process-performance interactions versus frequency of operation. Assembly of filters for integration into existing circuits requires additively printed metallization traces in addition to component attachment methods. Comparison of frequency response of the additively-printed filtering circuits vs conventional filters subtractively-fabricated on rigid substrates is needed to determine the performance parity of additive fabrication methods. In this paper, a micro-dispensing device is used to print conductive traces and electrically conductive adhesive (ECA) pads for the attachment of components. The effect of different print parameters on the width and height of the trace has been studied. Mechanical and electrical properties also play an important role in the study of different sintering conditions. Optimized parameters from the printing process and sintering analysis are used to print and compare commercially available LC filter circuitry using the Bode plot.

In this paper, the assembly of passive components on additively printed metallization using additively dispensed conductive adhesive is studied. Additively printing technologies are being increasingly explored in the realm of electronics manufacturing. These have found their way in various aspects of manufacturing starting from achieving reliable traces to multi-layer circuits with micro-via, and now surface mount devices. Different technologies have their merits and demerits based on the conductive material properties, substrate, and the printer's capability to incorporate multiple materials simultaneously. Previously, authors have shown how Aerosol Jet Printing (AJP) technology can be utilized to deposit very fine traces and realize multi-layer metallization with additively printed donut-vias. To move on to the other aspect of electronics manufacturing, assembly of Surface Mount Devices (SMD) on a flexible substrate is being looked into using additive methods. For the assembly, having an additively printed pad along with a conductive adhesive appears to be a suitable path and an alternative to solder reflow process where components suffer high temperatures. However, certain process limitations come to light in this aspect. For example, instead of copper being used as a conducting material, silver is used due to its availability and ease in thermal curing as opposed to copper because of oxide formation. Another example could be the depositing of conductive adhesive. Since the adhesive is extremely viscous, printer should be able to incorporate high viscosity materials without any blockage issues every minute. In this work, authors have utilized two different printing technologies: InkJet and micro-dispensing, to characterize the assembly process of SMD components. InkJet technology is used to deposit the metal conductive pad, on top of which conductive adhesive is pneumatically dispensed. SMD components are placed using a semi-automatic pick-and-place machine. Different SMD discreet components are used such as resistors, capacitors, and inductors. Process studies on the two state-of-art printing technologies in achieving a fine quality print are developed. With this work, authors aim to characterize the printing processes of two materials, and also providing a viable method for the assembly of SMDs on a flexible substrate.

In this paper, the process-recipes and process-performance relationships for additive-printing of copper circuits using direct-write methods have been studied. The process has been implemented on the direct write platform. Interest in the use of additive printing methods for the manufacture of micro-circuits has grown immensely in recent times. Direct write methods have been shown to have the ability to create circuits in a limited manner. However, the process recipes and the effect of process parameters on the manufactured properties are not well understood. Copper ink is a good and cost-effective alternative to silver ink but its use has lagged owing to an increased propensity for oxidation. In this paper, photonic curing has been used to sinter copper ink to make the traces conductive. The method flashes high energy light that sinter metal particles instantaneously and the temperature of the substrate remains low. The effect of the different photonic sintering profiles on the mechanical and electrical properties of the printed traces has been studied in this paper. The print process parameters also play an important role in the line width and height that has been studied to print with the desired line profile for the end application. An LC filter circuit is been printed with SMD components been attached using an electrically conductive adhesive (ECA). The manufactured flexible LC filter is been tested for its frequency sweep to compare with the commercially available LC filer with the help of the Bode plot.

Conventional rigid PCBs have been used in a variety of applications and have grown over the years to be the most reliable for the electronics industry. The rigid PCBs consist of thousands of surface-mounted devices that are attached to the circuitry using solder and traces. As far as flexible electronics is concerned, there has been not much study of the device attachment processes. Flexible substrates providing a higher degree of freedom, thus being very important to develop a reliable method for connecting components to circuits that can accommodate the various movements. In this paper, a micro dispensing device is used to print conductive traces and electrically conductive adhesive (ECA) pads for attachment of components. The effect of different print parameters on the width and height of the trace has been studied. Mechanical and electrical properties also play an important role in the study of different sintering conditions. Optimized parameters from the printing process and sintering analysis are used to print and compare commercially available LC filter circuitry using the Bode plot.