Self-assembled three dimensional network designs for soft electronics

Abstract

Low modulus, compliant systems of sensors, circuits and radios designed to intimately interface with the soft tissues of the human body are of growing interest, due to their emerging applications in continuous, clinical-quality health monitors and advanced, bioelectronic therapeutics. Although recent research establishes various materials and mechanics concepts for such technologies, all existing approaches involve simple, two-dimensional (2D) layouts in the constituent micro-components and interconnects. Here we introduce concepts in three-dimensional (3D) architectures that bypass important engineering constraints and performance limitations set by traditional, 2D designs. Specifically, open-mesh, 3D interconnect networks of helical microcoils formed by deterministic compressive buckling establish the basis for systems that can offer exceptional low modulus, elastic mechanics, in compact geometries, with active components and sophisticated levels of functionality. Coupled mechanical and electrical design approaches enable layout optimization, assembly processes and encapsulation schemes to yield 3D configurations that satisfy requirements in demanding, complex systems, such as wireless, skin-compatible electronic sensors.

Figure 1. Assembly of conductive 3D helical coils and their mechanical properties.

(a) Process for assembly illustrated by finite element analysis (FEA). A 2D filamentary serpentine structure bonded at selected locations to an underling, bi-axially stretched (ε_pre) soft elastomeric substrate (pre-streched). Corresponding 3D helical coils formed by relaxation of the substrate to its initial, unstretched state (strain released). The colour represents the magnitude of Mises stress in the metal layer. (b) Angled and cross-sectional optical images of an experimentally realized structure. The traces consist of lithographically defined multilayer ribbons of polyimide/Au or Cu/polyimide bonded to a silicone substrate. Scale bar, 1 mm. FEA results for the deformations and distributions of Mises stress in a 3D coil (c) and a 2D serpentine (d) with similar geometries at 0 and 50% uniaxial strain. (e) Distribution of maximum Mises stress for each cross section along the natural coordinate normalized by the arc length, for the 3D helical coil in c, for three different levels of applied strain (0, 25, 50%). (f) Similar results for the case of the 2D serpentine in d.

Figure 2. Three-dimensional network of helical coils as electrical interconnects for soft electronics.

(a) Optical image of the system at a bi-axially stretched state of 50%, showing ∼250 3D helices, ∼500 bonding sites, ∼50 component chips and elastomers for full encapsulation (Encapsulation material: Silbione 4717A/B, Bluestar Silicones, France; E=3 kPa; bottom substrate: Ecoflex 00-50A/B, Smooth-on, USA; E=20 kPa) Inset: optical image of the device under a complex state of deformation. Scale bar, 5 mm. Scanning electron micrographs and corresponding FEA results of representative regions of the 3D network, including (b) electrically isolated crossing points and (c,d) interfaces with chip components. Scale bars, 100 μm (b), 1 mm (c) and 1 mm (d). The widths and thicknesses of all coils throughout this system are 50 and 1 μm, respectively. (e) Optical image of a device supported by two fingers. (f) Block diagram of the functional components for a set of electrophysiological (EP) sensors with an analogue signal processing unit that includes filters and amplifiers, a three-axial digital accelerometer, a Bluetooth system on a chip (SoC) for signal acquisition and wireless communication, and a wireless power transfer system for battery-free operation. The signal acquisition involves sampling of the EP sensor output through an internal analogue-to-digital converter (ADC) and data acquisition of the accelerometer output via a serial peripheral interface (SPI). When operated using a custom graphical user interface on a smart phone, this system can capture and transmit a range of information related to physiological health, as shown in Supplementary Movie 1.

Figure 3. Strategy for soft encapsulation and system-level mechanics.

(a) Process for soft encapsulation illustrated with FEA results: first, a collection of electronic components joined by an interconnect network in a 2D serpentine design bond at selective sites to a bi-axially prestrained soft elastomeric substrate (Ecoflex 00-50A/B, Smooth-on, USA; E=20 kPa). Partial release of the prestrain mechanically transforms the 2D serpentines into 3D helices. Coating the entire structure with a thin, low-modulus silicone elastomer (Silbione 4717A/B, Bluestar Silicones, France; E=3 kPa) defines the encapsulation layer. Finally, releasing the remaining prestrain enhances the 3D geometry of the helices and completes the process. The insets correspond to magnified views of a local region. (b) Elastic stretchability of a 3D circuit system encapsulated at different states of partially released strain in this two-stage encapsulating process (blue for uniaxial stretching along X axis; red for uniaxial stretching along Y axis; black for radial stretching). (c) Bar graph of the elastic stretchability for a 3D circuit system formed by using a one-stage encapsulating process, two-stage encapsulating process and no encapsulation. (d) System-level deformation and distribution of stresses determined by FEA with encapsulation introduced after (d) full and (f) partial release of the prestrain. In all of the FEA images, the colour represents the magnitude of Mises stress in the metal layer. (e) Optical images of a device deformed in similar ways. Scale bars, 1 cm (a–d). The scale bar of the inset of (a) is 1 mm.

Figure 4. Mechanical and operational characteristics of the complete and encapsulated system.

(a) Optical images of a device deformed in different ways, with corresponding FEA results highlighted in red coloured boxes. (b) Representative recordings of three-axis acceleration from a device on the left forearm and inferred 3D patterns of motion. (c) Results for respiration rate extracted from frequency analysis of accelerometer data from a device placed on the chest, for cases at rest and after physical exercise. (d) Electrophysiological recordings with inset images of the device on the skin: electrocardiogram (ECG), electromyogram (EMG), electrooculogram (EOG) and electroencephalogram (EEG). (e) Wireless, multimodal monitoring of body activity, through simultaneous measurements of ECG (plotted in green colour) and movements of the chest by accelerometry (plotted in blue colour) during a time interval that includes standing (0–60 s), walking (60–120 s) and running (120–180 s).