Stretchable piezoelectric biocrystal thin films

Abstract

Stretchability is an essential property for wearable devices to match varying strains when interfacing with soft tissues or organs. While piezoelectricity has broad application potentials as tactile sensors, artificial skins, or nanogenerators, enabling tissue-comparable stretchability is a main roadblock due to the intrinsic rigidity and hardness of the crystalline phase. Here, an amino acid-based piezoelectric biocrystal thin film that offers tissue-compatible omnidirectional stretchability with unimpaired piezoelectricity is reported. The stretchability was enabled by a truss-like microstructure that was self-assembled under controlled molecule-solvent interaction and interface tension. Through the open and close of truss meshes, this large scale biocrystal microstructure was able to endure up to 40% tensile strain along different directions while retained both structural integrity and piezoelectric performance. Built on this structure, a tissue-compatible stretchable piezoelectric nanogenerator was developed, which could conform to various tissue surfaces, and exhibited stable functions under multidimensional large strains. In this work, we presented a promising solution that integrates piezoelectricity, stretchability and biocompatibility in one material system, a critical step toward tissue-compatible biomedical devices.

Fig. 1. Synthesis and growth mechanism of piezoelectric DL-alanine biocrystal network.

a Large-area optical microscope image of a DL-alanine truss-like network. Inset is a digital photograph of the network grown on a Si wafer substrate. b AFM topography images of a single DL-alanine MF (top) and a zoomed-in image showing the fibrous surface feature (bottom). c SEM image of an intersection area including both bifurcation and merging features of the MF network. d AFM topography image of a bifurcation region. Right-panel compares the height profiles of two MFs at the bifurcated region (top) and after bifurcation (bottom) as marked by the dashed lines in the AFM image. e XRD spectra of as-prepared truss-like DL-alanine MF network (red) and DL-alanine raw powders (black). f 2D XRD of DL-alanine network (top) and DL-alanine raw powders (bottom). g Schematics illustrating the branching process of DL-alanine MFs driven by solvent-molecular interaction and surface tension. L alanine and D alanine molecules are indicated by red arrows. Bottom left inset: optical microscope image showing the small contact angle at the DL-alanine MF and water interface. Bottom right inset: SEM image showing a MF with a developing branch.

Fig. 2. Stretchability of DL-alanine crystalline network.

a Schematics of a free-standing stretchable DL-alanine network film supported by PDMS (polydimethylsiloxane) elastomer. Top: DL-alanine network grown on a PDMS film. Middle: a truss-like network of package materials showing the principle of structure-enabled stretchability. Bottom inset: Schematic illustrating the tensile strains resulted from the opening and narrowing of truss units. b A DL-alanine network at 0% (left) and 40% (right) transverse tensile strain. Bottom enlarged images reveal a typical joint angle increase. c A DL-alanine network at 0% (left) and 20% (right) longitudinal tensile strain. Bottom enlarged images reveal a typical joint angle decrease. d Stress-strain curves of a PDMS-supported DL-alanine network and a PDMS film under transverse (top) and longitudinal (bottom) tensile strains. e FEA simulations of strain distribution on simplified MF networks under 40% transverse (top) and 20% longitudinal (bottom) tensile strains. Right insets are zoomed-in views showing strain distributions at intersections. f Schematics (i), simulations (ii), and SEM observations (iii) of releasing built-up strain energy by breaking hydrogen bonding between two joining MFs. L alanine and D alanine molecules are indicated by red arrows.

Fig. 3. Piezoelectricity and biocompatibility of DL-alanine biocrystal network.

a PFM amplitude (top) and phase (bottom) responses of a single MF (left), bifurcated MFs (middle), and a MF cross (right). b Large-area unpolarized second harmonic generation (SHG) image of DL-alanine network showing uniform polarization contrast. c Polarized SHG images of the MF networks under excitation linear polarization orientation varying from 0 degree to 90 degrees. d Current output and in-plane effective piezoelectric coefficient of the MF network under various tensile strains in response to a 1 Hz and 3 N normal tapping force applying at the center. Electrode configuration and effective directions are indicated in the top schematic figure. n = 5 for each group. The error bars represent standard deviations. All data in Fig. 3d are presented as mean ± s.d. e Quantitative cell viability analysis and comparison during a 3-day culturing period. n = 6 for each group. The error bars represent standard deviations. All data in Fig. 3e are presented as mean ± s.d. Colors represent the concentration of DL alanine in culture media solutions. f Fluorescence microscopy images showing the normal morphology evolution of cells cultured in a DM solution with various amounts of DL-alanine dissolved inside during a three-day period.

Fig. 4. Wearable and implantable electromechanical devices with tissue-mimicking stretchability.

a Schematics of an omnidirectionally stretchable piezoelectric NG fabricated by integrating the DL-alanine network with a stretchable Ag NWs electrode. Top inset: a SEM image of percolated Ag NWs. Bottom inset: digital images of a stretchable piezoelectric NG. b The voltage outputs measured from the stretchable NG without strain and with 20% tensile strains along various directions in response to 1 Hz pressure oscillation ( ~ 156 kPa). c A plot of average peak-to-peak voltage outputs as a function of the strain direction. n = 5 for each group. The error bars represent standard deviations. All data in Fig. 4c are presented as mean ± s.d. d The conformal attachment of a stretchable NG on human hand knuckles when fingers were bent and released (left). The stable voltage output under repeating finger bending (middle), and different voltage outputs in respond to different degrees of finger bending (right). e Implantation of a stretchable NG on the top thigh muscle of a swine (left), and its in vivo piezoelectric voltage outputs under different leg movements: (i) swing; (ii) lifting; (iii) rotating.