The piezoelectric anisotropy (i.e., the ratio between d 15 and d 33 ) is unknown for PNTs, but for a vast majority of ferroelectric materials, it varies between 1.3 (PZT) 29 and 10⫺12 (LNO). 26 It means that the expected value for the longitudinal piezocoefficient for PNTs (not accessible by our measurements) is between ⬇5⫺6 (as for LNO) and ⬇50 pm/V (as for PZT). In order to further prove the piezo- electric nature of the signal, we measured the ac voltage dependences of deformation for two oppositely oriented tubes (Figure 4a). As expected, two straight lines were ob- served without any nonlinearity or irreversibility even at a high ac voltage of 16 V (see also Figure S3 of Support- ing Information). It signifies that the observed piezore- sponse is very stable and PNTs can be driven under high excitation level. There was no visible degradation (within the measurement accuracy) of the topography after driv- ing during prolonged time (tens of minutes