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ARTICLE IN PRESS 3 What is nanomedicine

9 10 The early genesis of the concept of nanomedicine sprang from the visionary idea that tiny nanorobots 11 and related machines could be designed, manufactured, and introduced into the human body to perform 12 cellular repairs at the molecular level. Nanomedicine today has branched out in hundreds of different 13 directions, each of them embodying the key insight that the ability to structure materials and devices at 14 the molecular scale can bring enormous immediate benefits in the research and practice of medicine.

ARTICLE IN PRESS NANO-00003; No of Pages 8 Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx – xxx 1 2 Original article 3 What is nanomedicine? www.nanomedjournal.com Robert A. Freitas Jr. 4 Institute for Molecular Manufacturing, Box 605, Pilot Hill, CA 95664, USA Received 28 September 2004; accepted 23 November 2004 5 6 7 9 810 11 12 13 14 15 Abstract The early genesis of the concept of nanomedicine sprang from the visionary idea that tiny nanorobots and related machines could be designed, manufactured, and introduced into the human body to perform cellular repairs at the molecular level. Nanomedicine today has branched out in hundreds of different directions, each of them embodying the key insight that the ability to structure materials and devices at the molecular scale can bring enormous immediate benefits in the research and practice of medicine. D 2005 Elsevier Inc. All rights reserved. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 In his January 2000 State of the Union speech, the US president announced that he would seek $475 million for nanotechnology research and development (R&D) via the National Nanotechnology Initiative, effectively doubling federal nanotech funding for fiscal year (FY) 2001. The president never referred to bnanotechnologyQ by name, but he gushed about its capabilities, marveling at a technology that will someday produce bmolecular computers the size of a tear drop with the power of today’s fastest supercomputers.Q Annual US federal funding for nanotechnology R&D exceeded $500 million in 2002 [1], reached $849 million in FY 2004 [2], and may approach $1 billion in next year’s budget. The European Commission has set aside 1.3 billion euros for nanotechnology research during the 2003–2006 period [3], with annual nanotechnology investment worldwide reaching approximately $3 billion in 2003. Private sector analysts estimate that the worldwide market for nanoscale devices and molecular modeling should experience an average annual growth rate of 28% per year, rising from $406 million in 2002 to $1.37 billion in 2007, with a 35% per year growth rate in revenues from biomedical nanoscale devices [4]. In December 2002, the US National Institutes of Health (NIH) announced a 4-year program for nanoscience and nanotechnology in medicine [3]. Burgeoning interest in the medical applications of nanotechnology has led to the emergence of a new field called nanomedicine [3,5-12]. Most broadly, nanomedicine [5] is the process of diagnosing [13], treating, and preventing disease and traumatic injury, relieving pain, and preserving and improving human health, E-mail address: [email protected]. 1549-9634/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.nano.2004.11.003 using molecular tools and molecular knowledge of the human body. In short, nanomedicine is the application of nanotechnology to medicine. The NIH Roadmap’s new Nanomedicine Initiatives, first released in late 2003, benvision that this cutting-edge area of research will begin yielding medical benefits as early as 10 years from now Q and will begin with bestablishing a handful of Nanomedicine Centers . . . staffed by a highly interdisciplinary scientific crew including biologists, physicians, mathematicians, engineers and computer scientists . . . gathering extensive information about how molecular machines are built Q who will also develop ba new kind of vocabulary— lexicon—to define biological parts and processes in engineering terms Q [14]. Even state-funded programs have begun, such as New York’s Alliance for Nanomedical Technologies [15]. The first 12 doctoral candidates in bnanobiotechnology Q began laboratory work at Cornell University in June 2000, and many other universities have started similar programs as state, federal, and international funding has soared. