From the poem “They” by R.S. Thomas:
The new explorers don’t go
anywhere and what they discover
we can’t see. But they change our lives.
They interpret absence
as presence, measuring it by the movement
of its neighbours. Their world is
an immense place: deep down is as distant
as far out, but arrived at
in no time. These are the new
linguists, exchanging across closed
borders the currency of their symbols….
All the best for the New Year to all my readers.
Gene therapy holds out the promise of correcting a number of diseases whose origin lies in the deficiency of a particular gene – given our growing knowledge of the human genome, and our ability to synthesise arbitrary sequences of DNA, one might think that the introduction of new genetic material into cells to remedy the effects of abnormal genes would be straightforward. This isn’t so. DNA is a relatively delicate molecule, and organisms have evolved efficient mechanisms for finding and eliminating foreign DNA. Viruses, on the other hand, whose entire modus operandi is to introduce foreign nucleic acids into cells, have evolved effective ways of packaging their payloads of DNA or RNA into cells. One approach to gene therapy co-opts viruses to deliver the new genetic material, though this sometimes has unpredicted and undesirable side-effects. So an effective, non-viral method of wrapping up DNA, introducing it into target cells and releasing it would be very desirable. My colleagues at Sheffield University, led by Beppe Battaglia, have recently demonstrated an effective and elegant way of introducing DNA into cells, in work recently reported in the journal Advanced Materials (subscription required for full paper).
The technique is based on the use of polymersomes, which I’ve described here before. Polymersomes are bags formed when detergent-like polymer molecules self-assemble to form a membrane which folds round on itself to form a closed surface. They are analogous to the cell membranes of biology, which are formed from soap-like molecules called phospholipids, and the liposomes that can be made in the laboratory from the same materials. Liposomes are used to wrap up and deliver molecules in some commercial applications already, including some drug delivery systems and in some expensive cosmetics. They’ve also been used in the laboratory to deliver DNA into cells, though they aren’t ideal for this purpose, as they aren’t very robust. Polymersomes allow one a great deal more flexibility in designing polymersomes with the properties one needs, and this flexibility is exploited to the full in Battaglia’s experiments.
To make a polymersome, one needs a block copolymer – a polymer with two or three chemically distinct sections joined together. One of these blocks needs to be hydrophobic, and one needs to be hydrophilic. The block copolymers used here, developed and synthesised in the group of Sheffield chemist Steve Armes, have two very nice features. The hydrophilic section is composed of poly(2-(methacryloyloxy)ethyl phosphorylcholine) – this is a synthetic polymer that presents the same chemistry to the adjoining solution as a naturally occurring phospholipid in a cell membrane. This means that polymersomes made from this material are able to circulate undetected within the body for longer than other water soluble polymers. The hydrophobic block is poly(2-(diisopropylamino)ethyl methacrylate). This is a weak base, so it has the property that its state of ionisation depends on the acidity of the solution. In a basic solution, it is un-ionized, and in this state it is strongly hydrophobic, while in an acidic solution it becomes charged, and in this state it is much more soluble in water. This means that polymersomes made from this material will be stable in neutral or basic conditions, but will fall apart in acid. Conversely, if one has the polymers in an acidic solution, together with the DNA one wants to deliver, and then neutralises the solution, polymersomes will spontaneously form, encapsulating the DNA.
The way these polymersomes work to introduce DNA into cells is sketched in the diagram below. On encountering a cell, the polymersome triggers the process of endocytosis, whereby the cell engulfs the polymersome in a little piece of cell membrane that is pinched off inside the cell. It turns out that the solution inside these endosomes is significantly more acidic than the surroundings, and this triggers the polymersome to fall apart, releasing its DNA. This, in turn, generates an osmotic pressure sufficient to burst open the endosome, releasing the DNA into the cell interior, where it is free to make its way to the nucleus.
