Three-Dimensional Electronic Microfliers With Designs Inspired by Wind-Dispersed Seeds


 Large, distributed collections of miniaturized, wireless electronic devices may form the basis of future systems for environmental monitoring, population surveillance, disease management and other applications that demand coverage over expansive spatial scales. In this paper, we show that wind-dispersed seeds can serve as the bio-inspiration for unusual aerial schemes to distribute components for such networks via controlled, unpowered flight across natural environments or city settings. Techniques in mechanically guided assembly of three-dimensional (3D) mesostructures provide access to miniature, 3D fliers optimized for such purposes, in processes that align with the most sophisticated production techniques for electronic, optoelectronic, microfluidic and microelectromechanical technologies. We demonstrate a range of 3D macro-, meso- and microscale fliers produced in this manner, including those that incorporate active electronic payloads. Analytical, computational and experimental studies of the aerodynamics of high-performance structures of this type establish a set of fundamental considerations in bio-inspired design, with a focus on 3D fliers that exhibit controlled rotational kinematics and low terminal velocities. Battery-free, wireless devices for atmospheric measurements provide simple examples of a wide spectrum of applications of these unusual concepts.

Plants spread their seeds through a remarkable variety of passive strategies, each the result of sustained processes of natural selection. Botanists classify these strategies according to their vectors for dispersal, the main types of which are gravity, ballistic, wind, water, and animals. Among these, wind is one of the most powerful and widely applicable. The 3D shapes of seeds optimized to exploit air flow in such contexts can support stable dynamics in controlled free-fall and/or facilitate transport over distances of up to hundreds of kilometers [1][2][3] . Although certain interactions between airborne seeds and the ambient air are well known, few research studies quantitatively define the essential aerodynamics and none considers the potential relevance in microsystems technologies [4][5][6] . Just as plants use seeds and passive mechanisms for dispersal of genetic material to propagate the species, interesting opportunities might follow from use of similar approaches to distribute miniature electronic sensors, wireless communication nodes, energy harvesting components and/or various internet-of-things (IoT) technologies as monitors to track environmental processes, as aids to guide remediation efforts or as components to support distributed surveillance. This paper introduces the foundational engineering science for practical realization of these ideas [7][8][9][10][11] .
Wind-dispersed seeds adopt geometries that are shaped by forces of evolution to maximize dynamic stability and/or transport distance during passive free-fall. The character of motions induced by air flow defines three broad categories of seeds: (i) gliders such as those of the box elder (Acer negundo) and the big-leaf maple (Acer macrophyllum), (ii) parachuters such as those of the evergreen ash (Fraxinus uhdei) and the tipu tree (Tipuana tipu) and (iii) flutterers/spinners such as those of the empress tree (Paulownia tomentosa), the tree of heaven (Ailanthus altissima) and the jacaranda (Facaranda mimosifolia) (Fig. 1a). These designs serve as inspiration for man-made passive flier structures built using approaches introduced here and engineered to optimize aerial dispersal of functional payloads, including a range of electronic, optoelectronic, microfluidic and microelectromechanical systems technologies. The overall sizes span the microscale (half widths of wings < 1 mm; microfliers), mesoscale (half widths ~1 mm; mesofliers), and macroscale (half widths > 1 mm; macrofliers) with the capacity to integrate material elements and devices with critical feature sizes that extend into the nanometer regime. Fig. 1b compares the dimensions and the geometries of a representative 3D microflier to those of various seeds with elaborate designs.
The fabrication scheme exploits controlled mechanical buckling to convert planar precursor structures formed with state-of-the-art planar processing and lithographic techniques into desired 3D layouts. Specifically, releasing the strain in a prestretched elastomer substrate generates compressive forces on these precursors through a collection of bonding sites. The result affects geometrical transformation through a continuous sequence of in-and out-of-plane displacements and rotational motions (Figs. 1c~e). When implemented with shape memory polymers (SMPs; a mixture of epoxy monomer (E44; China Petrochemical Corporation) and curing agent (D230; Sigma-Aldrich)) and sacrificial thin layers (Mg ~ 50 nm) at the bonding sites, the resulting 3D objects can be released as free-standing passive fliers (Fig. 1f) 11 . The designs and choices of bonding sites define the overall 3D architectures; the magnitude of strain release determines the extent of three dimensionality, qualitatively defined by the ratio of the height of the structures to their lateral dimensions (small, 3D; large, 3D+), as in Fig. 1c. This scheme provides access to systems that behave in any of the three bio-inspired modalities mentioned previously, with flat and/or curved wings, solid and/or perforated structural elements, and various numbers of articulations. A simple identifying nomenclature includes (i) a number to indicate the number of wings, (ii) a letter to describe the shape of wings (R = ribbons, M = membranes, PM = porous membranes, and H = Hybrid, as a combination of ribbons and membranes), and (iii) a number to define the 3D aspect ratio (e.g., height divided by the width). Fig.   1d shows pictures of three 3D microfliers (widths~500 µm) placed on a fingertip. Fig. 1e highlights a 10 x 10 array of micro-, and mesofliers in various sizes (widths = 0.5~2 mm; Fig. S1) and geometries, formed via a single assembly process on a common substrate. Mass quantities of fliers can be formed at high throughput, as illustrated in Fig. 1f.
