Massimo Mastrangeli

Self-Assembly from Milli- to Nanoscales: Methods and Applications

The design and fabrication techniques for microelectromechanical systems (MEMS) and nanodevices are progressing rapidly. However, due to material and process flow incompatibilities in the fabrication of sensors, actuators and electronic circuitry, a final packaging step is often necessary to integrate all components of a heterogeneous microsystem on a common substrate. Robotic pick-and-place, although accurate and reliable at larger scales, is a serial process that downscales unfavorably due to stiction problems, fragility and sheer number of components. Self-assembly, on the other hand, is parallel and can be used for device sizes ranging from millimeters to nanometers. In this review, the state-of-the-art in methods and applications for self-assembly is reviewed. Methods for assembling three-dimensional (3D) MEMS structures out of two-dimensional (2D) ones are described. The use of capillary forces for folding 2D plates into 3D structures, as well as assembling parts onto a common substrate or aggregating parts to each other into 2D or 3D structures, is discussed. Shape matching and guided assembly by magnetic forces and electric fields are also reviewed. Finally, colloidal self-assembly and DNA-based self-assembly, mainly used at the nanoscale, are surveyed, and aspects of theoretical modeling of stochastic assembly processes are discussed.

Nanoscale topographical control of capillary assembly of nanoparticles

Predetermined and selective placement of nanoparticles onto large-area substrates with nanometre-scale precision is essential to harness the unique properties of nanoparticle assemblies, in particular for functional optical and electro-optical nanodevices. Unfortunately, such high spatial organization is currently beyond the reach of top-down nanofabrication techniques alone. Here, we demonstrate that topographic features comprising lithographed funnelled traps and auxiliary sidewalls on a solid substrate can deterministically direct the capillary assembly of Au nanorods to attain simultaneous control of position, orientation and interparticle distance at the nanometre level. We report up to 100% assembly yield over centimetre-scale substrates. We achieve this by optimizing the three sequential stages of capillary nanoparticle assembly: insertion of nanorods into the traps, resilience against the receding suspension front and drying of the residual solvent. Finally, using electron energy-loss spectroscopy we characterize the spectral response and near-field properties of spatially programmable Au nanorod dimers, highlighting the opportunities for precise tunability of the plasmonic modes in larger assemblies.

Conformal dip-coating of patterned surfaces for capillary die-to-substrate self-assembly

Capillarity-driven self-assembly of small chips onto planar target substrates is a promising alternative to robotic pick-and-place assembly. It critically relies on the selective deposition of thin fluid films on patterned binding sites, which is anyway normally non-conformal. We found that the addition of a thin wetting sidewall, surrounding the entire site perimeter, enables the conformal fluid coverage of arbitrarily shaped sites through dip-coating, significantly improves the reproducibility of the coating process and strongly reduces its sensitivity to surface defects. In this paper we support the feasibility and potential of this method by demonstrating the conformal dip-coating of square and triangular sites conditioned with combinations of different hydrophobic and hydrophilic surface chemistries. We present both experimental and simulative evidence of the advantages brought by the introduction of the wetting boundary on film coverage accuracy. Application of our surface preparation method to capillary self-assembly could result in higher precision in die-to-substrate registration and larger freedom in site shape design.

Small-scale soft-bodied robot with multimodal locomotion

Untethered small-scale (from several millimetres down to a few micrometres in all dimensions) robots that can non-invasively access confined, enclosed spaces may enable applications in microfactories such as the construction of tissue scaffolds by robotic assembly, in bioengineering such as single-cell manipulation and biosensing, and in healthcare such as targeted drug delivery and minimally invasive surgery. Existing small-scale robots, however, have very limited mobility because they are unable to negotiate obstacles and changes in texture or material in unstructured environments. Of these small-scale robots, soft robots have greater potential to realize high mobility via multimodal locomotion, because such machines have higher degrees of freedom than their rigid counterparts. Here we demonstrate magneto-elastic soft millimetre-scale robots that can swim inside and on the surface of liquids, climb liquid menisci, roll and walk on solid surfaces, jump over obstacles, and crawl within narrow tunnels. These robots can transit reversibly between different liquid and solid terrains, as well as switch between locomotive modes. They can additionally execute pick-and-place and cargo-release tasks. We also present theoretical models to explain how the robots move. Like the large-scale robots that can be used to study locomotion, these soft small-scale robots could be used to study soft-bodied locomotion produced by small organisms.

Challenges for Capillary Self-Assembly of Microsystems

Within the currently rising trend of heterogeneous microsystem integration and packaging, capillary self-assembly emerges as an innovative technique to enhance, complement and eventually replace pick-and-place assembly. Vast literature and experimental data support such claim. Still, the technique needs to overcome some important limitations in order to fully express its potential and earn wide industrial recognition. In this paper, we review and illustrate what are in our opinion the challenges ahead for making part-to-substrate capillary self-assembly reliable and seriously competitive with long-established assembly techniques. After setting self-assembly methods in the context of microsystem assembly and integration technologies, we focus on the standard embodiment of capillary self-assembly, and we describe in details the main, often novel technological steps required for its effective and reproducible performance. This preludes to an outline of what are presently, in our view, the major failure modes affecting the overall yield of the capillary self-assembly technique. Consequently, we propose solutions to face and overcome these challenges, which need to be met to foster the success of this technique.