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 Feynman’s early vision 67 The early genesis of the concept of nanomedicine sprang from the visionary idea that tiny nanorobots and related machines could be designed, manufactured, and introduced into the human body to perform cellular repairs at the molecular level. Although this idea was later championed in the popular writings of Drexler [16,17] in the 1980s and 1990s, and in the technical writings of Freitas [5,7] in the 1990s and 2000s, the first scientist to voice these possibilities was the late Nobel physicist Richard P. 68 69 70 71 72 73 74 75 76 ARTICLE IN PRESS 2 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx Feynman, who worked on the Manhattan Project at Los Alamos during World War II and later taught at CalTech for most of his professorial career. In his prescient 1959 talk, bThere’s Plenty of Room at the Bottom,Q Feynman proposed using machine tools to make smaller machine tools, these to be used in turn to make still smaller machine tools, and so on all the way down to the atomic level [18]. Feynman prophetically concluded that this is ba development which I think cannot be avoided.Q Such nanomachine tools, nanodevices, and nanorobots could ultimately be used to develop a wide range of atomically precise microscopic instrumentation and manufacturing tools—that is, nanotechnology. Feynman was clearly aware of the potential medical applications of the new technology that he was proposing. After discussing his ideas with a colleague, Feynman [18] offered the first known proposal for a nanomedical procedure of any kind—in this instance, to cure heart disease: bA friend of mine (Albert R. Hibbs) suggests a very interesting possibility for relatively small machines. He says that, although it is a very wild idea, it would be interesting in surgery if you could swallow the surgeon. You put the mechanical surgeon inside the blood vessel and it goes into the heart and looks around. (Of course the information has to be fed out.) It finds out which valve is the faulty one and takes a little knife and slices it out. Other small machines might be permanently incorporated in the body to assist some inadequately functioning organ.Q Later in his historic lecture in 1959, Feynman urged us to consider the possibility, in connection with biologic cells, bthat we can manufacture an object that maneuvers at that level!Q Without losing sight of Feynman’s original long-term vision of medical nanorobotics, nanomedicine today has branched out in hundreds of different directions, each of them embodying the key insight that the ability to structure materials and devices at the molecular scale can bring enormous immediate benefits in the research and practice of medicine. In general, miniaturization of our medical tools will provide more accurate, more controllable, more versatile, more reliable, more cost-effective, and faster approaches to enhancing the quality of human life [5]. Table 1 gives an overview of this rapidly expanding and exciting field. Over the next 5 to 10 years, nanomedicine will address many important medical problems by using nanoscale-structured materials and simple nanodevices that can be manufactured today. There is space here to briefly describe only a few of the most interesting and diverse current research projects within several of the 96 subcategories listed in Table 1 because each subcategory may represent up to a dozen or more projects of which I am aware. 128 Nanomedicine today 129 Many approaches to nanomedicine being pursued today 130 are already close enough to fruition that it is fair to say that their successful development is almost inevitable, and their subsequent incorporation into valuable medical diagnostics or clinical therapeutics is highly likely and may occur very soon. 131 132 133 134 Immunoisolation 135 One of the simplest medical nanomaterials is a surface perforated with holes, or nanopores. In 1997, Desai et al [19] created what could be considered one of the earliest therapeutically helpful nanomedical devices, using bulk micromachining to fabricate tiny chambers within single crystalline silicon wafers in which biologic cells can be placed. The chambers interface with the surrounding biologic environment through polycrystalline silicon filter membranes micromachined to present a high density of uniform nanopores as small as 20 nm in diameter. These pores are large enough to allow small molecules such as oxygen, glucose, and insulin to pass but are small enough to impede the passage of much larger immune system molecules such as immunoglobulins and graft-borne virus particles. Behind this artificial barrier, immunoisolated encapsulated rat pancreatic cells may receive nutrients and remain healthy for weeks, secreting insulin through the pores while remaining hidden from the immune system, which would normally attack and reject the foreign cells. Microcapsules containing easily harvested replacement pig islet cells could be implanted beneath the skin of some diabetes patients [20], temporarily restoring the body’s glucose control feedback loop, while avoiding the use of powerful immunosuppressants that can leave the patient at serious risk for infection. Supplying encapsulated new cells to the body could also be a valuable way to treat other enzyme- or hormone-deficiency diseases, including encapsulated neurons that could be implanted in the brain and then be electrically stimulated to release neurotransmitters, possibly as part of a future treatment for Alzheimer’s or Parkinson’s diseases. In conjunction with the biomedical company iMEDD (Columbus, Ohio), Desai has been active in continuing this work for immunoisolation [21], drug delivery [22,23] and cell-based sensing [24,25]. 