Specific binders comprised of nucleic acids, that is, RNA/DNA aptamers, are attractive functional biopolymers owing to their potential broad application in medicine, food hygiene, environmental analysis, and biological research. Despite the large number of reports on selection of natural DNA/RNA aptamers, there are not many examples of direct screening of chemically modified nucleic acid aptamers. This is because of (i) the inferior efficiency and accuracy of polymerase reactions involving transcription/reverse-transcription of modified nucleotides compared with those of natural nucleotides, (ii) technical difficulties and additional time and effort required when using modified nucleic acid libraries, and (iii) ambiguous efficacies of chemical modifications in binding properties until recently; in contrast, the effects of chemical modifications on biostability are well studied using various nucleotide analogs. Although reports on the direct screening of a modified nucleic acid library remain in the minority, chemical modifications would be essential when further functional expansion of nucleic acid aptamers, in particular for medical and biological uses, is considered. This paper focuses on enzymatic production of chemically modified nucleic acids and their application to random screenings. In addition, recent advances and possible future research are also described. 1. Introduction RNA/DNA aptamers, which are specific for a broad spectrum of targets, can be artificially created by systematic evolution of ligands by exponential enrichment (SELEX) methods [1, 2]. Large-scale chemical synthesis of RNA/DNA aptamers is possible, and synthesizing them is less expensive than producing antibodies; therefore, they have been considered as alternatives to therapeutic antibodies. Although RNA/DNA aptamers do not cause antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC), their specific binding abilities are expected to neutralize actions on the target and relieve symptoms. Indeed, the first example of an aptamer drug, “Macugen (pegaptanib sodium injection)” is being used for age-related macular degeneration (AMD) therapy [3]. Pegaptanib is a RNA-based aptamer that involves 2′-fluoropyrimidine nucleotides (U, C) and 2′-methoxy purine nucleotides (A, G) to remain intact under physiological conditions. In addition, a branched polyethylene glycol strand (40？kDa) and 3′-thymidylic acid are introduced at its 5′ and 3′ ends, respectively. The 5′-end modification is known to prolong circulation time in vivo as well as to enhance
References
[1]
A. D. Ellington and J. W. Szostak, “In vitro selection of RNA molecules that bind specific ligands,” Nature, vol. 346, no. 6287, pp. 818–822, 1990.
[2]
C. Tuerk and L. Gold, “Systemic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase,” Science, vol. 249, no. 4968, pp. 505–510, 1990.
[3]
J. Ruckman, L. S. Green, J. Beeson et al., “2'-fluoropyrimidine RNA-based aptamers to the 165-amino acid form of vascular endothelial growth factor (VEGF165): inhibition of receptor binding and VEGF-induced vascular permeability through interactions requiring the exon 7-encoded domain,” Journal of Biological Chemistry, vol. 273, no. 32, pp. 20556–20567, 1998.
[4]
N. Janji？, “Inhibition of receptor binding by high-affinity RNA ligands to vascular endothelial growth factor,” Biochemistry, vol. 33, no. 34, pp. 10450–10456, 1994.
[5]
C. E. Tucker, L. S. Chen, M. B. Judkins, J. A. Farmer, S. C. Gill, and D. W. Drolet, “Detection and plasma pharmacokinetics of an anti-vascular endothelial growth factor oligonucleotide-aptamer (NX1838) in rhesus monkeys,” Journal of Chromatography B, vol. 732, no. 1, pp. 203–212, 1999.
[6]
P. E. Burmeister, S. D. Lewis, R. F. Silva et al., “Direct in vitro selection of a 2′-O-methyl aptamer to VEGF,” Chemistry and Biology, vol. 12, no. 1, pp. 25–33, 2005.
[7]
J. Ju, C. Ruan, C. W. Fuller, A. N. Glazer, and R. A. Mathies, “Fluorescence energy transfer dye-labeled primers for DNA sequencing and analysis,” Proceedings of the National Academy of Sciences of the United States of America, vol. 92, no. 10, pp. 4347–4351, 1995.
[8]
B. Gharizadeh, T. Nordstr？m, A. Ahmadian, M. Ronaghi, and P. Nyrén, “Long-read pyrosequencing using pure 2′-deoxyadenosine-5′-O′-(1-thiotriphosphate) Sp-isomer,” Analytical Biochemistry, vol. 301, no. 1, pp. 82–90, 2002.
