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Bone is a hierarchical material primarily composed of collagen, water, and mineral that is organized into discrete molecular, nano-, micro-, and macroscale structural components. In contrast to the structural knowledge of the collagen and mineral domains, the nanoscale porosity of bone is poorly understood. In this study, we introduce a well-established pore characterization technique, positron annihilation lifetime spectroscopy (PALS), to probe the nanoscale size and distribution of each component domain by analyzing pore sizes inherent to hydrated bone together with pores generated by successive removal of water and then organic matrix (including collagen and noncollagenous proteins) from samples of cortical bovine femur. Combining the PALS results with simulated pore size distribution (PSD) results from collagen molecule and microfibril structure, we identify pores with diameter of 0.6 nm that suggest porosity within the collagen molecule regardless of the presence of mineral and water. We find that water occupies three larger domain size regions with nominal mean diameters of 1.1, 1.9, and 4.0 nm—spaces that are hypothesized to associate with intercollagen molecular spaces, terminal segments (d-spacing) within collagen microfibrils, and interface spacing between collagen and mineral structure, respectively. Subsequent removal of the organic matrix determines a structural pore size of 5–6 nm for deproteinized bone—suggesting the average spacing between mineral lamella. An independent method to deduce the average mineral spacing from specific surface area (SSA) measurements of the deproteinized sample is presented and compared with the PALS results. Together, the combined PALS and SSA results set a range on the mean mineral lamella thickness of 4–8 nm.

Swarms of micro- and milli-sized robots have the potential to advance biological micro-manipulation, micro- assembly and manufacturing, and provide an ideal platform for studying large swarm behaviors and control. Due to their small size and low cost, tens to hundreds of micro/milli robots can function in parallel to perform a task that otherwise would be too cumbersome or costly for a larger macroscopic robot. Here, we demonstrate a scalable system and modular circuit architecture for controlling and coordinating the motion of $ >$ 10's of magnetic micro/milli robots. By modifying the concepts of safety barrier certificates to our magnetic robot hardware, we achieve minimally invasive, collision-free, 2D position control (x, y) of up to N = 16 robots in a low-cost tabletop ( $\text{288 mm} \times \text{288 mm}$ ) magnetic milli-robot platform with up to 288 degrees of freedom. We show that the introduction of random dithering can achieve a $100\%$ success rate (i.e., no deadlocking), enabling the system to serve as a platform for the study of various swarm-like behaviors or multi-agent robotic coordination.

Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H2 storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H2 storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N2 cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.

Microbiological environments are a clear application area for microscale robotics, but targeted delivery of biochemical cues to particular cells or tissues remains a significant challenge. This challenge is compounded by the sheer number of cells that compose tissues. Multirobot system development has been limited due to challenges in discrete operation of several robots in a confined space. Here, we demonstrate the use of multiple diamagnetically levitated robots for delivering biochemicals dissolved in liquid in a manner compatible with light microscopy. Specifically, we show loading, transport, and diffusive release of fluorescent molecules in hydrogels with microscale precision, as well as induction of synthetically engineered bacterial cells with signaling molecules. This platform holds potential for impact and utility in fields, such as developmental biology, tissue engineering, and synthetic biology.

The process of activation, an essential step to reveal the porosity of metal–organic frameworks (MOFs), was examined for materials that produce surface areas in accord with prediction as well as for those failing to meet expectations. In situ two-dimensional powder X-ray diffraction, in combination with surface area measurements, was used to classify the structural outcomes of a variety of MOF activations. By modulation of activation solvent as well as linker length and flexibility, a variety of structural outcomes were obtained. In addition to pristine and amorphous products, MOFs that exhibit intermediate activation success were observed. Such MOFs were found to undergo reductions in crystallite size while varying in total amorphous content, which leads to vastly different product surface areas. Models to explain different activation outcomes are presented.

This paper describes the systematic study of metal–organic framework (MOF) catalysts for the reaction of propylene oxide (PO) with carbon dioxide (CO2) to generate propylene carbonate (PC). These studies began with the evaluation of MIL-101(Cr) as catalyst in a flow reactor. Under the developed flow conditions, MIL-101(Cr) was found to effectively catalyze PO carbonation in the absence of a halide co-catalyst. A systematic study of catalyst performance was then undertaken as a function of MOF synthesis technique, activation conditions, metal center, and node architecture. Ultimately, these investigations led to the identification of MIL-100(Sc) as a new, active, and stable catalyst for PO carbonation.