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 Gated nanosieves 170 The flow of materials through nanopores can also be externally regulated [26]. The first artificial voltage-gated molecular nanosieve was fabricated by Nishizawa et al [27] at Colorado State University in 1995; it had an array of cylindric gold nanotubules with inside diameters as small as 1.6 nm. When tubules were positively charged, positive ions were excluded and only negative ions were transported through the membrane; with a negative voltage, only positive ions could pass. Similar nanodevices are now combining voltage gating with pore size, shape, and charge constraints to achieve precise control of ion transport with significant molecular specificity [28]. Martin and Kohli’s [29] recent efforts have been directed at immobilizing biochemical molecular- recognition agents such as 171 172 173 174 175 176 177 178 179 180 181 182 183 184 ARTICLE IN PRESS R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx t1.1 t1.2 t1.3 t1.4 t1.5 3 Table 1 A partial nanomedicine technologies taxonomy Raw nanomaterials Nanoparticle coatings Nanocrystalline materials Cell simulations and cell diagnostics Cell chips Cell simulators Biological research Nanobiology Nanoscience in life sciences Nanostructured materials Cyclic peptides Dendrimers Detoxification agents Fullerenes Functional drug carriers MRI scanning (nanoparticles) Nanobarcodes Nanoemulsions Nanofibers Nanoparticles Nanoshells Carbon nanotubes Noncarbon nanotubes Quantum dots DNA manipulation, sequencing, diagnostics Genetic testing DNA microarrays Ultrafast DNA sequencing DNA manipulation and control Drug delivery Drug discovery Biopharmaceutics Drug delivery Drug encapsulation Smart drugs t1.6 t1.7 t1.8 t1.9 t1.10 t1.11 t1.12 t1.13 t1.14 t1.15 t1.16 t1.17 t1.18 t1.19 t1.20 t1.21 t1.22 t1.23 t1.24 t1.25 t1.26 t1.27 t1.28 t1.29 t1.30 t1.31 t1.32 t1.33 t1.34 t1.35 t1.36 t1.37 t1.38 t1.39 t1.40 t1.41 t1.42 t1.43 t1.44 t1.45 185 186 187 188 Artificial binding sites Artificial antibodies Artificial ezymes Artificial receptors Molecularly imprinted polymers Control of surfaces Artificial surfaces—adhesive Artificial surfaces—nonadhesive Artificial surfaces—regulated Biocompatible surfaces Biofilm suppression Engineered surfaces Pattern surfaces (contact guidance) Thin-film coatings Nanopores Immunoisolation Molecular sieves and channels Nanofiltration membranes Nanopores Separations Tools and diagnostics Bacterial detection systems Biochips Biomolecular imaging Biosensors and biodetection Diagnostic and defense applications Endoscopic robots and microscopes Fullerene-based sensors Imaging (cellular, etc.) Lab on a chip Monitoring Nanosensors Point of care diagnostics Protein microarrays Scanning probe microscopy Intracellular devices Intracellular assay Intracellular biocomputers Intracellular sensors/reporters Implants inside cells BioMEMS Implantable materials and devices Implanted bioMEMS, chips, and electrodes MEMS/Nanomaterials-based prosthetics Sensory aids (artificial retina, etc.) Microarrays Microcantilever-based sensors Microfluidics Microneedles Medical MEMS MEMS surgical devices enzymes, antibodies, and other proteins, and DNA, inside the nanotubes to make active biologic nanosensors [30-32] and also to perform drug separations [33,34] or to allow selected biocatalysis [34]. 189 Ultrafast DNA sequencing 190 191 192 193 194 195 196 197 198 199 Branton’s [35,36] team at Harvard University uses an electric field to drive a variety of RNA and DNA polymers through the central nanopore of an a-hemolysin protein channel mounted in a lipid bilayer similar to the outer membrane of a living cell. Branton first showed that the nanopore could rapidly discriminate between pyrimidine and purine segments along a single RNA molecule and then in 2000 demonstrated discrimination between DNA chains of similar length and composition differing only in base pair sequence. Reliability and resolution are the biggest chal- Molecular medicine Genetic therapy Pharmacogenomics Artificial enzymes and enzyme control Enzyme manipulation and control Nanotherapeutics Antibacterial and antiviral nanoparticles Fullerene-based pharmaceuticals Photodynamic therapy Radiopharmaceuticals Synthetic biology and early nanodevices Dynamic nanoplatform bnanosomeQ Tecto-dendrimers Artificial cells and liposomes Polymeric micelles and polymersomes Biotechnology and biorobotics Biologic viral therapy Virus-based hybrids Stem cells and cloning Tissue engineering Artificial organs Nanobiotechnology Biorobotics and biobots Nanorobotics DNA-based devices and nanorobots Diamond-based nanorobots Cell repair devices lenges, and Branton’s [37- 41] group continues to perfect this approach. Current research is directed toward fabricating pores with specific diameters and repeatable geometries at high precision [42- 45], understanding the unzipping of double-stranded DNA as one strand is pulled through the pore [46] and the recognition of folded DNA molecules passing through a pore [41], and investigating the benefits of adding electrically conducting electrodes to pores to improve longitudinal resolution bpossibly to the single-base level for DNAQ [41]. If these difficult challenges can be surmounted, nanopore-based DNA-sequencing devices could allow perpore read rates potentially up to 1000 bases per second [47]. 