Annual Review of Materials Research - Volume 31, 2001

Novel therapeutic strategies can be envisioned based on altering the expression level of target genes involved in cellular processes and disease progression; however, our ability to efficiently manipulate gene expression is limited. Non-viralbased gene therapy provides a relatively safe approach to increase or decrease the expression of a specific gene using DNA or antisense sequences; however, synthetic systems are required to direct plasmids and oligonucleotides to a specific tissue and to enhance cellular uptake and intracellular trafficking. Numerous materials are being developed that interact with DNA to enhance its properties (e.g. stability, charge density) and thus direct its biodistribution and facilitate cellular interactions. The development of synthetic delivery systems to manipulate gene expression efficiently is a powerful tool that will ultimately lead to novel therapeutic strategies for the treatment of numerous disorders.

Hydrogenated amorphous silicon (a-Si:H) exhibits a metastable light-induced degradation of its optoelectronic properties that is called the Staebler-Wronski effect, after its discoverers. This degradation effect is associated with the relatively high diffusion coefficient of hydrogen and the changes in local bonding coordination promoted by hydrogen. Reviewed are the fundamental aspects of the interplay between hydrogen and electronic energy states that form the basis of competing microscopic models for explaining the degradation effect. These models are tested against the latest experimental observations, and material and preparation parameters that reduce the Staebler-Wronski effect are discussed.

All materials intended for application in humans as biomaterials, medical devices, or prostheses undergo tissue responses when implanted into living tissue. This review first describes fundamental aspects of tissue responses to materials, which are commonly described as the tissue response continuum. These actions involve fundamental aspects of tissue responses including injury, inflammatory and wound healing responses, foreign body reactions, and fibrous encapsulation of the biomaterial, medical device, or prosthesis. The second part of this review describes the in vivo evaluation of tissue responses to biomaterials, medical devices, and prostheses to determine intended performance characteristics and safety or biocompatibility considerations. While fundamental aspects of tissue responses to materials are important from research and development perspectives, the in vivo evaluation of tissue responses to these materials is important for performance, safety, and regulatory reasons.

The use of energetic particles (ions and atoms) has become increasingly important in physical vapor deposition techniques. These deposition processes can be divided in two main classes: ion beam-assisted deposition and energetic condensation (or deposition). This review focuses on the latter, i.e. processes in which the actual depositing species have energies that far exceed ordinary thermal energies, namely energies greater than 20 eV. The phenomenology of the effect of these high-energy particles on the growth of thin films is first broadly presented, and then specific examples of film deposition are given. The examples drawn here are of films that have been prepared by metal plasma immersion implantation and deposition. The observed microstructures and functional properties of these films are discussed in terms of processing conditions.

A review of the field of photorefractive liquid crystals is presented. The first reports of photorefractive liquid crystals occurred in 1994, and the performance of these materials has dramatically improved since that time. Liquid crystalline materials have proven to be highly versatile, showing photorefractive character under a wide range of conditions. For example, new composites based on high-molar-mass liquid crystals are now capable of forming volume (Bragg) gratings with high photorefractive gain coefficients of >600 cm⁻¹. Formation times for photorefractive Bragg gratings of 15 ms with applied fields of only 0.1 V/μm have been reported. Low-molar-mass liquid crystals continue to be developed and show their largest photorefractive character in the thin (Raman-Nath) grating regime. Composites of nonmesogenic polymers and liquid crystals are also discussed. The experiments and theoretical work that have been used to characterize these materials are reviewed.

Photoinitiated polymerization and polymer crosslinking are viable strategies for biomaterial synthesis because of the mild temperatures and neutral pH environments in which these reactions typically take place. This review summarizes the relevant theories as well as current status of photoinitiated polymerizations in biomaterials. Photoinitiation, photoinitiated polymerization, and photoinitiators are discussed with consideration toward the biological nature of the intended application. Recent investigations into biomaterials, including hydrogels, biodegradable materials, and hard tissue resorbable scaffolds are presented. Lastly, studies of cell interactions with photoinitiated biomaterials are discussed. The work herein illustrates the potential use of photoinitiated polymerization in the development of novel biomaterials for tissue engineering.

The field of biomaterials has recently been focused on the design of intelligent materials. Toward this goal, materials have been developed that can provide specific bioactive signals to control the biological environment around them during the process of materials integration and wound healing. In addition, materials have been developed that can respond to changes in their environment, such as a change in pH or cell-associated enzymatic activity. In designing such novel biomaterials, researchers have sought not merely to create bio-inert materials, but rather materials that can respond to the cellular environment around them to improve device integration and tissue regeneration.