The terminal velocity (vT) associated with free-fall in still air serves as a simple metric to compare the aerodynamics of these fliers to seeds and other objects in nature. As described in the following, microfliers can exhibit values of vT that are 10 to 15 times small than other objects with similar sizes (~1 mm) and weights (~ 10 mg), including brown rice, sesame seeds, and snow ( Fig.   1g) 12  exhibit vT ~28 cm/s, which is a factor of 3 smaller than that of the seeds (vT ~100 cm/s; Supplementary while for meso-and micro-fliers, consistent with CFD simulations (Fig. S4). As might be expected, the behaviors of microfliers and macrofliers depend mainly on 0 and 1 , respectively; both parameters are important for mesofliers.
Mesofliers with different 3D configurations exhibit a common dependence of vT on fill factor (3), which is dominated by the viscous term, where load is the weight of the payload, is the density of the structural material, is the thickness, and is the gravity acceleration. This equation indicates the existence of an optimal fill factor, i.e., optimal = / 2 , that minimizes the terminal velocity for a given load .
Parachute type seeds incorporate bundles of filaments with high effective porosity ≈ 0.9.
Such configurations can be mimicked to a certain degree by introducing arrays of perforating holes Factors related to the properties of air, i.e., altitude, humidity, temperature or molecular makeup, influence the behaviors mainly through and . For example, increasing the altitude from 0 to 80 km decreases by a factor of 5, but the value of decreases by less than 25% (Fig. S13).
Therefore, as shown by the CFD simulation results in Fig. 2e, mesofliers exhibit small vT even at high altitudes (e.g., ~1.36 m/s at 80 km altitude for 2~2 mm). By comparison, macrofliers have large vT at such altitudes (e.g., > 100 m/s at 80 km altitude for 2~40 mm ). In a similar way, the temperature and molecular makeup of the air can lead to opposite effects for micro-and macrofliers (Figs. S14 and S15).
can characterize the stability, in which 0 / 0 and 4 / 0 2 account for the influences of material parameters, geometrical parameters and air properties (Fig. 2h, Supplementary Note 4), as given by where 1,2,3 are the moment of inertias with respect to directions 1, 2 and 3, and is the distance between the center of gravity and the center of pressure. A large positive value of Γ suggests that the structure can quickly recover to its balanced, stable state; a negative value of Γ indicates that the structure is unstable. Additionally, the overall maximum perturbed angle, i.e., max ( decreases monotonically with 0 / 0 ( Like seeds, these 3D platforms can transport payloads with passive or active functionality. The fabrication scheme affords many possibilities in functional integration, spanning nearly all forms of planar microsystems and semiconductor technologies. with a general-purpose-input/output (GPIO). An external reader device activates the SoC to measure the voltage across the SC, retrieve the corresponding digital data and to discharge the SC, all in a single operation (Fig. 4e, Fig. S30, S31, and Table S1). The measured dose depends on atmospheric conditions, including the pollution levels across altitudes, solar activity and other factors. Fig. 4f demonstrates the quantitative effect of air-born particles (Fig. S32).
The aerodynamics of these 3D IoT macrofliers (Figs. 4g~i and Fig. S33 ~ S36) are consistent with preceding discussions of the physics. The wakes exhibit oscillating tip vortices in the vicinity of the wings and a secondary vortex behind the center (Fig. 4g and Supplementary Video 7). Mean streamwise velocity fields (Fig. 4h) are similar to those of mesofliers with similar designs. Figure 4i shows that across a range of centimeter scale dimensions, the normalized transverse velocity profiles exhibit self-similarity, allowing for efficient dimensional analysis and modeling; inferred drag coefficients are shown in Fig. S33c and d). Layouts that combine these various design strategies may offer enhanced levels of performance, beyond those observed in nature. In addition to payloads that support active semiconductor functionality, responsive materials structures that change in color, shape or radio frequency signature according to environmental cues may serve as simple, complementary options for remote monitoring.
For many applications of distributed sensors and electronic components, efficient methods for recovery and disposal must be carefully considered. One solution is in devices constructed from materials that naturally resorb into the environment via a chemical reaction and/or physical disintegration to benign end products [21][22][23] . In these and other cases, eco-resorbable piezoelectric actuators or alternative active mechanical components may enhance control over flight dynamics.
Such possibilities represent promising directions for future work. Dissolving the PMMA layer with acetone released the devices from the silicon wafer. Lastly, the Si NMs device layer encapsulated by a film of PI was transfer printed onto a thin film of SMP (thickness ~ 5 μm) using a PDMS stamp. Additional steps to create 3D electronic mesofliers from these 2D precursors followed those outlined above. to produce long trajectories. Associated temporal derivatives were filtered and estimated using fourthorder B splines. Additional details of the PTV system can be found elsewhere 25 . The free-falling experiments involved 10 repetitions for each sample to obtain statistically significant measurements of the stability and kinematics of the falling behaviors (Fig. 3c). Tracer particles were tracked in 3D

Three-Dimensional (3D) Micro-, Meso-and Macrofliers. Fabrication of 2D precursors in thin
and converted into inferred 3D Eulerian velocity vector fields that defined the 3D induced flows.
Interpolating scattered Lagrangian flow particles at each frame based on the natural neighbor interpolation method yielded the 3D vector fields.

Experiments using High-Speed Particle Image Velocimetry (PIV) and a vertical wind tunnel.
Two sets of experiments used high-speed PIV above a vertical wind tunnel (Fig. S19b) to define the wake dynamics of (1) fixed 2D precursors and 3D