200 201 202 203 204 205 206 207 208 209 210 211 Fullerene-based pharmaceuticals 212 Soluble derivatives of fullerenes such as C60—a soccer- 213 ball–shaped arrangement of 60 carbon atoms per mole- 214 ARTICLE IN PRESS 4 215 216 217 218 219 220 221 222 223 224 225 226 R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx cule—show great promise as pharmaceutical agents. These derivatives, many already in clinical trials, have good biocompatibility and low toxicity even at relatively high dosages. Fullerene compounds may serve as antiviral agents (most notably against human immunodeficiency virus [48]), antibacterial agents (Escherichia coli [49], Streptococcus [50], Mycobacterium tuberculosis [51]), photodynamic antitumor [52,53] and anticancer [54] therapies, antioxidants and antiapoptosis agents as treatments for amyotrophic lateral sclerosis [55] and Parkinson’s disease, and other applications—most being pursued by C Sixty (www.csixty.com), the leading company in this area. 227 Nanoshells 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 Q2 249 250 251 252 253 254 255 256 Halas and West [56,57] at Rice University in Houston have developed a platform for nanoscale drug delivery called the nanoshell—dielectric metal (gold-coated silica) nanospheres whose optical resonance is a function of the relative size of the constituent layers. These nanoshells, embedded in a drug-containing tumor-targeted hydrogel polymer, and then injected into the body, accumulate near tumor cells. When heated with an infrared laser, the nanoshells (each slightly larger than a polio virus) selectively absorb a specific infrared frequency, melting the polymer and releasing the drug payload at a specific site. Nanoshells might prove useful in treating diabetes—a patient would use a ballpointpen–sized infrared laser to heat the skin site where nanoshell polymer had been injected, releasing a pulse of insulin. Unlike injections, which are taken several times a day, the nanoshell-polymer system could remain in the body for months. Nanospectra Biosciences (www.nanospectra.com) is conducting animal studies at the MD Anderson Cancer Center at the University of Texas in a related application specifically targeting micrometastases, tiny aggregates of cancer cells too small for surgeons to find and remove with a scalpel. The company hopes to start clinical trials for the cancer treatment in 2004 -2005 and for an insulin-delivery system by 2006. Rice University researchers have also developed a point-of-care whole-blood immunoassay using antibody-nanoparticle conjugates of gold nanoshells, successfully detecting subnanogram-per-milliliter quantities of immunoglobulins in saline, serum, and whole blood within 10 to 30 minutes of sample acquisition [58]. 257 Single-virus detectors 258 259 260 261 262 263 264 265 266 267 Lieber’s [59] group has recently reported direct, real-time electrical detection of single virus particles with high selectivity using nanowire field-effect transistors to measure discrete conductance changes characteristic of binding and unbinding on nanowire arrays modified with viral antibodies. The arrays detect viruses suspended in fluids, whether bodily or otherwise. The Lieber group tested nanowire arrays having receptors specific to influenza A, paramyxovirus, and adenovirus and found that the detectors could differentiate among the 3 viruses, both because of the specific receptors used to bind them and because each virus binds to its receptor for a characteristic length of time before dislodging, giving only a small risk of a false positive reading. Note the researchers’ comment: bThe possibility of large-scale integration of these nanowire devices suggests potential for simultaneous detection of a large number of distinct viral threats at the single virus level.Q Incorporation into practical clinical diagnostic devices seems within reach within the next few years. 268 269 270 271 272 273 274 275 276 Tectodendrimers 277 Starburst dendrimers [60] are tree-shaped synthetic molecules up to a few nanometers in diameter that are formed with a regular branching structure. Baker’s [61-63] and Tomalia’s [62-64] groups are synthesizing multicomponent nanodevices called tectodendrimers, which have a single core dendrimer to which additional dendrimer modules of different types are affixed, each type designed to perform a function necessary to a smart therapeutic nanodevice. A combinatorially large number of smart therapeutic nanodevices can easily be synthesized from a library of dendrimeric components performing the following tasks: (1) diseased cell recognition, (2) diagnosis of disease state, (3) drug delivery, (4) location reporting, and (5) reporting outcome of therapy. For instance, once