Abbasi A. Gandhi

Ferroelectric Polarization in Nanocrystalline Hydroxyapatite Thin Films on Silicon

Hydroxyapatite nanocrystals in natural form are a major component of bone- a known piezoelectric material. Synthetic hydroxyapatite is widely used in bone grafts and prosthetic pyroelectric coatings as it binds strongly with natural bone. Nanocrystalline synthetic hydroxyapatite films have recently been found to exhibit strong piezoelectricity and pyroelectricity. While a spontaneous polarization in hydroxyapatite has been predicted since 2005, the reversibility of this polarization (i.e. ferroelectricity) requires experimental evidence. Here we use piezoresponse force microscopy to demonstrate that nanocrystalline hydroxyapatite indeed exhibits ferroelectricity: a reversal of polarization under an electrical field. This finding will strengthen investigations on the role of electrical polarization in biomineralization and bone-density related diseases. As hydroxyapatite is one of the most common biocompatible materials, our findings will also stimulate systematic exploration of lead and rare-metal free ferroelectric devices for potential applications in areas as diverse as in vivo and ex vivo energy harvesting, biosensing and electronics.

Charge specific protein placement at submicrometer and nanometer scale by direct modification of surface potential by electron beam.

The understanding and the precise control of protein adsorption is extremely important for the development and optimization of biomaterials. The challenge resides in controlling the different surface properties, such as surface chemistry, roughness, wettability, or surface charge, independently, as modification of one property generally affects the other. We demonstrate the creation of electrically modified patterns on hydroxyapatite by using scanning electron beam to tailor the spatial regulation of protein adsorption via electrostatic interactions without affecting other surface properties of the material. We show that domains, presenting modulated surface potential, can be created to precisely promote or reduce protein adsorption.

Electrical properties of hydroxyapatite

Abstract Despite being one of the mostly studied biomaterials for orthopedic, dental, protein purification and stem cell applications, electrical properties of hydroxyapatite has received only limited attention. Since the prediction in 2005 of the possibility of piezo and pyroelectricity in hydroxyapatite several theoretical and experimental works in this field may lead to new understandings of electrical behaviors of calcified tissues in vertebrates. Also, the ability of creating discrete electrostatic domains on nanocrystalline films of hydroxyapatite will open the possibility of understanding how surface charge influences biological interactions. The outlook for future endeavours in this field will be discussed.

Biomedical Sensing with Hydroxyapatite Ceramics in GHz Frequency Range

Hydroxyapatite (HA) is a leading biocompatible material extensively used for bone implants as a porous ceramic graft and as a bioactive coating. Electrical characteristics of HA can be employed in implantable devices for real-time in vivo pressure sensor applications such as in knee or hip prosthesis. In particular, high piezo and pyroelectricity of HA, its polarisation by electron beam and selective adsorption of proteins on polarised domains indicate the potential for real-time biosensing applications of HA. For this purpose, a comprehensive understanding of the dielectric behaviour of different forms of HA over a frequency range relevant for biomedical sensing is critical. Such information for HA, especially its frequency dependent dielectric behaviour over the GHz range, is rare. To this end, we report on novel investigations of properties of HA in powder and film forms in the GHz frequency range.

Directly created electrostatic micro-domains on hydroxyapatite: probing with a Kelvin Force probe and a protein

Micro-domains of modified surface potential (SP) were created on hydroxyapatite films by direct patterning by mid-energy focused electron beam, typically available as a microprobe of Scanning Electron Microscopes. The SP distribution of these patterns has been studied on sub-micrometer scale by the Kelvin Probe Force Microscopy method as well as lysozyme adsorption. Since the lysozyme is positively charged at physiological pH, it allows us to track positively and negatively charged areas of the SP patterns. Distribution of the adsorbed proteins over the domains was in good agreement with the observed SP patterns.

A complementary contribution to piezoelectricity from bone constituents

In the light of the recent theoretic predictions and experimental evidences of piezoelectricity in synthetic hydroxyapatite, the analogue of the mineral component in bone, this study revisits the question of piezoelectricity in the two major constituents of bone: apatite and collagen. Structural and electromechanical properties have been studied at high lateral resolution on compact bovine femur, which has further been extracted to obtain the organic matrix (collagen fibres) and the inorganic matrix (apatite nanocrystals). X-ray data analyses by Rietveld method indicates the presence of piezoelectric phase in bone apatite. While piezoresponse has been detected in bone collagen and apatite, the response in bone apatite is still inconclusive.

The direct piezoelectric effect in the globular protein lysozyme

Here, we present experimental evidence of the direct piezoelectric effect in the globular protein, lysozyme. Piezoelectric materials are employed in many actuating and sensing applications because they can convert mechanical energy into electrical energy and vice versa. Although originally studied in inorganic materials, several biological materials including amino acids and bone, also exhibit piezoelectricity. The exact mechanisms supporting biological piezoelectricity are not known, nor is it known whether biological piezoelectricity conforms strictly to the criteria of classical piezoelectricity. The observation of piezoelectricity in protein crystals presented here links biological piezoelectricity with the classical theory of piezoelectricity. We quantify the direct piezoelectric effect in monoclinic and tetragonal aggregate films of lysozyme using conventional techniques based on the Berlincourt Method. The largest piezoelectric effect measured in a crystalline aggregate film of lysozyme was approxi...

Pyroelectric, piezoelectric and photoeffects in hydroxyapatite thin films on silicon

Tofail et al.3 have found that the dipole of HA is the hydroxyl (OH) ion, which lies along the crystallographic c-axis within the tunnel formed by phosphate (PO 4 ) tetrahedra. In adjacent tunnels, the OH ions could point in a parallel direction or in an anti-parallel one. We believe that the high-temperature calcination crystallized the HA film on silicon and converted most of the OH pairs to a parallel configuration. Thus the HA developed a domain structure with randomly oriented dipoles. Upon cooling, sufficient domains reoriented in polarity so as to result in a net polar structure. This is due to the crystalline character of the silicon substrate and texturing. The photocurrent is caused by the silicon alone.

Piezoelectricity of bone from a new perspective

In 1957, Eiichi Fukada and Iwao Yasuda [1] related to piezoelectricity the empirical evidence of stress generated electrical potential in bone. This discovery, together with Yasudas earlier observation of a link between an electrical stimulus and bone growth [2], paved the way to many curious studies in the three decades that followed. For example, Bassett and Becker [3] conjectured that electrical potentials might be the basic link in the clinically observed adaptive response in bone and Shamos and Lavine [4] have explained the importance of physiological functions of such electrical potentials in bone remodelling. Researchers in the past five decades are content that bone piezoelectricity originates from collagen fibre, the main organic constituent of bone. Collagen has also been found to be piezoelectric macroscopically, and very recently, microscopically [5].

Converse piezoelectricity and ferroelectricity in crystals of lysozyme protein revealed by piezoresponse force microscopy

ABSTRACT This work investigates the converse piezoelectric effect in crystals of the protein lysozyme using Piezoresponse Force Microscopy (PFM) in contact and Hybrid modes. The mechanical properties of lysozyme crystals were mapped at the surface by means of Hybrid mode. In addition, ferroelectric loops were measured by the switching-spectroscopy PFM method (SS-PFM). We explore these findings using crystallographic principles and propose that the presence of defects within the crystal may lower the symmetry of lysozyme to a polar one. Our findings point towards the potential of exploiting lysozyme and other proteins in technical applications, especially those in which biocompatibility is critical.