[9]
J. Eid, A. Fehr, J. Gray et al., “Real-time DNA sequencing from single polymerase molecules,” Science, vol. 323, no. 5910, pp. 133–138, 2009.
[10]
T. Kajiyama, M. Kuwahara, M. Goto, and H. Kambara, “Optimization of pyrosequencing reads by superior successive incorporation efficiency of improved 2′-deoxyadenosine-5′-triphosphate analogs,” Analytical Biochemistry, vol. 416, no. 1, pp. 8–17, 2011.
[11]
Z. Zhu, J. Chao, H. Yu, and A. S. Waggoner, “Directly labeled DNA probes using fluorescent nucleotides with different length linkers,” Nucleic Acids Research, vol. 22, no. 16, pp. 3418–3422, 1994.
[12]
A. Anne, B. Blanc, and J. Moiroux, “Synthesis of the first ferrocene-labeled dideoxynucleotide and its use for 3′-redox end-labeling of 5′-modified single-stranded oligonucleotides,” Bioconjugate Chemistry, vol. 12, no. 3, pp. 396–405, 2001.
[13]
J. A. Piccirilli, S. E. Moroney, and S. A. Benner, “A C-nucleotide base pair: methylpseudouridine-directed incorporation of formycin triphosphate into RNA catalyzed by T7 RNA polymerase,” Biochemistry, vol. 30, no. 42, pp. 10350–10356, 1991.
[14]
S. Matsuda, A. A. Henry, and F. E. Romesberg, “Optimization of unnatural base pair packing for polymerase recognition,” Journal of the American Chemical Society, vol. 128, no. 19, pp. 6369–6375, 2006.
[15]
M. Kimoto, T. Mitsui, R. Yamashige, A. Sato, S. Yokoyama, and I. Hirao, “A new unnatural base pair system between fluorophore and quencher base analogues for nucleic acid-based imaging technology,” Journal of the American Chemical Society, vol. 132, no. 43, pp. 15418–15426, 2010.
[16]
T. Ono, M. Scalf, and L. M. Smith, “2'-Fluoro modified nucleic acids: polymerase-directed synthesis, properties and stability to analysis by matrix-assisted laser desorption/ionization mass spectrometry,” Nucleic Acids Research, vol. 25, no. 22, pp. 4581–4588, 1997.
[17]
K. Raines and P. A. Gottlieb, “Enzymatic incorporation of 2'-thio-CTP into the HDV ribozyme,” RNA, vol. 4, no. 3, pp. 340–345, 1998.
[18]
K. Sakthivel and C. F. Barbas III, “Expanding the potential of DNA for binding and catalysis: highly functionalized dUTP derivatives that are substrates for thermostable DNA polymerases,” Angewandte Chemie, vol. 37, no. 20, pp. 2872–2875, 1998.
[19]
R. Padilla and R. Sousa, “Efficient synthesis of nucleic acids heavily modified with non-canonical ribose 2'-groups using a mutant T7 RNA polymerase (RNAP),” Nucleic Acids Research, vol. 27, no. 6, pp. 1561–1563, 1999.
[20]
N. K. Vaish, A. W. Fraley, J. W. Szostak, and L. W. McLaughlin, “Expanding the structural and functional diversity of RNA: analog uridine triphosphates as candidates for in vitro selection of nucleic acids,” Nucleic Acids Research, vol. 28, no. 17, pp. 3316–3322, 2000.
[21]
M. L. Andreola, C. Calmels, J. Michel, J. J. Toulmé, and S. Litvak, “Towards the selection of phosphorothioate aptamers: optimizing in vitro selection steps with phosphorothioate nucleotides,” European Journal of Biochemistry, vol. 267, no. 16, pp. 5032–5040, 2000.
[22]
K. Vastmans, S. Pochet, A. Peys et al., “Enzymatic incorporation in DNA of 1,5-anhydrohexitol nucleotides,” Biochemistry, vol. 39, no. 42, pp. 12757–12765, 2000.
[23]
A. M. Noronha, C. J. Wilds, C. N. Lok et al., “Synthesis and biophysical properties of arabinonucleic acids (ANA): circular dichroic spectra, melting temperatures, and ribonuclease H susceptibility of ANA·RNA hybrid duplexes,” Biochemistry, vol. 39, no. 24, pp. 7050–7062, 2000.
