Cornelius J. Pings Chair in Biomedical Sciences, Professor of Ophthalmology, Biomedical Engineering, and Integrative Anatomical Sciences.
Director of the USC Ginsburg Institute for Biomedical Therapeutics, and Co-Director of the USC Roski Eye Institute
Department of Physics, Coventry CV4 7AL, UK
Advanced Retinal Implants for Ophthalmology
Abiotic- Biotic interfaces in Ophthalmology have played and will in the future play an important role in not only restoring vision but hopefully also preventing vision loss. These interfaces can be wearable or implantable and are wirelessly connected. They can be diagnostic and/or therapeutic and in the future will benefit from artificial intelligence algorithms. This talk will focus mostly on a bioelectronic retinal implant but also briefly describe some other implants for Ophthalmology. Bioelectronic implants are those that are implanted in the eye either epiretinally (ganglion cell side) or subretinally (in between the retina and eye wall). Also, these implants can be situated at the visual cortex. Argus II epiretinal implant is the only FDA and EMA approved medical implant. It has 60 electrodes and both data and power are delivered via inductive coupling. This device is intended to restore useful vision for people suffering from retinitis pigmentosa, a genetic condition that leads to retinal blindness. The most recent results from the Argus II retinal Prosthesis (clinicaltrials.gov NCT00407602). The subjects of the clinical trials implanted with a Second Sight Argus II implant had severe outer retinal degenerations (photoreceptor loss). In the clinical trial, visual function was evaluated by visual function tests presented on an LCD screen, including Square Localization, Direction of Motion, and Grating Visual Acuity. Assessments of functional vision included controlled Orientation and Mobility (O&M) tasks, and the Functional Low-Vision Observer Rated Assessment (FLORA). The talk will cover some of the engineering challenges as well as surgical and clinical learnings. Pixium Prima is a subretinal implant that is in early clinical trials for dry age-related macular degeneration and is a photovoltaic based device and this will also be covered. Visual Cortical implants like the Second Sight ORION and the Utah device which are in early clinical trials will also be discussed. Lastly, some other non-bioelectronic devices such as scaffolds form stem cells which are also in early clinical trials will be discussed.
Ferroelectric Memories At Last
In spite of being one of the first (or perhaps the first) non-volatile semiconducting memory demonstrated almost 70 years ago, ferroelectrics have struggled to compete in the race towards miniaturization and it is only recently that ferroelectric memories can be scaled down sufficiently to be introduced at the industrial scale. The enabler of this success is the family of hafnia-based thin films, until recently a material used in transistors simply as insulating layer, which can be stabilized in a polar state, at sizes as small as a few nanometers. After a period of incredulity, in which multiple proofs of robust switching were collected, the first challenge has been to understand how ferroelectricity is achieved in these materials: What, at first, seemed like a puzzling set of miscellaneous mechanisms (size, doping, strain etc.), is now rationalized as volume changing routes that induce low molar volume, fluorite-like, metastable phases, among which two different polar phases, with orthorhombic and rhombohedral symmetries, have been reported. More recently, the scientific focus has moved to understanding the device behavior, as the properties of the ferroelectric layer strongly depend on the thickness, the electrode configuration and chemistry, as well as the magnitude and duration of the applied electric field pulses, challenging the robustness and reliability of future devices.
Here we present results on two-terminal LSMO/Hf0.5Zr0.5O2/LSMO multiferroic tunnel junctions showing both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), and their four associated resistance states by magnetic and electric field switching. Upon electric field cycling, the TER displays progressive enhancement reaching values as large as 106 %. Simultaneously, sign reversal of the TMR develops allowing electrical control of spin polarization. The epitaxial nature of these heterostrucutres (grown on SrTiO3 substrates) allows for an in-depth structural and microstructural investigation, including atomic resolution imaging in operando TEM and synchrotron experiments with electric field applied in-situ, that have allowed to directly demonstrate the crucial role of oxygen exchange in the switching characteristics in hafnia-based devices.
