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Review Article

Flexoelectricity: A Perspective on an Unusual Electromechanical Coupling

[+] Author and Article Information
Sana Krichen, Pradeep Sharma

Department of Mechanical Engineering,
University of Houston,
Houston, TX 77204

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received December 25, 2015; final manuscript received December 28, 2015; published online January 20, 2016. Editor: Yonggang Huang.

J. Appl. Mech 83(3), 030801 (Jan 20, 2016) (5 pages) Paper No: JAM-15-1701; doi: 10.1115/1.4032378 History: Received December 25, 2015; Revised December 28, 2015

The ability of certain materials to convert electrical stimuli into mechanical deformation, and vice versa, is a prized property. Not surprisingly, applications of such so-called piezoelectric materials are broad—ranging from energy harvesting to self-powered sensors. In this perspective, written in the form of question-answers, we highlight a relatively understudied electromechanical coupling called flexoelectricity that appears to have tantalizing implications in topics ranging from biophysics to the design of next-generation multifunctional nanomaterials.

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Figures

Grahic Jump Location
Fig. 1

Illustration of induced polarization due to nonuniform deformation of a centrosymmetric (nonpiezoelectric) material

Grahic Jump Location
Fig. 2

Flexoelectricity in membranes (a) Bending of a (dielectric) graphene nanoribbon: bending deformation leads to symmetry breaking of the electron distribution at each atomic site leading to the development of a dipole moment normal to the ribbon plane. (b) Bending of a lipid bilayer membrane: Due to bending, the asymmetry in both the charge and dipole densities in the upper and lower layers causes the normal polarization in the bilayer membrane. (Reproduced from Deng et al. [26] with permission from Elsevier.)

Grahic Jump Location
Fig. 3

The first figure depicts a nonpiezoelectric 2D sheet with circular pores. Under uniform stretching, strain gradients develop in the vicinity of the holes, and therefore, the local polarization due to flexoelectricity is nonzero; however, the net or average polarization remains zero, and thus, overall there is no emergent piezoelectric response. The second figure shows the same sheet with triangular pores. In this case, again, locally, in the vicinity of the triangular holes, polarization develops. Unlike the previous case, however, there also exists now a net nonzero polarization, and thus, this hypothetical material with triangular holes exhibits an apparent piezoelectricity even though the native material itself is nonpiezoelectric. (Reproduced from Ahmadpoor and Sharma [9] with permission from the National Center for Nanoscience and Technology (NCNST) and The Royal Society of Chemistry.)

Grahic Jump Location
Fig. 4

Graphene nitride nanosheet, riddled by triangular holes, was experimentally and computationally shown to exhibit an apparent piezoelectric response (Reprinted with permission from Ahmadpoor and Sharma [9] with permission from the National Center for Nanoscience and Technology (NCNST) and The Royal Society of Chemistry.)

Grahic Jump Location
Fig. 6

Hair bundles consist of several stereocilia that are connected by thin fibers called tip links and organized in rows of decreasing height. The axes of hair bundles point away from the center of the cochlea. Mechanosensitive ion channels are located within the wall of the stereocilia near the top and tethered to adjacent stereocilia by tip link tension. Bending of the hair bundle toward the tallest row imposes tip link tension on channels in the shorter neighbor causing them to open and make the cellular inner environment more electrically positive. Similarly, bending the bundle in the opposite direction closes the channel, causing the cell to become more negative. During these processes, a voltage difference emerges across the thickness of the stereocilia membrane, and due to the flexoelectric response of the cellular membrane, the radius of the stereocilia changes. Accordingly, the height of the stereocilia increases (or decreases) to maintain the fixed volume. Caption quoted from the text of Ahmadpoor and Sharma [9].

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