The test of the theory is to see whether one can introduce a section of DNA into a cell and then demonstrate how effectively the corresponding gene is expressed. The DNA used in these experiments was the gene that codes for a protein that fluoresces – the famous green fluorescent protein, GFP, originally obtained from certain jelly-fish – making it easy to detect whether the protein coded for by the introduced gene has actually been made. In experiments using cultured human skin cells, the fraction of cells in which the new gene was introduced was very high, while few toxic effects were observed, in contrast to a control experiment using an existing, commercially available gene delivery system, which was both less effective at introducing genes and actually killed a significant fraction of the cells.
A switchable polymersome as a vehicle for gene delivery. Beppe Battaglia, University of Sheffield.
My book, Soft Machines: nanotechnology and life, has now been released in the UK as a paperback, with a price of £9.99. It should be available in the USA early in the new year. It’s available from from Amazon UK here
Having an opportunity to make corrections, I re-read the book in the summer. One very embarrassing numerical error needed correcting, and anything to do with the dimensions of semiconductor processes needed to be updated to account for four more years of Moore’s law. But in general I think what I wrote has stood the test of time pretty well.
I’m on my way back from India, where I’ve been at the conference Bangalore Nano 07. The enthusiasm for nanotechnology in India has been well publicised; it’s traditional to bracket the country with China as two rising science powers that see nano as an area in which they can compete on equal terms with the USA, Europe and Japan. So it was great to get an opportunity to see for myself something of what’s going on.
I’ll just mention a couple of highlights from the conference itself. Prof Ramgopal Rao from the Indian Institute of Technology Bombay described a very nice looking project to make an inexpensive point of care system for cardiac diagnostics. He began with the gloomy thought that soon more than half the cases of cardiac disease in the world will be India. If acute myocardial infarction can be detected early enough a heart attack can be prevented, but this currently needs expensive and time consuming tests. The need, then, is for a simple test that’s cheap and reliable enough to be done in a doctor’s office or clinic.
To do this one needs to integrate a microfluidic system to handle the blood sample, a sensor array to detect the appropriate biochemical markers, and a box of electronics to analyse the results. The sensor array and fluid handling system needs to be disposable, and to cost no more than a few hundred rupees (i.e. a couple of dollars), while the box should only cost a few thousand rupees, even though the protocols for diagnosis need to be quite sophisticated and robust. Rao is aiming for a working prototype very soon; the biosensor is based on a cantilever which bends when the marker binds to a bound antibody. He uses a polymer photoresist to make the cantilever, with an embedded poly-silicon piezo-resistor to measure the deflection (this isn’t trivial at all, as the change in resistance amounts to about 10 parts per million).
Another nice talk was from Prof T. Pradeep, a surface chemist from the Indian Institute of Technology Madras. He described a water filter incorporating gold and silver nanoparticles mounted on a substrate of alumina, which is particularly effective at removing halogenated organic compounds such as pesticide residues. This is already marketed, with the filter cartridge costing about a few hundred rupees. He also mentioned a kit that can test for such pesticide residues with a detection limit of around 25 parts per billion.
The closing talk was given by Prof CNR Rao, and consisted of reflections on the future of nanotechnology. His opinions are worth paying attention to, if for no other reason than that he is undoubtedly the most powerful and influential scientist in India, and his views will shape the way nanotechnology is pursued there. What follows are my notes on his talk, tidied up but not verbatim.