[24]
K. U. Schoning, P. Scholz, S. Guntha, X. Wu, R. Krishnamurthy, and A. Eschenmoser, “Chemical etiology of nucleic acid structure: the α-threofuranosyl-(3'→2') oligonucleotide system,” Science, vol. 290, no. 5495, pp. 1347–1351, 2000.
[25]
H. Sawai, A. N. Ozaki, F. Satoh, T. Ohbayashi, M. M. Masud, and H. Ozaki, “Expansion of structural and functional diversities of DNA using new 5-substituted deoxyuridine derivatives by PCR with superthermophilic KOD Dash DNA polymerase,” Chemical Communications, no. 24, pp. 2604–2605, 2001.
[26]
O. Thum, S. J？ger, and M. Famulok, “Functionalized DNA: a new replicable biopolymer,” Angewandte Chemie, vol. 40, no. 21, pp. 3990–3993, 2001.
[27]
S. E. Lee, A. Sidorov, T. Gourlain et al., “Enhancing the catalytic repertoire of nucleic acids: a systematic study of linker length and rigidity,” Nucleic Acids Research, vol. 29, no. 7, pp. 1565–1573, 2001.
[28]
H. A. Held and S. A. Benner, “Challenging artificial genetic systems: thymidine analogs with 5-position sulfur functionality,” Nucleic Acids Research, vol. 30, no. 17, pp. 3857–3869, 2002.
[29]
R. Padilla and R. Sousa, “A Y639F/H784A T7 RNA polymerase double mutant displays superior properties for synthesizing RNAs with non-canonical NTPs,” Nucleic Acids Research, vol. 30, no. 24, p. e138, 2002.
[30]
S. M. Lato, N. D. S. Ozerova, K. He, Z. Sergueeva, B. R. Shaw, and D. H. Burke, “Boron-containing aptamers to ATP,” Nucleic Acids Research, vol. 30, no. 6, pp. 1401–1407, 2002.
[31]
M. Kuwahara, Y. Takahata, A. Shoji, A. N. Ozaki, H. Ozaki, and H. Sawai, “Substrate properties of C5-substituted pyrimidine 2′-deoxynucleoside 5′-triphosphates for thermostable DNA polymerases during PCR,” Bioorganic and Medicinal Chemistry Letters, vol. 13, no. 21, pp. 3735–3738, 2003.
[32]
T. Tasara, B. Angerer, M. Damond et al., “Incorporation of reporter molecule-labeled nucleotides by DNA polymerases. II. High-density labeling of natural DNA,” Nucleic Acids Research, vol. 31, no. 10, pp. 2636–2646, 2003.
[33]
J. B. J. Pavey, A. J. Lawrence, I. A. O'Neil, S. Vortler, and R. Cosstick, “Synthesis and transcription studies on 5′-triphosphates derived from 2′-C-branched-uridines: 2′-homouridine-5′-triphosphate is a substrate for T7 RNA polymerase,” Organic and Biomolecular Chemistry, vol. 2, no. 6, pp. 869–875, 2004.
[34]
T. Ohbayashi, M. Kuwahara, M. Hasegawa, T. Kasamatsu, T. Tamura, and H. Sawai, “Expansion of repertoire of modified DNAs prepared by PCR using KOD Dash DNA polymerase,” Organic and Biomolecular Chemistry, vol. 3, no. 13, pp. 2463–2468, 2005.
[35]
S. J？ger, G. Rasched, H. Kornreich-Leshem, M. Engeser, O. Thum, and M. Famulok, “A versatile toolbox for variable DNA functionalization at high density,” Journal of the American Chemical Society, vol. 127, no. 43, pp. 15071–15082, 2005.
[36]
V. Kempeneers, M. Renders, M. Froeyen, and P. Herdewijn, “Investigation of the DNA-dependent cyclohexenyl nucleic acid polymerization and the cyclohexenyl nucleic acid-dependent DNA polymerization,” Nucleic Acids Research, vol. 33, no. 12, pp. 3828–3836, 2005.
