Harris, NR

Hill, M

Townsend, RJ

English

Southampton (e-Prints Soton)

Ultrasonic Force Ultrasonic Force- -fields for the  fields for the Micro Micro- -manipulation of Cells  manipulation of Cells Rosemary J Townsend1, Martyn Hill1, Nicholas R Harris21School of Engineering Sciences, 2Electronics and Computer Science, University of Southampton, Southampton, SO17 1BJ, UK.  Symmetry planeReflectorWater layerFigure 8: Cross-section of particle separator illustrating the yz plane simulated.ACKNOWLEDGEMENTSThis work was funded through EPSRC grant GR/R13333/01  with additional support from Dstl, Porvair Plc and EPSRC network grants (USWNet).  Some of this work was completed in collaboration with Cardiff University and the University of Manchester.INTRODUCTION TO ACOUSTIC FORCESUniversity of SouthamptonGlass reflectorWaterAcoustic fieldNodal planePiezoelectric TransducerFigure 1:  Schematic showing how particles move to the nodal plane of an acoustic standing wave.APPLICATIONSThis  phenomenon  is  being  used  to  control  the  movement  of particles  and  cells  for  various  bio-sensing  and  life  science applications.  For example:Biohazard detection          Tissue engineeringWe  are  developing  ultrasonic  devices  to  develop  these applications.    Such  devices  can  be  realised  using  micro-engineering  fabrication  techniques  and  can  ultimately  be incorporated  into  Micro-total-analysis-systems (µTAS)  for rapid processing and sensing of small fluid samples.   This is one of the most rapidly advancing and exciting fields in sensing andanalysis.BiohazardsporesPiezoelectric TransducerSensor surfaceGlass reflectorLiver cellsPiezoelectric TransducerGlass reflectorFigure 3: Pushing spores up to a surface where they can be detected and the environment monitored.Figure 4:  Agglomerating liver cells and controlling their growth for artificial organ development.NUMERICAL SIMULATIONSWhere  particles  are  to  be  forced  towards  the  reflector surface, acoustic simulations programmed in MATLAB show that the reflector thickness has a significant influence on the node position (figure 5).  To get particles on the surface, the node needs  to  be  positioned  within  the  reflector  which  was  only possible  using  reflector  thicknesses  of  ≥1mm.  These  findings are  supported  by  experimental  data  collected  at  Cardiff University  (figure  6)  which  shows  that  the  proportion  of particles captured on the surface is low for thicknesses <1mm.  Predicted  capture  was  calculated  by  simulation  particle trajectories as they pass through the acoustic field.Figure 5:  Pressure field in biohazard detection system for a range of reflector thicknesses.OUR RESEARCH – PREDICTING THE MOVEMENT OF PARTICLESThe challenge is to design reliable ultrasonic devices which can successfully control particle movement.  The movement of particles depends upon the 3-dimensional acoustic and fluid characteristics of the device.  We use numerical models to predict the exact nature of the acoustic field and fluid flow within these devices, then the resulting particle trajectories can be predicted.  The influence of device geometry and materials used has been assessed and experimentally validated, and reveals how sensitive the performance of ultrasonic devices are to geometric changes.Figure 6:  Particle capture in biohazard detection system for a range of reflector thicknesses (■ - modelled, ♦ - measured).0204060801001201401601802000.94 0.96 0.98 1 1.02 1.04 1.06Slide depth (mm)Capture per image05101520253035404550% Particle CaptureAs well forming a standing wave between the transducer and reflector, a standing wave may also form across the width  of  the  water  layer.    This  makes  particles agglomerate into striations (figure 7).  To understand the causes of these lateral standing waves, a finite element analysis  of  one  of  our  device  was  carried  out.    This predicted  that  the  side-walls  induce  a  complex  2-d acoustic field resulting in striations with similar spacing to that observed (figure 9).  Further analysis has shown that the side wall geometry has a strong influence upon the occurrence of these striations.Side wallFigure 7:  Striations across the width of water layer (looking down through glass reflector).Particles  within  an  ultrasonic standing  wave  experience acoustic radiation forces.  These forces  move  the  particles  to acoustic  nodes  within  the acoustic  field,  as  demonstrated schematically  in  Figure  1.    The particles are typically held within a layer of water, across which an acoustic  standing  wave  is generated.  The acoustic source is  a  piezo-electric  transducer, and a glass layer is typically used to reflect the acoustic wave and help  form  the  standing  wave.    Figure 2 illustrates how this has been  used  to  separate  particles from the water.TransducerSilicon layerGlass reflectorParticle suspension inClarified fluid outParticles outParticles collecting along nodal planeFigure 2:  Acoustic particle separation in a micro-engineered device.Figure 9: Predicted acoustic pressure showing variations across width of water layer.FUTURE WORKFuture work will look at realising the biohazard detection and tissue growth applications.  This  involves  more  extensive  analysis  of  the  acoustic  field  using  the  techniques described above.  Part of this work aims to maximise the acoustic energy and therefore force acting on a particle, thus making the techniques more viable.  Fabrication of these devices is an issue, too, and new micro-milling facilities at the university are now being used to make such devices.This  technique  lends  itself  to  many  other  applications.    For  example,  Southampton University has EPSRC funding to develop a technique to evaluate DNA sequences using fluorescent markers.    This  relies  on  an  acoustic  field  to  control  the  position  and orientation of the DNA strands attached to microbeads.CONCLUSIONSNumerical  simulations  have  improved  our understanding  of  the  use  of  acoustic  radiation forces.    This  has  helped  us  improve  the  design  of these  ultrasonic  systems;  for  example,  adjusting geometry to reliably position the acoustic node.  Although not described here, similar fluid simulation have been used to improve the  ‘microfluidic’ design of the separator device and uniformity of the fluid flow.

Ultrasonic Force Field for the micro-manipulation of cells

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