We present measurements of the time development of the dielectric displacement and the remanent polaroization in PVDF for poling times ranging from 1 ~s to 1000 s and poling fields between 0.8 and 2.0 MV /cm. For longer times (0.1 to 1000 s) we determined also the time dependence of the polarization distribution across the film thickness. After applying a steep rectangular HV pulse, the sample is shorted to .aero voltage. The remanent polarization under short-circuit conditions is compared with the maximum dielectric displacement under the external poling field. We observed a significant time delay of the build-up of the remanent polarization as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal we observed for shorter poling times of up to 200 IlS a

Objective. Brain functions such as perception, motor control, learning, and memory arise from the coordinated activity of neuronal assemblies distributed across multiple brain regions. While major progress has been made in understanding the function of individual neurons, circuit interactions remain poorly understood. A fundamental obstacle to deciphering circuit interactions is the limited availability of research tools to observe and manipulate the activity of large, distributed neuronal populations in humans. Here we describe the development, validation, and dissemination of flexible, high-resolution, thin-film (TF) electrodes for recording neural activity in animals and humans. Approach. We leveraged standard flexible printed-circuit manufacturing processes to build high-resolution TF electrode arrays. We used biocompatible materials to form the substrate (liquid crystal polymer; LCP), metals (Au, PtIr, and Pd), molding (medical-grade silicone), and 3D-printed housing (nylon). We designed a custom, miniaturized, digitizing headstage to reduce the number of cables required to connect to the acquisition system and reduce the distance between the electrodes and the amplifiers. A custom mechanical system enabled the electrodes and headstages to be pre-assembled prior to sterilization, minimizing the setup time required in the operating room. PtIr electrode coatings lowered impedance and enabled stimulation. High-volume, commercial manufacturing enables cost-effective production of LCP-TF electrodes in large quantities. Main Results. Our LCP-TF arrays achieve 25× higher electrode density, 20× higher channel count, and 11× reduced stiffness than conventional clinical electrodes. We validated our LCP-TF electrodes in multiple human intraoperative recording sessions and have disseminated this technology to >10 research groups. Using these arrays, we have observed high-frequency neural activity with sub-millimeter resolution. Significance. Our LCP-TF electrodes will advance human neuroscience research and improve clinical care by enabling broad access to transformative, high-resolution electrode arrays.

The polymer PVDF and ita copolymers with TrFE show a strong piesoelectric effect ane r poling in high e lectric ficldl ( 100 MV / m). In order to . tudy the dynamiu of the poling proce •• we have applied HV impubes of deftnite duration to the polymer. The dielectrie di.placement during the HV impul.e i. r e corded. Aner tbe impul.e tbe remanent pollll'iBation under . bo rt eircuit condit ion i. mea.ured. Thu. it i. po .. ible to obtain the minimum poling time dependent on the applied field strength nece .. ary to I tabilise the remanent polarillation. By comparing the time development of t he dielectric di.placement with the corre.ponding r emanent polaris.tion we find. time delay between the orientation o f the dipole. and their .tabili.ation. Therefore we conclude that the procell o f orientation of the dipole. ihelfi. not .uffteient to le ad to a r emane nt polarization. An additional interaction between trapped cblll'g"" and the orie nted dipoles can explain the . tability of the remanent pola risation and the ob.erved time d e lay. INTRODUCTI ON KAWAI di aeovered piuoelectricity in the polymer of polyvinylidenefluoride (PV OF) in 1969. Later it wu shown that the copoly m~rl of vi nyl idenefluoride (V OF) with trifluoroethylene (TrFE) alto ex hi bit piezoelect rici ty TlFE) the cry.taUite di poles can be permanently aligned. Unpoled filma .how no piesoeleetricity. The pieloeleetrie reapon ... can be deaeribed mainly by a ehange in the num· ber density or aligned crYltallite dipole moment. under strain. While PVDF crYl tallize8 in the unpolar o·phMe, the P (VOF / TrFE) copolymer cry. taUilu directly in the ,8-phase, if the TrFE portion i. greater than 15 mole% [6J. o·PVOF it Iran.rormed to the polar ,8·ph3le by stretch· ing the fi lma.[21· By poling with lower field strength spatial inhomoge_ neOUI polarilation distributions are found in PVDF. The invest igation of t he development of these inhomogeneous distributions during t he poling process gives strong evi_ dence that injee~ion and migration of lpace charges dete rmine the polarisation distribution [9].Another point is the surprising stability of the remanent polarisation in both polymers al room temperature for many year •. In addi tion, the cocrdtive field Ilrength is th ree orders of magnitude latger than in other ferroelt<:triel [3]. In Figure 2 the expected behavior il.ho",n for the case of polarisation rever .... l of pre polarised filma. At t.) the di· polel begin to invert under t he field and the displacement I t arts to grow as in the ease of unpoled films (compare Figure 1). At t\ all dipole. are inverted under the field and the displlLCement attain. itl maximum. For long er poling time. it remain. conltant. Under .hort circuit the remanent polari1ation in the original dir ection fi rst decreuel to ,eTO li nd then growl in the new, opposite direction. The remanent polatilation in the opposi te direetion latu ratu again o.t h.Since the values of the remanent polarization are the lame (but in oPP01lite direction) before the application ...

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The spatial distribution of the piezoelectric coefficient in polarized PVDF has been investigated by several authors using different methods / 1-6/. In addition also the time deve lopment of the polarization distribution in PVDF under external electfic fields was studied for a variety of commercially available PVDF films from different suppliers using the piezoelectric pressure step (PPS) technique / 7 J. For a field strength ranging from 0.5 to 1 MV fem, samples from different suppliers developped either i) a centra! polarization zone with no piezoeffect in the boundary regions close to the fi lm surface or Ii) a polarization maximum close to the positive charg ing electrode (anode) without polarization from about the centre of the film to the negative electrode. Since sample inhomogeneities can be excluded , the development of polarization zones are to be explained by charge injection and trapping / 3,9 f.The different behaviour i) and ii) in principle must be attributed either to possible differences in the general chem ical composition of the films of different suppliers or to differences in the crystall ite structu re. So far it was found / 11 / that the central polarization zone i) appeared in films with a finite content o f /3-form crystallites, whereas the polarization maximum at the positive electrode ii) appeared for pure a-material. In order to check the possible influence of the crystall ite structure independently from the' chemical sample composition, we investigated the po larizatron distributions for the same material as received first with dominant a-crystallite content. then after biaxially stretching in order to increase the ,8-crvstallite content and flnaiiy after anneal ing again in order to reduce the /3-crystallite content. Experimental TechniqueThe spatial distribution of the polarization in PVDF ~i lms was measured at room temperature with the PPS-method / 3.7 f. In order to investigate the time development under external fields a thin insulating PET -film covering an evaporated aluminium electrode was inserted between sample and the measuring electrode. This additional polymer ads as a broadband coupling capacitor to the measured displacement current. It does not influence the shape of the signal. The crystallite phase composition of the PVOF films was determined by IR absorption. The ratio between the relative absorption at 530 em-1 (a-crystallites) and 510 cm-1 t,B -crystalli1es) is taken as a standard for the a/,B-phase content (according to /8/ ).