apoptosis-reporting, contrast-enhancing, and chemotherapeutic-releasing dendrimer modules are made and attached to the core dendrimer, it should be possible to make large quantities of this tectodendrimer as a starting material. This framework structure can be customized to fight a particular cancer simply by substituting any one of many possible distinct cancer recognition or btargeting Q dendrimers, creating a nanodevice customized to destroy a specific cancer type and no other, while also sparing the healthy normal cells. In 3 nanodevices synthesized using a 5generation, ethylenediamine-core polyamidoamine dendrimer with folic acid, fluorescein, and methotrexate covalently attached to the surface to provide targeting, imaging, and intracellular drug delivery capabilities, the btargeted delivery improved the cytotoxic response of the cells to methotrexate 100-fold over free drug Q [61]. At least a half-dozen cancer cell types have already been associated with at least one unique protein that targeting dendrimers could use to identify the cell as cancerous, and as the genomic revolution progresses it is likely that proteins unique to each kind of cancer will be identified, thus allowing the design of recognition dendrimers for each type of cancer, although practical clinical therapeutics are probably at least 3 to 5 years away. The same cell-surface protein recognition–targeting strategy could be applied against virus-infected cells and parasites. 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 Radio-controlled biomolecules 318 Jacobson’s [65] group has attached tiny radiofrequency 319 (RF) antennas—1.4-nm gold nanocrystals of b 100 atoms— 320 ARTICLE IN PRESS R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 to DNA. When a ~1-GHz RF magnetic field is transmitted into the tiny antennas, alternating eddy currents induced in the nanocrystals produce highly localized inductive heating, in seconds causing the double-stranded DNA to separate into 2 strands in a fully reversible dehybridization process that leaves neighboring molecules untouched. The long-term goal is to apply the antennas to living systems and control gene expression via remote electronic switching. This requires attaching gold nanoparticles to specific oligonucleotides that, when added to a sample of DNA, would bind to complementary gene sequences, blocking the activity of those genes and effectively turning them off. Applying the RF magnetic field would then heat the gold particles, causing their attached DNA fragments to detach, turning the genes back on. One observer noted [66]: bYou can even start to think of differential receivers—different radio receivers that respond differently to different frequencies. By dialing in the right frequency, you can turn on tags on one part of DNA but not other tags.Q The gold nanocrystals can also be attached to proteins, opening up the possibility of electronically controlling more complex biologic processes such as protein folding and enzymatic activity. In one case [67], an RNAhydrolyzing enzyme called ribonuclease S was separated into 2 pieces: a large segment made up of 104 amino acids and a small 18-amino-acid strand called the S-peptide. The ribonuclease (RNAase) enzyme is inactive unless the small strand sits in the mouth of the protein. Gold nanoparticles were linked to the end of S-peptide strands and served as a switch to turn the enzyme on and off—in the absence of the RF field, the S-peptides adopted their usual conformation and the RNAase remained active, but with the external RF field switched on, the rapidly spinning nanoparticles prevented the S-peptide from assembling with the larger protein, thereby inactivating the enzyme. 355 Biologic robots 356 357 358 359 360 361 362 363 364 Q3 365 366 367 368 369 370 371 372 373 374 375 Engineered bacterial bbiorobots Q may be constructed from as few as 300 highly conserved genes (~150,000 nucleotide bases) that constitute the minimum possible genome for a functional microbe [68]. Used in medicine, these synthetic microbes could be designed to produce useful vitamins, hormones, enzymes, or cytokines in which a patient’s body was deficient or to selectively absorb and metabolize into harmless end products harmful substances such as poisons, toxins, or indigestible intracellular detritus or even to perform useful mechanical tasks. In 2003, Egea Biosciences (www.egeabiosciences.com) received bthe first [patent] [69] to include broad claims for the chemical synthesis of entire genes and networks of genes comprising a genome, the doperating systemT of living organisms.Q Egea’s proprietary GeneWriter and Protein Programming technology have assembled libraries of N 1 million programmed proteins, produced more than 200 synthetic genes and proteins, and synthesized the largest gene ever chemically synthesized (N 16,000 bases). Egea’s software allows researchers to author new DNA sequences that the 5 company’s hardware can then manufacture to specification with a base-placement error of only ~10 4, which Egea calls bword processing for DNAQ [70]. The goal