[37]
M. Kuwahara, J. I. Nagashima, M. Hasegawa et al., “Systematic characterization of 2′-deoxynucleoside- 5′-triphosphate analogs as substrates for DNA polymerases by polymerase chain reaction and kinetic studies on enzymatic production of modified DNA,” Nucleic Acids Research, vol. 34, no. 19, pp. 5383–5394, 2006.
[38]
M. Kuwahara, K. Hanawa, K. Ohsawa, R. Kitagata, H. Ozaki, and H. Sawai, “Direct PCR amplification of various modified DNAs having amino acids: convenient preparation of DNA libraries with high-potential activities for in vitro selection,” Bioorganic and Medicinal Chemistry, vol. 14, no. 8, pp. 2518–2526, 2006.
[39]
N. Inoue, A. Shionoya, N. Minakawa, A. Kawakami, N. Ogawa, and A. Matsuda, “Amplification of 4′-thioDNA in the presence of 4′-thio-dTTP and 4′-thio-dCTP, and 4′-thioDNA-directed transcription in vitro and in mammalian cells,” Journal of the American Chemical Society, vol. 129, no. 50, pp. 15424–15425, 2007.
[40]
C. H. Tsai, J. Chen, and J. W. Szostak, “Enzymatic synthesis of DNA on glycerol nucleic acid templates without stable duplex formation between product and template,” Proceedings of the National Academy of Sciences of the United States of America, vol. 104, no. 37, pp. 14598–14603, 2007.
[41]
H. Sawai, J. Nagashima, M. Kuwahara, R. Kitagata, T. Tamura, and I. Matsui, “Differences in substrate specificity of C(5)-substituted or C(5)-unsubstituted pyrimidine nucleotides by DNA polymerases from thermophilic bacteria, archaea, and phages,” Chemistry and Biodiversity, vol. 4, no. 9, pp. 1979–1995, 2007.
[42]
P. ？apek, H. Cahová, R. Pohl, M. Hocek, C. Gloeckner, and A. Marx, “An efficient method for the construction of functionalized dna bearing amino acid groups through cross-coupling reactions of nucleoside triphosphates followed by primer extension or PCR,” Chemistry, vol. 13, no. 21, pp. 6196–6203, 2007.
[43]
C. G. Peng and M. J. Damha, “Polymerase-directed synthesis of 2′-deoxy-2′-fluoro-β-D- arabinonucleic acids,” Journal of the American Chemical Society, vol. 129, no. 17, pp. 5310–5311, 2007.
[44]
R. N. Veedu, B. Vester, and J. Wengel, “Polymerase chain reaction and transcription using locked nucleic acid nucleotide triphosphates,” Journal of the American Chemical Society, vol. 130, no. 26, pp. 8124–8125, 2008.
[45]
M. Kuwahara, S. Obika, J. I. Nagashima et al., “Systematic analysis of enzymatic DNA polymerization using oligo-DNA templates and triphosphate analogs involving 2′,4′-bridged nucleosides,” Nucleic Acids Research, vol. 36, no. 13, pp. 4257–4265, 2008.
[46]
R. N. Veedu, B. Vester, and J. Wengel, “Efficient enzymatic synthesis of LNA-modified DNA duplexes using KOD DNA polymerase,” Organic and Biomolecular Chemistry, vol. 7, no. 7, pp. 1404–1409, 2009.
[47]
V. Borsenberger, M. Kukwikila, and S. Howorka, “Synthesis and enzymatic incorporation of modified deoxyuridine triphosphates,” Organic and Biomolecular Chemistry, vol. 7, no. 18, pp. 3826–3835, 2009.
[48]
H. Yu, S. Zhang, and J. C. Chaput, “Darwinian evolution of an alternative genetic system provides support for TNA as an RNA progenitor,” Nature Chemistry, vol. 4, no. 3, pp. 183–187, 2012.
[49]
J. F. Milligan, D. R. Groebe, G. W. Witherell, and O. C. Uhlenbeck, “Oligoribonucleotide synthesis using T7 RNA polymerase and synthetic DNA templates,” Nucleic Acids Research, vol. 15, no. 21, pp. 8783–8798, 1987.
[50]
S. O. Gudima, D. A. Kostyuk, O. I. Grishchenko, V. L. Tunitskaya, L. V. Memelova, and S. N. Kochetkov, “Synthesis of mixed ribo/deoxyribopolynucleotides by mutant T7 RNA polymerase,” FEBS Letters, vol. 439, no. 3, pp. 302–306, 1998.
