53 Essays on Infinite Lifespans   João Pedro de Magalhães mesenchymal stem cells into the bone marrow has shown that they can travel through the body and become bone or muscle cells where needed. [29] These experiments demonstrate how a few cells can impact on whole organs by fostering regenera- tion, how a few tiny cells can transmit massive amounts of information to the human body. Although much research is necessary and stem cells are still too expensive for widespread use, the basics for using these techniques are known and we can expect more practical appli- cations to emerge in a near future. The ability stem cells have to sprout regeneration and repair tissues makes them an excel- lent candidate for anti-aging therapies. NANOTECHNOLOGY An adult human, once a tiny cell, is a self-assembling machine made  of  trillions  of  microscopic  components.  Roughly, a human being consists of ~7x10²⁷ atoms and ~10⁵ different molecular species, mostly proteins [30]. Genes and proteins are organic nanostructures working with molecular precision to form complex components such as human cells. The con- cept of nanotechnology, first proposed by Richard Feynman and later developed by the pioneering work of Eric Drexler, is our ability to manipulate matter and energy at smaller scales (one billionth of a specified unit is called a ‘nano’). This capac- ity will increase until we reach and surpass our own biological nanostructures [4;31]. One key concept in nanotechnology is the molecular assembler, a machine capable of assembling other molecules given a set of instructions and the necessary resources. Ribosomes, the sites where proteins are built based on the instructions of the genes, are known molecular assem- blers. A man-made molecular assembler capable of building molecule-scale machines to guide specific chemical reactions