is the synthesis of ba gene of 100,000 bp . . . from one thousand 100-mers. The overlap between ’pairs’ of plus and minus oligonucleotides is 75 bases, leaving a 25 base-pair overhang. In this method, a combinatorial approach is used where corresponding pairs of partially complementary oligonucleotides are hybridized in the first step. A second round of hybridization then is undertaken with appropriately complementary pairs of products from the first round. This process is repeated a total of 10 times, each round of hybridization reducing the number of products by half. Ligation of the products then is performed.Q The result would be a strand of DNA 100,000 bp in length, long enough to make a very simple bacterial genome [70]. The Institute for Biological Energy Alternatives (www.bioenergyalts.org) also has a $3 million, 3-year grant from the US Department of Energy to create a related minimalist organism, starting with the Mycoplasma genitalium microorganism [71]. Scientists from the Institute for Biological Energy Alternatives (Rockville, Md) are removing all genetic material from the organism, then synthesizing an artificial string of genetic material resembling a naturally occurring chromosome that they hope will contain the minimum number of M genitalium genes needed to sustain life. The artificial chromosome will be inserted into the hollowed-out cell, which will then be tested for its ability to survive and reproduce. To ensure safety, the cell will be deliberately hobbled to render it incapable of infecting people, and will be strictly confined and designed to die if it does manage to escape into the environment. Development of biologic robots seems inevitable, with clinical trials likely in the 3- to 5-year time frame. 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 Medical nanorobotics of tomorrow 409 In the longer term, perhaps 10 to 20 years from today, the earliest molecular machine systems and nanorobots may join the medical armamentarium, finally giving physicians the most potent tools imaginable to conquer human disease, ill health, and aging. Organic building materials (eg, proteins, polynucleotides) are very good at self-assembly, but the most reliable and high-performance molecular machines may be constructed out of diamondoid materials, the strongest substances known. Many technical challenges must be surmounted before medical nanorobots can become a reality. Building diamondoid nanorobots—the most aggressive objective—will require both massive parallelism in molecular fabrication and assembly processes [72] and programmable positional assembly including molecularly precise manufacture of diamond structures using molecular feedstock [73-75]. Positionally controlled single-atom covalent bonding (mechanosynthesis) has been achieved experimentally for hydrogen [76] and silicon [77] atoms, but at present only computational simulations support the same expectation for carbon atoms and diamond structures. 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 ARTICLE IN PRESS 6 430 431 432 433 434 435 436 R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx As a result, the prospect for diamond nanorobotics remains controversial, although considerably less so for other approaches to medical nanorobotics that might use biologic components [72,78]. Yet if it can be done, the ability to build diamond-based molecular machine systems in large numbers leads, ultimately, to the most powerful kinds of medical nanorobots. 437 Respirocytes 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 One example of such a future device is the artificial mechanical red blood cell or brespirocyteQ [79], a bloodborne, spherical, 1-Am diamondoid, 1000-atm–pressure vessel with active pumping powered by endogenous serum glucose, able to deliver 236 times more oxygen to the tissues per unit volume than natural red blood cells and to manage carbonic acidity. The nanorobot is made of 18 billion atoms precisely arranged in a diamondoid pressure tank that can be pumped full of up to 3 billion oxygen (O2) and carbon dioxide (CO2) molecules. Later on, these gases can be released from the tank in a controlled manner using the same molecular pumps. Respirocytes mimic the action of the natural hemoglobin-filled red blood cells. Gas concentration sensors on the outside of each device let the nanorobot know when it is time to load O2 and unload CO2 (at the lungs), or vice versa (at the tissues). An onboard nanocomputer and numerous chemical and pressure sensors enable complex device behaviors remotely reprogrammable by the physician via externally applied acoustic signals. The injection of a 5-mL therapeutic dose of 50% respirocyte saline suspension, a total of 5 trillion individual nanorobots, into the human bloodstream would exactly duplicate the gas-carrying capacity of the patient’s entire 5.4 L of blood. Primary medical applications of respirocytes would include transfusable blood substitution; partial treatment for anemia, perinatal/neonatal, and lung disorders; enhancement of cardiovascular/neurovascular