[51]
M. F. Goodman, S. Keener, S. Guidotti, and E. W. Branscomb, “On the enzymatic basis for mutagenesis by manganese,” Journal of Biological Chemistry, vol. 258, no. 6, pp. 3469–3475, 1983.
[52]
R. A. Beckman, A. S. Mildvan, and L. A. Loeb, “On the fidelity of DNA replication: manganese mutagenesis in vitro,” Biochemistry, vol. 24, no. 21, pp. 5810–5817, 1985.
[53]
Y. Lin, Q. Qiu, S. C. Gill, and S. D. Jayasena, “Modified RNA sequence pools for in vitro selection,” Nucleic Acids Research, vol. 22, no. 24, pp. 5229–5234, 1994.
[54]
X. Yang, S. Fennewald, B. A. Luxon, J. Aronson, N. K. Herzog, and D. G. Gorenstein, “Aptamers containing thymidine 3'-O-phosphorodithioates: synthesis and binding to nuclear factor-κB,” Bioorganic and Medicinal Chemistry Letters, vol. 9, no. 23, pp. 3357–3362, 1999.
[55]
A. Shoji, M. Kuwahara, H. Ozaki, and H. Sawai, “Modified DNA aptamer that binds the (R)-isomer of a thalidomide derivative with high enantioselectivity,” Journal of the American Chemical Society, vol. 129, no. 5, pp. 1456–1464, 2007.
[56]
K. Ohsawa, T. Kasamatsu, J. I. Nagashima et al., “Arginine-modified DNA aptamers that show enantioselective recognition of the dicarboxylic acid moiety of glutamic acid,” Analytical Sciences, vol. 24, no. 1, pp. 167–172, 2008.
[57]
M. Li, N. Lin, Z. Huang et al., “Selecting aptamers for a glycoprotein through the incorporation of the boronic acid moiety,” Journal of the American Chemical Society, vol. 130, no. 38, pp. 12636–12638, 2008.
[58]
M. Sassanfar and J. W. Szostak, “An RNA motif that binds ATP,” Nature, vol. 364, no. 6437, pp. 550–553, 1993.
[59]
P. L. Sazani, R. Larralde, and J. W. Szostak, “A small aptamer with strong and specific recognition of the triphosphate of ATP,” Journal of the American Chemical Society, vol. 126, no. 27, pp. 8370–8371, 2004.
[60]
N. K. Vaish, R. Larralde, A. W. Fraley, J. W. Szostak, and L. W. McLaughlin, “A novel, modification-dependent ATP-binding aptamer selected from an RNA library incorporating a cationic functionality,” Biochemistry, vol. 42, no. 29, pp. 8842–8851, 2003.
[61]
D. E. Huizenga and J. W. Szostak, “A DNA aptamer that binds adenosine and ATP,” Biochemistry, vol. 34, no. 2, pp. 656–665, 1995.
[62]
T. R. Battersby, D. N. Ang, P. Burgstaller et al., “Quantitative analysis of receptors for adenosine nucleotides obtained via in vitro selection from a library incorporating a cationic nucleotide analog,” Journal of the American Chemical Society, vol. 121, no. 42, pp. 9781–9789, 1999.
[63]
M. F. Kubik, A. W. Stephens, D. Schneider, R. A. Marlar, and D. Tasset, “High-affinity RNA ligands to human α-thrombin,” Nucleic Acids Research, vol. 22, no. 13, pp. 2619–2626, 1994.
[64]
R. White, C. Rusconi, E. Scardino et al., “Generation of species cross-reactive aptamers using “toggle” SELEX,” Molecular Therapy, vol. 4, no. 6, pp. 567–574, 2001.
[65]
N. Minakawa, M. Sanji, Y. Kato, and A. Matsuda, “Investigations toward the selection of fully-modified 4′-thioRNA aptamers: optimization of in vitro transcription steps in the presence of 4′-thioNTPs,” Bioorganic and Medicinal Chemistry, vol. 16, no. 21, pp. 9450–9456, 2008.
[66]
P. E. Burmeister, C. Wang, J. R. Killough et al., “2′-deoxy purine, 2′-O-methyl pyrimidine (dRmY) aptamers as candidate therapeutics,” Oligonucleotides, vol. 16, no. 4, pp. 337–351, 2006.