Rao is a materials chemist, and he started by observing that now we can make pretty much any material in any form. But the question is, how can we use them, how can we assemble them and integrate them into devices? This is the biggest gap – we need products, devices and machines from nano-objects, and this is still probably at least 10 years, maybe 15 years, away. But we shouldn’t worry just about products and devices – nanotechnology is a new type of science, which will dissolve barriers between physics and chemistry and biology and bring in engineering. As an example – many people have made molecular motors. But… can they be connected together to do something? This sort of thing needs combinations of molecular science and nanoscience. Soft matter is another area with many good people and interesting work, including some in Bangalore. But there’s still a gap in applying them, what about active gels? Similarly, we see big successes in sensors, imaging but there’s much left to do. As an example of one very big challenge, many people suffer in Bangalore and everywhere else from dementia; we know this is related to the nanoscale phenomenon of peptide aggregation, but we need to understand why it happens and how to stop it. Drug delivery and tissue engineering are other examples where nanotechnology can make a real impact on human suffering. If one wants a role model, Robert Langer is a great example of someone who has produced many new results in tissue engineering and drug delivery, many graduate students and many companies; science in India should be done like this. We must remove the barriers and bureaucracy to give more freedom to scientists and engineers. At the moment, public servants like academics, cannot get involved in private enterprise, and this must change. Nanotechnology doesn’t take much money – it’s the archetypal knowledge based industry, and as such it should lead to much more linkage between industry and academia.
One of my engagements in a slightly frantic period last week was to go to a UK-Korea meeting on collaboration in nanotechnology. This had some talks which gave a valuable insight into how the future of nanotechnology is seen in Korea. It’s clearly seen as central to their program of science and technology; according to some slightly out-of-date figures I have to hand about government spending on nanotechnology, Korea ranks 5th, after the USA, Japan, Germany and France, and somewhat ahead of the UK. Dr Hanjo Lim, of the Korea Science and Engineering Foundation, gave a particularly useful overview.
He starts out by identifying the different ways in which going small helps. Nanotechnology exploits a confluence of 3 types of benefits – nanomaterials exploit surface matter, in which benefits arise from their high surface to volume ratio, with most obvious benefits from catalysis. They exploit quantum matter, size dependent quantum effects that are so important for band gap engineering and making quantum dots. And they can exploit soft matter, which is so important for the bio-nano interface. As far as Korea is concerned, as a small country with a well-developed industrial base, he sees four important areas. Applications in information and communication technology will obviously directly impact the strong position Korea has in the semiconductor industry and the display industry, as well as having an impact on automobiles. Robots and ubiquitous devices play to Korea’s general comparative advantage in manufacturing, but applications in Nano foods and medical science are relatively weak in Korea at the moment. Finally, the environmentally important applications in Fuel/solar cells, air and water treatments will be of growing importance in Korea, as everywhere else.
Korea ranks 4th or 5th in the world in terms of nano-patents; the plan is, up to 2010, to expand existing strength in nanotechnology and industrialise this by developing technology specific to applications. Beyond that the emphasis will be on systems level integration and commercialisation of those developments. Clearly, in electronics we are already in the nano- era. Korea has a dominant position in flash memory, where Hwang’s law – that memory density doubles every year – represents a more aggressive scaling than Moore’s law. To maintain this will require perhaps carbon nanotubes or silicon nanowires. Lim finds nanotubes very attractive but given the need for control of chirality and position his prediction is that this is still more than 10 years until commercialisation. An area that he thinks will grow in importance is the integration of optical interconnects in electronics. This, in his view, will be driven by the speed and heat issues in CPU that arise from metal interconnects – he reminds us that a typical CPU has 10 km of electrical wire, so it’s no wonder that heat generation is a big problem, and Google’s data centres come equipped with 5 story cooling towers. Nanophotonics will enable integration of photonic components within silicon multi-chip CPUs – but the problem that silicon is not good for lasers will have to be overcome. Either lasers off the chip will have to be used, or silicon laser diodes developed. His prognosis is, recognising that we have box to box optical interconnects now, and board to board interconnects are coming, that we will have chip to chip intercoonnects on the 1 – 10 cm scale by 2010, with intrachip connects by 2010-2015.
Anyone interested in more general questions of the way the Korean innovation system is developing will find much to interest them in a recent Demos pamphlet: Korea: Mass innovation comes of age. Meanwhile, I’ll be soon reporting on nanotechnology in another part of Asia; I’m writing this from Bangalore/Bengalooru in India, where I will be talking tomorrow at Bangalore Nano 2007.
