Metallic Nanoislands on Graphene for Biomechanical Sensing

Abstract

This minireview describes a nanomaterial-based multimodal sensor for performing biomechanical measurements. The sensor consists of ultrathin metallic films on single-layer graphene. This composite material exhibits physical properties that neither material possesses alone. For example, the metal, deposited by evaporation at low (≤10 nm) nominal thicknesses, renders the film highly sensitive to mechanical stimuli, which can be detected using electrical (i.e., resistance) and optical (i.e., plasmonic) modalities. The electrical modality, in particular, is capable of resolving deformations as small as 0.0001% engineering strain, or 1 ppm. The electrical and optical responses of the composite films can be tailored by controlling the morphology of the metallic film. This morphology (granular or island-like when deposited onto the graphene) can be tuned using the conditions of deposition, the identity of the substrate beneath the graphene, or even the replacement of the graphene for hexagonal boron nitride (hBN). This material responds to forces produced by a range of physiological structures, from the contractions of heart muscle cells, to the beating of the heart through the skin, to stretching of the skin due to the expansion of the lungs and movement of limbs. Here, we provide an update on recent applications of this material in fields ranging from cardiovascular medicine (by measuring the contractions of 2D monolayers of cardiomyocytes), regenerative medicine (optical measurements of the forces produced by myoblasts), speech pathology and physical therapy (measuring swallowing function in head and neck cancer survivors), lab-on-a-chip devices (using deformation of sidewalls of microfluidic channels to detect transiting objects), and sleep medicine (measuring pulse and respiration with a wearable, unobtrusive device). We also discuss the mechanisms by which these films detect strain.

Figure 1

Overview of sensor devices comprising nanocracked metallic films (a) Wearable sensors for measuring calorie expenditure during exercise. Reproduced (adapted) with permission from ref (6). Copyright (2019) Wiley-VCH. (b) Tactile sensing device comprising metallized nanofibril networks for resistive strain sensing. Reproduced (adapted) with permission from ref (7). Copyright (2018) Wiley-VCH. (c) Wearable sensor comprising a cracked metal network for the monitoring of finger movements, which were mapped to a virtual environment. Reproduced (adapted) with permission from ref (8). Copyright (2020) Nature Publishing Group.

Figure 2

Overview of metallic films on 2D substrates for biomechanical measurements. (a) Wearable sensors for the detection of mechanical biosignals on the skin. Reproduced (adapted) with permission from ref (9). Copyright (2018) American Chemical Society. (b) Optical detection of the contractions in musculoskeletal cells using changes in intensity of the surface-enhanced Raman scattering signal of a monolayer of reporter molecules adsorbed to the metallic films. Reproduced (adapted) with permission from ref (10). Copyright (2017) Royal Society of Chemistry. (c) Detection of particles and cells flowing through microfluidic channels. Reproduced (adapted) with permission from ref (11). Copyright (2018) American Chemical Society.

Figure 3

Piezoresistive biomechanical sensing with wearable devices comprising graphene/metal composites. Graphene/palladium (Gr/Pd) devices have been used for (a) the piezoresistive detection of swallowing activity in head and neck cancer patients for the monitoring of the onset of swallowing dysfunction due to radiation. The swallowing data acquired by the sensor were used to (b) develop machine learning algorithms designed to (c) distinguish swallows for the same food type between a healthy human (bottom plot, blue) and a dysphagic patient (top plot). Reproduced (adapted) with permission from ref (9). Copyright (2018) American Chemical Society. By combining (d) Gr/Pd with PEDOT:PSS, stretchable devices could be used for the simultaneous measurement of human pulse pressure and respiration waveforms by placing the device on the torso of a human participant (e). (f) The deformation in the structure was modeled using finite element analysis. Reproduced (adapted) with permission from ref (19). Copyright (2019) American Chemical Society.

Figure 4

Use of metal–graphene composite strain gauges for cellular biomechanics. (a) Metallic nanoislands supported by graphene undergo a change in plasmonic resonance under strain. This change is reflected in a modulation of the surface-enhanced Raman scattering (SERS) intensity of a self-assembled monolayer of benzene thiolate bonded to the metal. (b) A device comprising a film of graphene/silver is used to stimulate contractions of musculoskeletal cells (C2C12 myoblast) electrically, while (c) attenuation of the SERS signal corresponds to the contractions. Reproduced (adapted) with permission from ref (10). Copyright (2017) Royal Society of Chemistry. (d) Particles passing through microfluidic channels produce small deflections of the sidewalls of the channels that can be detected by bending of sensors embedded in the sidewalls. (e) A change in resistance caused by the channel deformation due to flowing human mesenchymal stem cells is monitored. (f) The image shows cells as they pass through the fluidic channel. Reproduced (adapted) with permission from ref (11). Copyright (2018) American Chemical Society.

Figure 5

Elucidating mechanism of piezoresistive strain detection. (a) Transmission electron micrographs of disconnected and percolated films of palladium on single-layer graphene, with nominal thicknesses of 2 and 8 nm, respectively. (b) Schematic diagrams showing the electron path across the films of graphene/metal and hBN/metal films. (c) Cantilever apparatus for measuring the piezoresistive response of graphene/metal and hBN/metal films as they undergo step bending strains of 1 ppm (0.0001%). The magnitude of sensitivity is determined by calculating the gauge factor of the films at a chosen bending strain. Reproduced from ref (30).