Multifold Control Of Magnetoelectric States In Multiferroic Nanodot Array
The first-priority application potentials of multiferroic/ferroelectric materials would be associated with the ultra-density data storages, and therefore various approaches along this line become particularly attractive. Recently, interest in ferroelectric/multiferroic topological domain structures is rapidly increasing with findings of a wealth of emerging exotic phenomena and prospect applications not only for future nanoelectronic devices. Certainly, the associated emerging fundamental issues of multiferroic physics and materials science are also attractive in the community. For example, observations of a number of fascinating domain structures in ferroelectric nanostructures have been reported, and additional topology associated with order parameters is discussed. Besides, various types of excitations and dynamic responses in these domain structures are expected. Among all of these emerging phenomena, we are particularly interested in topological domain structures and their emerging functionalities.
In fact, it is still challenging to characterize and manipulate various topological states and their related physical properties. In this presentation, we will address our recent works on manipulation of various ferroelectric topological states, e.g. quadrant vortex domains, central domains (monopole-like polarization texture with polarization pointing toward/from the central core), and fascinating domain wall properties, in epitaxial BiFeO3 (BFO) nanodots / nanoislands under well-controlled and combined preparation conditions. We have also been involved in domain switching and domain wall conductivity in well-prepared BFO nanostructures. These works as a whole package represent a comprehensive step towards understanding of the ferroelectric/multiferroic nanostructures and their application potentials.
Induced Functionalities by Symmetry Breaking
Symmetry lies at the heart of the laws of nature and determines material properties at the fundamental level. We all know that breaking the inversion symmetry is directly mapped into materials properties by inducing a plethora of effects such as dielectric polarisation along with pyro- and ferroelectricity, piezoelectricity, bulk photovoltaic effect, electro-optic effect and second harmonic generation, etc. Material symmetry in chiefly determined by its pristine crystallographic structure, but external stimuli can also lower symmetry or even break the inversion symmetry. A well-known example of such stimulus is the strain gradient that breaks the inversion and induces electric polarisation in any material, including centrosymmetric materials, by the so-called flexo-electric effect.
In this talk, I will focus on inducing the effects associated with inversion symmetry breaking in native centrosymmetric materials. I will show that strain gradients not only induce electric polarisation but also convert any semiconductor in a photovoltaic/photogalvanic generator by the flexo-photovoltaic effect. Similarly, built-in electrical fields within ubiquitous Schottky contacts break the symmetry at the interface inducing piezo- and pyroelectricity with completely different tweaking parameters. I will also show that ferroelectric polarisation breaks locally the symmetry in contiguous materials, especially magnetic oxides, inducing/enhancing effects such as topological Hall effect.
Piezoelectricity: Symmetry Breaking, Disorder, Charge Transport And Multiproperty Coupling
The world of piezoelectric materials looks very different today than it did just 25 years ago: materials based on wurtzite and fluorite structures are today ferroelectric, centrosymmetric materials are considered for piezoelectric applications, monoclinic phases have been recognized in Pm3m perovskites, lead-free materials are said to show a promise to replace PZT, and flexoelectricity has been proposed as a viable alternative to piezoelectricity. These discoveries and perceptions are based on new theoretical approaches, advances in sophisticated characterization techniques, are driven in part by societal pressures, are sometimes revived old ideas, but, above all, are a result of readiness to look beyond the established interpretations. In fact, some of these breakthroughs and advances required “only” scratching of the surface” and looking deeper into the underlaying complexity of the material. The electro-mechanical coupling is always more complex than it looks at the first sight, has multiple origins and disentanglement of the resulting contributions to the properties is in the center of the science and applications of piezoelectric materials.
In that spirit, the focus of this presentation is on a second look at (i) the local atomic symmetry and disorder in oxide perovskites, (ii) nanoscale motion of domains and their mutual interactions in canonical ferroelectrics, (iii) piezoelectric effect in nonferroelectric oxides with fluorite structure and (iv) multicoupling of the electrical, chemical, elastic, thermal and optical processes in organometallic halide perovskites. The electro-mechanical response originating from the long-range and short-range displacements of electrons and atoms will be contrasted and emergence of the apparent giant electrostrictive and piezoelectric effects in some of these materials will be discussed.