Exploratory research into the critical steps in metal-organic framework (MOF) activation involving solvent exchange and solvent evacuation are reported. It is discovered that solvent exchange kinetics are extremely fast, and minutes rather days are appropriate for solvent exchange in many MOFs. It is also demonstrated that choice of a very low surface tension solvent is critical in successfully activating challenging MOFs. MOFs that have failed to be activated previously can achieve predicted surface areas provided that lower surface tension solvents, such as n-hexane and perfluoropentane, are applied. The insights herein aid in the efficient activation of MOFs in both laboratory and industrial settings and provide best practices for avoiding structural collapse.

Magnetic micro-robots have a variety of potential applications for biomedical manipulation and microassembly. However, providing independent degrees of freedom to individual magnetic micro-robots can be hindered by the large numbers of local coils or traces required to achieve all degrees of freedom. In this paper, we explore the use of novel control strategies and mechanical constraints to provide additional rotational degrees of freedom to underactuated multipole magnetic robots originally designed with only translational degrees of freedom. By carefully applying repulsive impulse forces and changing the center of rotation, we show that a dynamic in-plane rotation of 3500 deg/s can be achieved. By using other geometries of mechanical constraints such as a wheel on axle, we can also demonstrate 1800 deg/s out-of-plane rotation.

Metal organic frameworks (MOFs) are promising materials for the storage of hydrogen fuel due to their high surface areas, tunable properties, and reversible gas adsorption. Although several MOFs are known to exhibit high hydrogen densities on a gravimetric basis, realizing high volumetric capacities – a critical attribute for maximizing the driving range of fuel cell vehicles – remains a challenge. Here, MOFs that achieve high gravimetric and volumetric H2 densities simultaneously are identified computationally, and demonstrated experimentally. The hydrogen capacities of 5309 MOFs drawn from databases of known compounds were predicted using empirical (Chahine rule) correlations and direct atomistic simulations. A critical assessment of correlations between these methods, and with experimental data, identified pseudo-Feynman–Hibbs-based grand canonical Monte Carlo calculations as the most accurate predictive method. Based on these predictions, promising MOF candidates were synthesized and evaluated with respect to their usable H2 capacities. Several MOFs predicted to exhibit high capacities displayed low surface areas upon activation, highlighting the need to understand the factors that control stability. Consistent with the computational predictions, IRMOF-20 was experimentally demonstrated to exhibit an uncommon combination of high usable volumetric and gravimetric capacities. Importantly, the measured capacities exceed those of the benchmark compound MOF-5, the record-holder for combined volumetric/gravimetric performance. Our study illustrates the value of computational screening in pinpointing materials that optimize overall storage performance.

The use of microporous coordination polymers with coordinatively unsaturated metal centers in the removal of dimethyl ether from chloromethane via flow was explored as an alternative to current industrial methods. Three different materials examined, Co/DOBDC, MIL-100(Fe), and HKUST-1, were able to efficiently purify chloromethane through selective adsorption of dimethyl ether. The recyclability of Co/DOBDC after separation was also examined, and little loss of capacity for dimethyl ether over the course of multiple regeneration cycles was observed.

The cationic rhodium complexes (dppe)Rh(COD)BF4 and (MeCN)2Rh(COD)BF4 have been supported in metal–organic frameworks bearing anionic nodes (ZJU-28) and anionic linkers (MIL-101-SO3) via ion exchange. These MOF-supported Rh species serve as recyclable catalysts for the hydrogenation of both the terminal alkene substrate 1-octene and the internal alkene substrate 2,3-dimethylbutene. The nature of the MOF support impacts various aspects of catalysis, including: (i) the rate of 1-octene hydrogenation, (ii) the activity and recyclability of the catalyst in 2,3-dimethylbutene hydrogenation, and (iii) the size selectivity of hydrogenation with alkene substrates appended to calixarenes.

In this article, we study the use of ferrofluids for levitating magnetic micro/milli-robots. With the addition of a ferrofluid, the robots improved their weight-carrying ability (xI2, >12 grams) and precision (x2 ~5 um) without any associated increase in actuation power of the robot relative to sliding versions without the ferrofluid. To better understand the longterm stability of a ferrofluid on the micro/milli-robot, we perform motion cycle testing on two different surfaces (graphite and Teflon), ultimately achieving a motion repeatability and pose angular stability of 5 um and 0.05 °, close to the limits of our measurement setup. The underdamped motion of the ferrofluid robot showed a natural resonance of ~58–60 Hz. Through careful control of the evaporation of the ferrofluid, we show long-term stability of a ferrofluid micro/milli-robot over the course of >11 hours. While the performance of the robot does change due to evaporation of the particular ferrofluid used, we also show that robots can easily be replenished with new ferrofluid after mechanically cleaning away the remaining ferrofluid. Overall, the application of ferrofluids toward magnetically actuated micro/milli-robots can enable low-cost and high-precision motion for future high-precision micro-manipulation or positioning tasks.

