Predictive design of biomolecular crystal piezoelectricity

Piezoelectricity is the linear relationship between applied stress and induced electrical charge, and conversely, applied electric field and mechanical strain. It is exploited in a wide variety of applications in our daily lives, but through inorganic materials such as aluminium nitrate (AlN), zinc oxide (ZnO), and quartz (SiO2). In consumer electronics the majority of the piezoelectric materials used are still lead-based, even for potential biomedical applications. Here we present biomolecular crystals as the next generation of piezoelectric materials, using simulation-guided experiments on a variety of crystalline organic materials. The majority of organic materials do not have a centre of symmetry, and so are likely to possess properties such as piezoelectricity, ferroelectricity, pyroelectricity, and non-linear optical (NLO) behaviour. Already a wide variety of large biomolecular structures have been shown to be piezoelectric, such as collagen, elastin, deoxyribonucleic acid (DNA), and viruses. In order to understand the piezoelectric response in structures such as these, we focus the majority of this work on amino acids, which are the building blocks of proteins and other biomolecules. We use quantum mechanical models based on density functional theory (DFT), to predict and understand the piezoelectric response of amino acid crystals, and relate this to the observed hierarchy of electromechanical coupling in peptide and protein structures. Based on our predictions, we have experimentally verified an extremely high transverse shear piezoelectricity in the β-glycine crystal polymorph of 178 pC/N. We also report a d25 value of 25 pC/N in hydroxy-L-proline single crystals. A number of amino acid crystal films have been grown and studied, including the racemic amino acid DL-alanine, which is shown here to generate up to 800 mV of electricity under manual compression. DFT calculations, in combination with X-ray diffraction (XRD) and scanning electron microscopy (SEM), have been utilised to deconstruct the effective piezoelectric constants of orthorhombic amino acid films. Finally, we use DFT to reconcile a decades old observation that the centrosymmetric biomineral calcite is weakly piezoelectric. We quantitatively relate DFT predictions to previous second harmonic generation (SHG) measurements, and show that calcite based macrostructures can demonstrate longitudinal piezoelectric constants of up to 0.07 pC/N.