The Fluid Joint: The Soft Spot of Micro- and Nanosystems.

Fluid bridges are ubiquitous soft structures of finite size that conform to and link the surfaces of neighboring objects. Fluid joints, the specific type of fluid bridge with at least one extremity constrained laterally, display even more pronounced reactivity and self-restoration, which make them remarkably suited for assembly, actuation, and manipulation purposes. Their peculiar surface and bulk properties place fluid joints at the rich intersection of diverse scientific interests, and foster their widespread use throughout micro- and nanotechnology. A critical survey of the mechanics and of the manifold applications of fluid bridges and joints in micro- and nanosystems is presented here, along with current challenges and multidisciplinary perspectives.

Capillary Self-Alignment of Mesoscopic Foil Components for Sensor-Systems-in-Foil

This paper reports on the effective use of capillary self-alignment for low-cost and time-efficient assembly of heterogeneous foil components into a smart electronic identification label. Particularly, we demonstrate the accurate (better than 50 μm) alignment of cm-sized functional foil dies. We investigated the role played by the assembly liquid, by the size and the weight of assembling dies and by their initial offsets in the self-alignment performance. It was shown that there is a definite range of initial offsets allowing dies to align with high accuracy and within approximately the same time window, irrespective of their initial offset.

Real-time automated modeling and control of self-assembling systems

We present the M3 framework, a formal and generic computational framework for modeling and controlling stochastic distributed systems of purely reactive robots in an automated and real-time fashion. Based on the trajectories of the robots, the framework builds up an internal microscopic representation of the system, which then serves as a blueprint of models at higher abstraction levels. These models are then calibrated using a Maximum Likelihood Estimation (MLE) algorithm. We illustrate the structure and performance of the framework by performing the online optimization of a bang-bang controller for the stochastic self-assembly of water-floating, magnetically latching, passive modules. The experimental results demonstrate that the generated models can successfully optimize the assembly of desired structures.

In-Plane Mode Dynamics of Capillary Self-Alignment

We present an experimental study of the complete in-plane dynamics of capillary self-alignment. The two translational (shift) and single rotational (twist) in-plane modes of square millimetric transparent dies bridged to shape-matching receptor sites through a liquid meniscus were selectively excited by preset initial offsets. The entire self-alignment dynamics was simultaneously monitored over the three in-plane degrees of freedom by high-speed optical tracking of the alignment trajectories. The dynamics of the twist mode is shown to qualitatively follow the sequence of dynamic regimes also observed for the shift modes, consisting of initial transient wetting, acceleration toward, and underdamped harmonic oscillations around the equilibrium position. Systematic analysis of alignment trajectories for individually as well as simultaneously excited modes shows that, in the absence of twist offset, the dynamics of the degenerate shift modes are mutually independent. In the presence of twist offset, the three modes conversely evidence coupled dynamics, which is attributed to a synchronization mechanism affected by the wetting of the bounding surfaces. The experimental results, justified by energetic, wetting, and dynamic arguments, provide substantial benchmarks for understanding the full dynamics of the process.

Modeling Self-Assembly Across Scales: The Unifying Perspective of Smart Minimal Particles

A wealth of current research in microengineering aims at fabricating devices of increasing complexity, notably by (self-)assembling elementary components into heterogeneous functional systems. At the same time, a large body of robotic research called swarm robotics is concerned with the design and the control of large ensembles of robots of decreasing size and complexity. This paper describes the asymptotic convergence of micro/nano electromechanical systems (M/NEMS) on one side, and swarm robotic systems on the other, toward a unifying class of systems, which we denote Smart Minimal Particles (SMPs). We define SMPs as mobile, purely reactive and physically embodied agents that compensate for their limited on-board capabilities using specifically engineered reactivity to external physical stimuli, including local energy and information scavenging. In trading off internal resources for simplicity and robustness, SMPs are still able to collectively perform non-trivial, spatio-temporally coordinated and highly scalable operations such as aggregation and self-assembly (SA). We outline the opposite converging tendencies, namely M/NEMS smarting and robotic minimalism, by reviewing each field’s literature with specific focus on self-assembling systems. Our main claim is that the SMPs can be used to develop a unifying technological and methodological framework that bridges the gap between passive M/NEMS and active, centimeter-sized robots. By proposing this unifying perspective, we hypothesize a continuum in both complexity and length scale between these two extremes. We illustrate the benefits of possible cross-fertilizations among these originally separate domains, with specific emphasis on the modeling of collective dynamics. Particularly, we argue that while most of the theoretical studies on M/NEMS SA dynamics belong so far to one of only two main frameworks—based on analytical master equations and on numerical agent-based simulations, respectively—alternative models developed in swarm robotics could be amenable to the task, and thereby provide important novel insights.