The development of high-capacity methane adsorbents would accelerate the adoption of natural gas as a vehicular fuel, thereby lowering CO2 emissions from the combustion of gasoline. In this regard metal–organic frameworks (MOFs) have emerged as promising methane storage materials due to their high capacities and tunable properties. Within this class, HKUST-1 ([Cu3(BTC)2]n, BTC = 1,3,5-benzenetricarboxylate) is an important benchmark, as it exhibits methane densities that are among the highest reported. Furthermore, uptake in HKUST-1 can potentially be tuned by altering the methane–MOF interaction through metal substitution on coordinatively unsaturated sites (CUS). Predicting the impact of metal substitution remains a challenge, however, because general interatomic potentials commonly used in calculating uptake do not properly describe interactions involving CUS. Here, a new interatomic potential that explicitly accounts for these interactions is derived from quantum-mechanical calculations. The potential...

A strategy that allows the tuning of pore size in microporous coordination polymers (MCPs) through modification of their organic linkers is presented. When large substituents are introduced onto the linker, these pendent groups partially occupy the pores, thus reducing pore size while serving as additional adsorption sites for gases. The approach takes advantage of the fact that, for methane storage materials, small pores (0.4-0.8 nm in diameter) are more desirable than large pores since small pores promote optimal volumetric capacity. This method was demonstrated with IRMOF-8, a MCP constructed from Zn4O metal clusters and 2,6-naphthalenedicarboxylate (NDC) linkers. The NDC was functionalized through the addition of substituents including tert-butylethynyl or phenylethynyl groups. High pressure methane uptake demonstrates that the IRMOF-8 derivatives have significantly better performance than the unfunctionalized material in terms of both excess volumetric uptake and deliverable capacity. Moreover, IRMOF-8 derivatives also give rise to stronger interactions with methane molecules as shown by higher heat of adsorption values.

This paper describes the systematic study of metal-organic framework (MOF) catalysts for the reaction of propylene oxide (PO) with carbon dioxide (CO

Examination of the microstructure of metal–organic frameworks (MOFs) after postsynthetic exchange (PSE) reveals that the exchanged ligand is concentrated at the edges of the crystal and decreases in concentration with crystal depth, resulting in a core–shell arrangement. Diffusion studies of the carboxylate ligand into MOF-5 indicate that diffusion is limiting to the exchange process and may ultimately be responsible for the observed core–shell structure. Examination of PSE in UMCM-8 and single crystals of UiO-66 shows a similar trend, illustrating the applicability of PSE as a method for the creation of core–shell MOFs.

In this article, we demonstrate the application of diamagnetically levitated milli-robots for the 2D micro-assembly of 10-μm polymer microspheres and other silicon microfabricated parts. By using an optical microscope for feedback (imaged at 27 Hz), we are able to demonstrate long-term open-loop stability (up to 78 hr) and sub-micron stability of the levitated micro-robots. Furthermore, due to the low hysteresis and high compliance in the magnetic drive of the milli-robots, we are able to directly use the milli-robots in conjunction with machine vision as a force sensor. Soft polymer-based end effectors are used for the micromanipulation of parts and show modest reliability of pick (>70%) and high reliability of place (>99%) that is insensitive to the pick surface material. Finally, we implement autonomous micro-assembly from randomly deposited microspheres into ordered arrays.

Multi-agent robotic systems on small scales typically use, or envision using, small mobile robots. This paper takes a broad-brush look at a relatively recent development in milli and micro robots, Diamagnetic Micro Manipulation (DM3) systems, in a multi-agent context. We report various multi-agent operations, such as multi-agent part manipulation, robots-acting-on-other-robots for enhanced functionality, and robot-supplied local feed, to assess the state of the art of this new type of system. In addition, we explore and analyze the multi-agent implications of known DM3 properties and earlier single-agent results to motivate future research in this area. As reported here, some of these multi-agent implications include mobile high-precision sensor transducers, near-field communications channels using robots as both transmitters and receivers, and novel swarm architectures.

Air drying is a widespread and critical industrial process. Removal of water from air is commonly accomplished by passage through a desiccant such as alumina; modest water capacity and energy intensive regeneration are limitations of currently used sorbents. Microporous coordination polymers (MCPs) are demonstrated here to be efficient desiccants for the dehumidification of air, and a comparison of their capacity, regenerability, and efficiency with commercial activated alumina is conducted. Complete regeneration using dry air with mild heating is achieved. The attainment of high capacity for the adsorption of water coupled to facile regeneration indicates that gas dehumidification may be an important application for MCPs.