Piezoelectric Nanomaterials for Biomedical Applications

Smart Materials, with their ability to change some of their properties in response to an external stimulus or to changes in conditions of their surrounding environment, have gained considerable attention in the biomedical community because of the interest in applications that could be foreseen for them in a multitude of active structures and devices.

A short introduction to Smart Materials is given in this chapter as well as some summary of recent achievements in biomedicine is also given. An overview of the different classes of Smart Materials, with a special emphasis on smart polymers is presented and classification is proposed based on the different chemistry. Biomedical applications of selected Smart Materials are also considered.

Due to their exceptional properties, the PZT–type materials have become the most important piezoelectric materials, having an extremely large area of applications in many fields. Their high conversion factors of 60–70 % makes them the most remarkable materials for ultrasound transducers. There are numerous methods to prepare such materials. Two physical methods are the most usual: the conventional mixed route in which the stoichiometric amounts of oxides are mixed together, followed by calcination to accomplish the solid–state reaction and the mechanochemical synthesis, where the chemical reaction takes place during milling, being activated by the mechanical energy of collisions. No calcination is necessary in this case. The resulted powders are more homogenous both structurally and chemically. Other relevant methods are: coprecipitation, hydrothermal and sol-gel routes. Here the reactions take place in solution, at molecular level, thus producing materials with a high degree of homogeneity. The precipitate product or the gel resulted is subjected to a calcination process at low temperatures and the powders are very homogenous and in the nanometric range (10–200 nm).

In this chapter nanoscale characterization techniques for piezoelectric materials are discussed, with a focus on nanomechanical and electromechanical methods for one-dimensional nanomaterials. One-dimensional nanostructures have been the focus of recent nanoscale research due to their potential application in future nanoelectronics and nanodevices. However, their small size and special geometry impose a challenge especially from experimental point of view and renders their characterization nontrivial. In this chapter, the common methods of nanomechanical and electromechanical characterization of these nanostructures are discussed, with an emphasis on piezoelectric one-dimensional materials. Advantages and limitations of each method are discussed and the relevant literature is presented.

Since fabrication, characterization, and integration into practical devices of nanostructures is unavoidably complex and expensive, accurate models are crucial for designing high performance nanostructures-based devices. Moreover, piezoelectric nanotransducers may have several crucial advantages when compared with the correspondent macro- or micro-devices. For these reasons, after reviewing both piezoelectric constitutive equations and equivalent circuits for piezoelectric transducers, we show how these tools can be applied to analysis and design of practical piezoelectric nanodevices. As an important example, we choose piezoelectric nanogenerators; however, by analyzing this type of devices, we discuss the key general concepts and challenges for modeling piezoelectric nanodevices.

This chapter reviews the fundamental principles and recent development of piezoelectric nanogenerators that convert mechanical energy into electricity at the nanometer scale. Theoretical prediction of the piezoelectric potential output from a single nanowire is discussed first. The semiconductor-piezoelectric coupling effect, enhanced mechanical property, and the nanoscale flexoelectric enhancement are important aspects that make nanowire advantageous for mechanical energy harvesting. Several representative approaches are presented for characterizing the piezoelectric potential on nanowires. The ultrasonic wave-driven nanogenerator and power fiber are reviewed as two important prototypes. The most recent strategies integrating nanowires at macroscopic scale for providing sufficient electric energy for small electronic devices are also summarized and discussed. Challenges and future research opportunities are suggested at the end of this chapter.

Biological structures, at different organization levels (macromolecules, tissues, etc.) present a typical spiral shape. Spirals do not have a center of symmetry and hence almost all biological matter possesses piezoelectricity properties. Thus, the possibility to convert mechanical signals into electric ones, and viceversa, is not only ascribed to minerals or ceramics. In this chapter, after reminding essential elements on piezoelectricity and its connection with material structure, the piezoelectric properties of two typical macromolecular components, cellulose (for plants) and collagen (for animals) are introduced, and their role in biological tissues is described.

Piezoelectricity is one of the common ferroelectric material properties, along with pyroelectricity, optical birefringence phenomena, etc. There has been widespread observation of piezoelectric and ferroelectric phenomena in many biological systems and molecules, and these are referred to as biopiezoelectricity and bioferroelectricity. Investigations have been made of these properties in biological and organic macromolecular systems on the nanoscale, by techniques such as atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). This chapter presents a short overview of the main issues of piezoelectricity and ferroelectricity, and their manifestation in organic and biological objects, materials and molecular systems. As a showcase of novel biopiezomaterials, the investigation of diphenylalanine (FF) peptide nanotubes (PNTs) is described in more detail. FF PNTs present a unique class of self-assembled functional biomaterials, owing to a wide range of useful properties, including nanostructural variability, mechanical rigidity and chemical stability. The discovery of strong piezoactivity and polarization in aromatic dipeptides [ACS Nano 4, 610, 2010] opened up a new perspective for their use as nanoactuators, nanomotors and molecular machines as well for possible biomedical applications.

In this Chapter, recent results about studies of interactions between piezoelectric nanoparticles and living systems will be discussed. As extremely innovative materials, great importance is devoted to the investigations of their stabilisation in physiological environments and to their biocompatibility. Applications as drug carriers and nanovectors will be thereafter described, and special attention will be dedicated to tissue engineering applications. Finally, preliminary results achieved by our group on “wireless” cell stimulation will be approached.

This chapter represents the conclusive part of the book. We present the most recent researches on piezoelectric materials for nanomedicine applications and non-invasive wireless stimulation of tissues and cells. Particular attention is devoted to the ongoing research in our laboratories. Concluding remarks and perspectives are also reported.