Chemical Technology July 2015

NANOTECHNOLOGY

anyway, and the human trials process has been so long and – for the most part – has not demonstrated unequivocal benefits for the substitute blood. Fluosol, a PFC-based product, is still the only blood substitute approved by the US FDA for use, and it was only used during cardiac angioplasty (that is, during a very brief procedure, and only because it allowed surgeons to extend the procedure and give themselves more time). However, problems with the emulsion storage used meant that it stopped being used by 1992. In fact, it’s kind of difficult to find an early entrant to the industry that has managed to stick out the lengthy clinical trial process, mostly because the early efforts have been only marginally as good as donated blood. Despite this, the British National Health Service has announced that it will be using artificial blood (in very specific-use cases) by 2017, with wider use by 2020. Promising new generation of replacements Enter the new generation of blood replacements that look tremendously promising. The NHS approach is to use stem cells harvested from umbilical cords to produce red blood cells. IIT-Madras scientists claim that they can produce tril- lions of cells from millions of stem cells. However, so far, this is only in a petri dish in a lab. The most far advanced approaches can produce about three units of blood for every unit of stem cells. The ad- vantage here is that the resultant erythrocytes can be used for any blood type, and they behave as normal in the body. There is also Oxycyte, a third-generation PFC carrier, invented by Leland Clark shortly before he died. This is undergoing stage II trials. Scientists at the University of Sheffield are developing ‘plastic blood’; a dendrimer composed of repetitively- branched molecules forming a tree-like structure around an iron atom at the core. Dendrimers have been used in drug and gene delivery to protect their payload (or prevent ad- verse side-effects in the wrong place). Creating a designed version of haemoglobin (which has its own Fe core), seems a logical next step. Now, if that isn’t sufficiently sophisticated for you, there’s the respirocyte. This hypothetical artificial blood cell was developed by Robert Freitas and described in his 1998 paper, “A Mechanical Artificial Red Blood Cell: Exploratory Design in Medical Nanotechnology”. They are spherical nanorobots composed of 18 billion atoms designed as a tiny pressure tank able to carry oxygen and carbon dioxide and deliver these around the body. Thus, a person could fill up his blood with respirocytes and sprint for 15 minutes without breathing. There is a darkish side to all this technology. In 2004, it emerged that Spanish Tour de France cyclist Jesus Manzano had used Oxyglobin during the 2003 race. That’s the artificial blood used for animals. Unfortunately, but maybe predictably, he became ill and crashed during the race. The likelihood is that we will soon have a useful artificial blood that will reduce the risk of disease and incompatibility. Whether we’ll be ready for it is another story. z

of the right blood type even as one discards blood of the ‘wrong’ blood type. It is most vexing. Human Hb is extracted from erythrocytes but cannot be simply transfused. Hb is held in place within the erythrocyte by a scaffold-like tissue called the stroma. Hb isolated from compromised erythrocytes still contains stroma, and stroma – outside of its cellular environment – is toxic to kidneys. Yet, isolating Hb from stroma results in Hb which does not release oxygen. It also circulates far too rapidly through the body, causing problems with hydrostatic pressure. Hb can be cross-linked with bis-fumarate, and further polymerised, to result in a tetrameric haemoglobin which has lower oxygen affinity and a longer circulation time. To further increase circulation time, Hb can be linked to thiols or they can be encapsulated in biodegradable polymer vesicles. The result is also typeless and usable by anyone. However, getting enough human stroma-free Hb is a problem, and so bovine Hb is being used as a starter. This leads to new problems of potential introduction of cow diseases as well as incompatibility. A product developed from cows was used in South Africa until 2008, when it was de-authorised for use in humans (although its use in veterinary treatment continues). There are also efforts around recombinant production of human haemoglobin. E. coli and yeast have been tried as vectors to express human fused α-globins, but trials were stopped since the resulting molecules resulted in vasocon- striction and other harmful side-effects. Transgenic mice and pigs have been created that con- tain human α and β globin genes. The only problem is that these animals also have their own native haemoglobin. Haemoglobin has proven problematic. An alternative to haemoglobin An older alternative is that of perfluorocarbons (PFCs). Leland Clark, one of the foremost biochemists and ‘father of biosensors’ (he invented a device for measuring oxygen in blood, as well as the precursor to the modern glucose sensor), experimented in the 1960s with fluorocarbon- based liquid that could be breathed by mice in place of air. The fluorine-based polymer is similar to synthetic materials like Teflon, and can potentially carry 100 times more oxygen around the body than does haemoglobin. The problems (obviously) are numerous, including that PFC is biologically inert and so accumulates in the liver, is also oil-like and cannot carry water-soluble salts and metabolic substrates. PFCs must be emulsified in albumin or phospholipids and triglycerides. The most active investor in artificial blood is the US military and for obvious reasons. Getting blood to injured soldiers would be a lot easier if one didn’t also have to worry about expiry-dates and blood types. Biopure (makers of Hemopure, a Hb approach), North- field (makers of Polyheme, another Hb variant), and Sangart (makers of Hemospan, also Hb), have all received fund- ing. And they’re all bankrupt. Biopure was bought by OPK Biotech, which shut down, and its IP was bought by HbO 2 Therapeutics … which is still going. The problem is that patients requiring major transfusions are not in good shape

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Chemical Technology • July 2015