The future is small: nanotech meets biotech

The convergence of nanotechnology with biotechnology will be one of the hot topics of AusBiotech 2006. Dr John Kapeleris takes a look at recent developments in nanotechnology and how they may be put to use in the biotech world.

Many concepts in nanotechnology are derived from biological systems that can produce components from genetic code and then self-assemble into larger structures. Examples include viruses, ribosomes and DNA molecules.

Some of the key developments in nanotechnology are being further developed for potential applications in biotechnology, including anti-viral reagents, anti-cancer targets, diagnostic tests and drug delivery systems. They also have applications in material science, computing and nanoelectronics.

Biotechnology has been recognised as the driving force of a new wave of technological developments. On the edge of the latest technological frontier, nanotechnology is also emerging. While biotechnology largely involves the application of biological knowledge and techniques to develop products and services, it is interconnected with nanotechnology.

The concept of nanotechnology was first proposed by Richard Feynman in his landmark speech in 1959 titled "There's Plenty of Room at the Bottom". Feynman talked about miniaturisation in general and concluded that there was no reason why the miniaturisation of the manufacturing process couldn't be taken down to the atomic scale.

Nanotechnology was later popularised by Eric Drexler in his book Engines of Creation. In Drexler's vision, nanomachines would cruise through the body to fight viruses; nanobots would circulate through the bloodstream of a person repairing cells and destroying cancer cells before they form a tumour; and we would have the ability to patch wounded nerves and muscles in paraplegics and quadriplegics.

How much of the vision is real or not is now becoming clear. The long-term scope of nanotechnology staggers the imagination. The ability to build self-replicating molecular robots or "assemblers" could in turn create or manufacture anything out of spare molecules or atoms.

Molecular engineering

Molecular engineering, the ability to work at the molecular level, creating structures one atom at a time, was described by Drexler in 1981. This implies building matter, atom by atom from the bottom up, unlike traditional manufacturing systems that construct material from the top down.

Biochemical systems are the closest example to molecular engineering since proteins and other compounds are produced at the molecular level rather than built down from the macroscopic level. Gene synthesis and recombinant DNA technology can re-direct the ribosomes of bacterial and eukaryotic cells to produce novel proteins.

At present, this technology is limited to synthesising small proteins, which are components of larger molecular structures. Predicting the three dimensional conformation of a natural protein from its amino acid sequence is still a difficult task.

With the invention by IBM in the 1980s of the scanning tunneling microscope (STM) and the atomic force microscope (AFM), came the tools to observe and manipulate individual atoms. These microscopes use a probe composed of a single atom to "read" the surface of material.

The STM measures a quantum effect called tunneling. The metal probe of an STM carries a tiny charge and when it comes within 1nm of a conducting surface, the STM measures electrons that tunnel between the nanoscale spaces to map the sample.

Scanning probe microscopes (SPMs) have helped to revolutionise nanoscience but are only capable of manipulating atoms on a two-dimensional scale. In the future three-dimensional manipulation will be required to fully exploit nanotechnology applications.

Biological nanodevices

Nanotechnology is not a new concept in nature. Viruses are an example of naturally occurring nanomachines. Viruses convert human or bacterial cells into factories that serve to produce viral components at the expense of the cell's own metabolic functions. They then assemble and package these components into new viruses that move on to infect other cells.

The most studied virus from a nanomachine perspective has been the bacteriophage T4. The T4 bacteriophage consists of an elongated icosahedral head that contains the virus DNA and a tail with short and tall fibres (or leg-type structures) and a hexagonal baseplate.

A recent study revealed the mechanism of the T4 virus binding to the surface of its host (an E. coli cell), puncturing the surface of the cell wall with a syringe-like tube and injecting its DNA into the host.

Further analysis of the T4 needle-like syringe reveals a structure that may be useful as a probe in an atomic force microscope for future applications in nanotechnology.

Carbon nanotubes

A key focus of research in nanotechnology has involved carbon nanotubes. These are nanoscale tubes only a few carbon molecules thick and are created from a thin sheet of graphite that is cut and rolled into a tube. They are much stronger than steel, conduct electricity as effectively as copper and have the diameter of a DNA molecule.

The immediate application for carbon nanotubes has been in composite materials where their addition improves electrical conductivity and provides increased strength. In the future, carbon nanotubes are predicted to be the wires and building blocks for nanoengineered applications.

When placed in a living entity, carbon nanotubes continue to display their unique properties. They can conduct electricity in a similar fashion to copper wires that transfer the flow of electrons, unlike biological solutions that use ion exchange.

In April 2001 researchers at IBM announced that they had created the world's first array of transistors out of carbon nanotubes. The transistor nanotubes used in the array measured 10 atoms wide, were considerably smaller than most silicon-based transistors and were a thousand times stronger than steel.