[67]
L. C. Bock, L. C. Griffin, J. A. Latham, E. H. Vermaas, and J. J. Toole, “Selection of single-stranded DNA molecules that bind and inhibit human thrombin,” Nature, vol. 355, no. 6360, pp. 564–566, 1992.
[68]
D. M. Tasset, M. F. Kubik, and W. Steiner, “Oligonucleotide inhibitors of human thrombin that bind distinct epitopes,” Journal of Molecular Biology, vol. 272, no. 5, pp. 688–698, 1997.
[69]
J. A. Latham, R. Johnson, and J. J. Toole, “The application of a modified nucleotide in aptamer selection: novel thrombin aptamers containing 5-(1-pentynyl)-2'-deoxyuridine,” Nucleic Acids Research, vol. 22, no. 14, pp. 2817–2822, 1994.
[70]
J. D. Vaught, C. Bock, J. Carter et al., “Expanding the chemistry of DNA for in vitro selection,” Journal of the American Chemical Society, vol. 132, no. 12, pp. 4141–4151, 2010.
[71]
M. T. Stubbs and W. Bode, “The clot thickens: clues provided by thrombin structure,” Trends in Biochemical Sciences, vol. 20, no. 1, pp. 23–28, 1995.
[72]
I. R. Krauss, A. Merlino, C. Giancola, A. Randazzo, L. Mazzarella, and F. Sica, “Thrombin-aptamer recognition: a revealed ambiguity,” Nucleic Acids Research, vol. 39, no. 17, pp. 7858–7867, 2011.
[73]
S. M. Nimjee, S. Oney, Z. Volovyk et al., “Synergistic effect of aptamers that inhibit exosites 1 and 2 on thrombin,” RNA, vol. 15, no. 12, pp. 2105–2111, 2009.
[74]
S. B. Long, M. B. Long, R. R. White, and B. A. Sullenger, “Crystal structure of an RNA aptamer bound to thrombin,” RNA, vol. 14, no. 12, pp. 2504–2512, 2008.
[75]
K. Padmanabhan and A. Tulinsky, “An ambiguous structure of a DNA 15-mer thrombin complex,” Acta Crystallographica Section D, vol. 52, no. 2, pp. 272–282, 1996.
[76]
L. Gold, D. Ayers, J. Bertino et al., “Aptamer-based multiplexed proteomic technology for biomarker discovery,” PLoS ONE, vol. 5, no. 12, Article ID 15004, 2010.
[77]
M. Kuwahara, Y. Takano, Y. Kasahara et al., “Study on suitability of KOD dna polymerase for enzymatic production of artificial nucleic acids using base/sugar modified nucleoside triphosphates,” Molecules, vol. 15, no. 11, pp. 8229–8240, 2010.
[78]
P. Wittung, P. E. Nielsen, O. Buchardt, M. Egholm, and B. Norden, “DNA-like double helix formed by peptide nucleic acid,” Nature, vol. 368, no. 6471, pp. 561–563, 1994.
[79]
J. Summerton, D. Stein, S. B. Huang, P. Matthews, D. Weller, and M. Partridge, “Morpholino and phosphorothioate antisense oligomers compared in cell-free and in-cell systems,” Antisense and Nucleic Acid Drug Development, vol. 7, no. 2, pp. 63–70, 1997.
[80]
V. B. Pinheiro, A. I. Taylor, C. Cozens et al., “Synthetic genetic polymers capable of heredity and evolution,” Science, vol. 336, no. 6079, pp. 341–344, 2012.
[81]
F. J. Ghadessy, J. L. Ong, and P. Holliger, “Directed evolution of polymerase function by compartmentalized self-replication,” Proceedings of the National Academy of Sciences of the United States of America, vol. 98, no. 8, pp. 4552–4557, 2001.
[82]
S. D. Mendonsa and M. T. Bowser, “In vitro evolution of functional DNA using capillary electrophoresis,” Journal of the American Chemical Society, vol. 126, no. 1, pp. 20–21, 2004.
[83]
X. Lou, J. Qian, Y. Xiao et al., “Micromagnetic selection of aptamers in microfluidic channels,” Proceedings of the National Academy of Sciences of the United States of America, vol. 106, no. 9, pp. 2989–2994, 2009.
