Miniature Joints Research Articles

Human Mesenchymal Stem Cell-Derived Miniature Joint System for Disease Modeling and Drug Testing.

Diseases of the knee joint such as osteoarthritis (OA) affect all joint elements. An in vitro human cell‐derived microphysiological system capable of simulating intraarticular tissue crosstalk is desirable for studying etiologies/pathogenesis of joint diseases and testing potential therapeutics. Herein, a human mesenchymal stem cell‐derived miniature joint system (miniJoint) is generated, in which engineered osteochondral complex, synovial‐like fibrous tissue, and adipose tissue are integrated into a microfluidics‐enabled bioreactor. This novel design facilitates different tissues communicating while still maintaining their respective phenotypes. The miniJoint exhibits physiologically relevant changes when exposed to interleukin‐1β mediated inflammation, which are similar to observations in joint diseases in humans. The potential of the miniJoint in predicting in vivo efficacy of drug treatment is confirmed by testing the “therapeutic effect” of the nonsteroidal anti‐inflammatory drug, naproxen, as well as four other potential disease‐modifying OA drugs. The data demonstrate that the miniJoint recapitulates complex tissue interactions, thus providing a robust organ chip model for the study of joint pathology and the development of novel therapeutic interventions.

Size dependent creep deformation of elastically constrained compliant metallic joints

The present study proposes a novel method to predict the size dependent creep behavior of bi-material joints formed by constraining a creep compliant metal, such as Sn, having varied thickness ranging from 1.4 mm to 170 µm, between stiff elastic substrates, such as Cu. A dramatic reduction in the secondary or minimum creep rate was observed with decrease in joint thickness. Finite element (FE) analysis using continuum formulations attributed this strengthening to the geometric constraints imposed by Cu, which increases the triaxiality in the joints and hence reduces the effective stress. While FE results were in close agreement with experiment in thick joints, it significantly overpredicted the creep rate of miniature joints. Further, orientation imaging revealed that microstructure varied with length scale, from bulk Sn with multiple grains to miniature joints having a few grains. The additional strengthening was captured using dislocation-based crystal plasticity (CP) creep modeling of Sn-Cu joints by incorporating this microstructural length scale, in addition to the geometric constraints. CP simulations revealed that orientation anisotropy of Sn and the constraints imposed by substrates on dislocation motion lead to higher strength in the miniature joints. Using the insights from FE and CP modeling a unified length scale sensitive model, incorporating both geometric or continuum and microstructural factors, was developed that can accurately predict the creep response of the joints of macro and meso length scale.

Modification of Sn58Bi solder based on graphite

A novel composite solder paste with 0·07 wt-% graphite addition was fabricated. The mechanical properties of matrix solder and composite solder alloys were tested using a kind of miniature joint. The results showed that the tensile strength of the Sn58Bi+graphite composite solder joint declined slightly compared with the Sn58Bi solder joint. However, the elongation of the Sn58Bi+graphite composite solder joint was ∼13 times as much as that of the Sn58Bi solder joint. Stress relaxation method, which was used to test the creep performance, was verified by nanoindentation method. The result showed that the stress relaxation method was suitable for creep testing. The creep testing results of Sn58Bi and Sn58Bi+0·07 wt-% graphite solder alloys showed that the anticreep performance of Sn58Bi was improved by the addition of graphite.

Nanoindentation measurements and mechanical testing of as-soldered and aged Sn–0.7Cu lead-free miniature joints

Nanoindentation testing has been used to analyze local mechanical parameter changes across interfaces formed between the solder and the Cu substrate and also in the eutectic solder, in miniature lead-free soldered joints. Effects of plastic deformation and aging were investigated. The lead-free solder alloy used in this study is commercially available and has nominal composition by weight of 99.3% Sn and 0.7% Cu. During aging, the joints were exposed to 150 °C for 1000 h in an inert atmosphere. Tensile testing of as-soldered miniature joints shows strong positive dependence of the plastic flow stresses on strain rate, meaning that the stresses increase with strain rate. Similar behavior was observed for aged miniature soldered joints with ∼10% decrease in the level of maximum stresses, compared to as-soldered joints. Indentation hardness and modulus were measured in soldered joint components, in the as-soldered, aged conditions, and tensile tested with strain rates in the range of 1.8 × 10 −3–1.8 × 10 −1 s −1. Scanning electron and optical microscopy were employed to analyze the fracture paths and microstructure of the as-soldered and aged miniature joints, as well as the location and shape of the indentations. The measured indentation hardness and modulus agreed well with previous studies on similar alloys. The tested modulus of the intermetallic phase in the eutectic area exhibited a considerable reduction as compared to the intermetallic phase at the interface between the solder and the Cu substrate. Strain rate strongly influences local mechanics: for both as-soldered and deformed miniature joints; close to the fracture face in the eutectic solder, the indentation modulus values were 53% higher and hardness more than 100% higher in joints exposed to the highest strain rate relative to the smallest rate.

Solderability Testing of Lead-Free Solder Joints Using Miniature Specimens

Abstract A new method for mechanical testing of lead-free solder joints solderability is presented. Copper blanks are soldered with a known thermal cycle. Miniature tensile solder joints specimens are produced in large numbers from the blanks having a gage section made of lead-free solder alloy with dimensions that represent actual solder joints. Mechanical properties of Sn-3.5Ag-0.7Cu in tension at room temperature and strain-rates between 2.8×10−4–2.7×10−1s−1 have been studied. It is concluded that the solder section of the miniature joints is the weakest link in this complex structure. It was also found that the experimental results for solder strength and strain-rate sensitivity are in partial agreement with data obtained using solder bulk specimens. The role of microstructure across the joint, especially intermetallic compounds along the copper/solder interface is also discussed.