Buckminsterfullerenes

Buckminsterfullerenes are soccer-ball-shaped molecules that are made up of 60 carbon atoms. They have fascinated and frustrated scientists since their discovery in 1985 because they have been a challenge to prepare and handle.

Practical applications for these structures still remain elusive. Fullerenes have the ability to stabilise reactive species such as metal atoms inside their cage. This provides an opportunity to explore fullerenes as potential contrast-enhancing agents for magnetic resonance imaging (MRI).

Contrast agents enhance the quality of MRI aiding in the detection and diagnosis of injuries or abnormalities. Because the metallic ions used in MRI are toxic they are usually injected into the body as organic chelates.

Fullerenes may be able to make these metallic ions even safer. By adding up to three metal atoms into each fullerene, this has the ability to increase the potency of MRI agent. Fullerenes can be made soluble by attaching hydroxyl groups or dendrimers to the outer surface of the cage.

A fullerene complex with dendrimers attached has been discovered to find its way to the active site of reverse transcriptase. This enzyme is critical to the functioning of the HIV virus and this discovery offers the use of fullerenes as potential anti-HIV drugs.

Quantum nanodots

Semiconductor quantum dots measure between five and 10 nanometres across and consist of three components. The core is made of paired clusters of atoms such as cadmium and selenium. Even though quantum dots are made up of a cluster of atoms they behave like one single gigantic atom.

These central clusters are surrounded by a shell of an inorganic substance. The outer component consists of a coating of an organic surface that allows the attachment of proteins or DNA molecules. When stimulated by ultraviolet light, quantum dots release light of a specific colour. By varying the number of atoms in the core (i.e. changing the size), quantum dots can be made to emit light of different colours. The luminescent dots have hydrophobic surfaces and therefore can be used in aqueous solutions.

When attached to DNA, proteins and antibodies quantum dots can be used as biological tags to locate target cells or molecules. They can be used to replace traditional organic dyes that tend to fade over time. In addition, each different organic dye must be illuminated by a specific wavelength of light making it difficult to monitor different molecules simultaneously.

It is possible to track up to 10 different quantum dot-tagged molecules simultaneously under the microscope. Quantum dots, therefore, can be used in diagnostic assays as markers for specific biological molecules. The different sized quantum dots producing different colours makes them suitable for multianalyte testing.

Dendrimers

Dendrimers are highly branched mono-disperse polymers forming a tree-like structure that originates from a central core of ammonia or pentaerythritol. The branching units form highly packed surface structures and internal cavities and channels. These characteristics of dendrimers play an important role in life sciences - both in drug delivery and in the diagnosis of infectious diseases.

The unique characteristics of dendrimers result in true biomimicry of proteins. When they enter the body dendrimers are not degraded like proteins and do not elicit an immune response. This makes them suitable candidates as carriers or drug delivery systems for small therapeutic molecules to specific disease sites in the human body.

Small toxic molecules can be placed in the internal cavities where the dense surface structures provide protection to the external environment. Targeting groups, such as specific antibodies, can be attached to active sites on the surface of dendrimers.

In addition to active sites, dendrimers also contain surface modifying groups. When the dendrimer reaches the specific target site, the antibody binds to the target (either a cancer cell or a microorganism), causing surface modifying groups to break the dendrimer apart releasing the drug of interest at the intended site.

Dendrimers can also be used as diagnostic tools for the replacement of conjugated microparticles. By linking dendrimers to antibodies they can be used as carriers for different detector ligands. The surface area of the dendrimer nanoparticles results in an increased amplification of the detector ligands thereby providing improved sensitivity in diagnostic applications.

Implications for the future

Living systems are governed by the behaviour of molecules at the nanoscale level. Multidisciplinary convergence between chemistry, physics and biology is stimulating new applications in biotechnology.

New nanostructured materials such as dendrimers and fullerenes able to mimic biological systems will provide new approaches to drug delivery and the further development of therapeutic drugs that target specific diseases.

Nanotechnology will also provide tools for analysing cells and cellular components. New analytical devices will be capable of probing the inconsistencies in cells and tissues, locate the abnormalities and deliver appropriate therapy thereby facilitating extended life span in humans.

Quantum dots will play a role in the visualisation of tagged molecules and in new diagnostic assays. In vivo nanodevices will be implanted to provide continuous monitoring of biological functions and to proactively deliver controlled release doses of therapeutics necessary to treat disease.

The penultimate vision is the release of nanobots into humans where they continuously monitor and repair damaged cells and tissues or fight foreign intruders. The development of new rejection-resistant implants, tissues and organs will ultimately extend the lifespan of humans and slow down the aging process. Nanotechnology will ultimately shift the focus of patient care from disease treatment to proactive early detection and prevention.