Continuing increases in the areal density of hard disk drives will be limited by thermal instability of the thin film medium. Patterned media, in which data are stored in an array of single-domain magnetic particles, have been suggested as a means to overcome this limitation and to enable recording densities of up to 150 Gbit cm⁻² (1 Tbit inch⁻²) to be achieved. However, the implementation of patterned media requires fabrication of sub-50-nm features over large areas and the design of recording systems that differ substantially from those used in conventional hard drives. This review describes patterned media, including the fabrication of arrays of small magnetic particles and their magnetic properties, such as domain structure, reversal mechanisms, thermal stability, and interactions. The practical implementation of patterned media recording schemes is assessed.

This review summarizes recent advances and future research trends in the field of phospholipid-based biomaterials. Lipids play an important role in biomineralization and countless other biological processes, and they are receiving increasing attention for the synthesis of new biomimetic biomaterials. Several emerging strategies in biomaterials research take advantage of phospholipids to compartmentalize and/or template chemical reactions via self-assembled structures such as liposomes and tubules. Still others exploit the inherent biocompatibility of phospholipids and phospholipid-mimetic materials for use as novel tissue-contacting biomaterials that mimic biological membranes. In the future, phospholipid-based materials may be increasingly utilized as tools for the manipulation of cell and tissue responses to biomaterials, for controlled drug release, for reconstructive surgery, and as tissue-engineered constructs.

Recently there have been significant advances in understanding the magnetic properties of epitaxial ferrite films that are not found in bulk ferrites. Much effort has been expended on trying to achieve bulk properties in thin films for a wide range of applications. From a fundamental science perspective, epitaxial thin films and heterostructures have provided model systems in which novel phenomena, such as modified super-exchange interactions, nearly ideal exchange coupling, and perpendicular exchange coupling, have been observed. These magnetic phenomena and other anomalous magnetic properties are interesting in their own right and are highlighted here.

Energetic materials are chemical compounds or mixtures that store significant quantities of energy. In this review, we explore recent approaches to property prediction and new material synthesis. We show how the successful design of new energetic materials with tailored properties is becoming a practical reality.

A two-part review of research concerning block copolymer thin films is presented. The first section summarizes experimental and theoretical studies of the fundamental physics of these systems, concentrating upon the forces that govern film morphology. The role of film thickness and surface energetics on the morphology of compositionally symmetric, amorphous diblock copolymer films is emphasized, including considerations of boundary condition symmetry, so-called hybrid structures, and surface chemical expression. Discussions of compositionally asymmetric systems and emerging research areas, e.g., liquid-crystalline and A-B-C triblock systems, are also included. In the second section, technological applications of block copolymer films, e.g., as lithographic masks and photonic materials, are considered. Particular attention is paid to means by which microphase domain order and orientation can be controlled, including exploitation of thickness and surface effects, the application of external fields, and the use of patterned substrates.

Osteoblasts respond to surface topography with altered morphology, proliferation, and differentiation. The effects observed vary with cell culture model and the topographical features of the surface. In general, increased surface roughness is associated with decreased proliferation and increased differentiation. Cell responses to hormones, growth factors, and cytokines are altered as well, as is autocrine production of these factors. The cells interact with the surface via integrin receptors, and their expression is also surface roughness-dependent. Integrin binding to cell attachment proteins activates signal transduction cascades, including those mediated by protein kinase C and phospholipase A2. These signaling pathways are also used by regulatory factors, which results in synergistic responses. Prostaglandins are important mediators of the surface effects, and both constitutive and inducible cyclooxygenase are involved.

Materials research has been applied successfully to the study of archaeological ceramics for the last fifty years. To learn about our history and the human condition is not just to analyze and preserve the objects but also to investigate and understand the knowledge and skills used to produce and use them. Many researchers have probed the limits and methods of such studies, always mindful that a glimpse at ancient reality lies in the details of time and place, context of finds, and experimentally produced data, usually compared with standards that were collected in an equivalent ethnographic setting or that were fabricated in a laboratory in order to elucidate the critical questions in a technology that could be understood in no other way. The basis of most studies of ancient technology has been established as microstructure; composition and firing; methods and sequence of manufacture; differentiation of use; use-wear and post-depositional processes; technological variability that can be interpreted as a pattern of stasis or innovation, which can be related to cultural continuity or change; and interpretation that can involve technology, subsistence trade, organization, and symbolic group- and self-definition.

We summarize developments in the construction of synthetic cells made from polymers, with a particular focus on mimicking the structure and behavior of blood cells. Two basic themes emerge—the use of block copolymers to make polmer vesicles and the functionalization of colloidal or polymeric microspheres with cell-like adhesive properties. Both platforms provide a means for building the complex hierarchy that is characteristic of biological cells, while also incorporating novel and perhaps superior properties of material strength, specific targeting, and controlled release.