procedures, tumor therapies and diagnostics; prevention of asphyxia; artificial breathing; and a variety of sports, veterinary, battlefield, and other uses. 468 Microbivores 469 470 471 472 473 474 475 476 477 478 479 480 481 482 An artificial mechanical white blood cell of microscopic size, called a bmicrobivore,Q has as its primary function to destroy microbiologic pathogens found in the human bloodstream using a digest and discharge protocol [80]. The benchmark microbivore nanorobot design is an oblate spheroidal 200-pW device measuring 3.4 Am in diameter along its major axis and 2.0 Am in diameter along its minor axis. During each cycle of nanorobot operation, the target bacterium is bound to the surface of the bloodborne microbivore like a fly on flypaper, via speciesspecificreversible-binding sites [5]. Telescoping robotic grapples emerge from silos in the device surface, establish secure anchorage to the microbe’s plasma membrane, then transport the pathogen to the ingestion port at the front of the device where the pathogen cell is internalized into a 2Am3 morcellation chamber. After mechanical mincing, the remains of the cell are pistoned into a separate 2-Am3 digestion chamber where a preprogrammed sequence of 40 engineered enzymes are successively injected and extracted 6 times, progressively reducing the morcellate ultimately to monoresidue amino acids, mononucleotides, glycerol, free fatty acids, and simple sugars. These simple molecules are then harmlessly discharged back into the bloodstream through an exhaust port at the rear of the device, completing the 30-second digestion cycle. The nanorobots would be ~80 times more efficient as phagocytic agents than macrophages in terms of volume/second digested per unit volume of phagocytic agent and would have far larger maximum lifetime capacity for phagocytosis than natural white blood cells. An infusion of a few milliliters of microbivores would fully eliminate septicemic infections in minutes to hours, whereas natural phagocytic defenses—even when aided by antibiotics—can often require weeks or months to achieve complete clearance of target bacteria from the bloodstream. Hence, microbivores look to be up to ~1000 times faster acting than either unaided natural or antibiotic-assisted biologic phagocytic defenses and able to extend the therapeutic competence of the physician to the entire range of potential bacterial threats, including locally dense infections. The microbivores would be removed from the body once their mission was completed. 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 Chromosome replacement therapy 510 Medical nanorobots may also be able to intervene at the cellular level, performing in vivo cytosurgery. The most likely site of pathologic function in the cell is the nucleus— more specifically, the chromosomes. In one simple cytosurgical procedure called bchromosome replacement therapy,Q a nanorobot controlled by a physician would extract existing chromosomes from a particular diseased cell and insert new ones in their place, in that same cell [9,81]. The replacement chromosomes will be manufactured to order, outside of the patient’s body, in a laboratory bench-top production device that includes a molecular assembly line, using the patient’s individual genome as the blueprint. The replacement chromosomes are appropriately demethylated, thus expressing only the appropriate exons that are active in the cell type to which the nanorobot has been targeted. If the patient chooses, inherited defective genes could be replaced with nondefective base-pair sequences, permanently curing a genetic disease. 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 Conclusion 529 Our near-term ability to structure materials and devices at the molecular scale brings enormous immediate benefits and will revolutionize the research and practice of medicine. Early theoretical and experimental studies of the biocompatibility of nanomaterials and advanced nanodevices have begun [7]. Taking Feynman’s long-term vision 530 531 532 533 534 535 ARTICLE IN PRESS R.A. Freitas / Nanomedicine: Nanotechnology, Biology, and Medicine xx (2005) xxx–xxx 536 537 538 539 540 541 542 543 544 545 546 of medical nanorobots to heart, our present knowledge tells us that these things violate no known laws of physics, chemistry, biology, or engineering. Complex issues relating to future US Food and Drug Administration approval of nanomedical materials, devices, and even the possibility of medical nanorobots are already being addressed in mainstream legal journals [82,83]. One hopes that our society will be able to muster the collective financial and moral courage to allow such extraordinarily powerful medicine to be deployed for human betterment, with due regard to essential ethical considerations. 547 Acknowledgements 548 I thank the Institute for Molecular Manufacturing, Alcor 549 Foundation, and Kurzweil Foundation for their financial 550 support of this work. 551 References 552 Q5 553 [1] National Science Foundation. 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