利用DNA建造工厂以及电脑(2)

There  is  a  problem,  however,  in  making  more  complicated  components.  To  keep  negatively-charged  DNA  stable,  researchers  add  positive  ions  to  their  solutions.  But  these  ions  can  interfere  with  the  functional  materials  needed  to  build  electronics.

"It  is  difficult  to  keep  all  these  things  happy  at  the  same  time,"  LaBean  says.

A  solution  might  be  to  use  a  DNA-like  molecule  that  is  uncharged  and  yet  has  the  same  code  as  DNA.  There  are  about  1000  "flavors"  of  DNA  derivatives,  Seeman  says,  so  one  of  these  might  do  the  trick.

Trouble  is  these  alternatives  can  be  10  times  more  expensive  to  make  than  regular  DNA,  according  to  LaBean.  It  could  be  worth  it,  however,  as  computer  chip  manufacturing  techniques  currently  cannot  go  smaller  than  tens  of  nanometers.

Self-assembling  arrays  of  DNA-like  molecules  could  go  beyond  this  limitation,  by  providing  the  scaffolds  for  nanometer-scale  circuits.  This  would  not  only  make  our  computers  and  other  devices  more  compact,  but  faster  as  well.

Nano  robots

Besides  controlling  the  shape  of  DNA  assemblages,  researchers  can  use  specific  DNA  attachments  to  move  other  DNA  molecules.

One  of  the  first  demonstrations  of  this  came  in  2000,  when  a  group  from  Lucent  Technologies  in  New  Jersey  fabricated  a  short  V-shaped  DNA  molecule  that  acted  like  molecular  tweezers.

Placing  several  copies  of  their  molecule  in  solution,  the  researchers  could  snap  the  tweezers  shut  by  mixing  in  another  DNA  molecule,  called  a  "set  strand,"  that  bonds  specifically  to  the  two  ends  of  the  "V"  and  pulls  it  closed.  To  reopen  the  tongs,  the  science  team  added  an  "unset  strand,"  which  links  to  the  set  strand  and  pulls  it  off  the  tweezers.

Using  a  similarly  orchestrated  movement,  Seeman  and  his  colleagues  in  2004  made  a  two-legged  DNA  molecule  that  could  walk.  The  feet  were  anchored  to  a  DNA-studded  floor  by  set  strands.  The  tiny  biped  took  a  step  whenever  the  group  introduced  unset  strands  that  freed  one  leg  at  a  time.

Assembly  line

More  recently,  Seeman  and  colleagues  have  put  DNA  robots  to  work  by  incorporating  them  into  a  self-assembling  array.  The  composite  device  grabs  various  molecular  chains,  or  "polymers,"  from  a  solution  and  fuses  them  together.  By  controlling  the  position  of  the  nano-bots,  the  researchers  can  specify  the  arrangement  of  the  finished  polymer.

Seeman  hopes  this  tiny  assembly  line  can  be  expanded  into  nano-factories  that  would  synthesize  whole  suites  of  polymers  in  parallel.  The  major  challenge  now  is  going  from  2D  arrays  to  3D  structures.  The  extra  dimension  would  allow  the  fabrication  of  more  elaborate  molecules,  as  well  as  denser  electronic  circuits.

In  the  future,  doctors  might  inject  variants  of  these  automated  DNA  machines  into  the  body,  either  as  bio-sensors  or  as  drug  delivery  systems  that  can  target  specific  sites  like  tumors  or  blood  clots,  LaBean  said.

Although  some  of  these  applications  may  be  several  years  down  the  road,  progress  in  DNA  nanotech  "has  become  a  lot  faster  now  that  there  are  20  or  more  groups  doing  it  rather  than  just  my  own,"  Seeman  said. (责任编辑：泉水)

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利用DNA建造工厂以及电脑(2)