Imbibition in Paper-based Microfluidic Devices

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, December 2016
Dr. Hideaki Tsutsui, Chairperson

Point-of-care  technologies  provide  innovative  solutions  that  improved  treatment.  Healthcare systems including some low-resource settings have begun implementing these technologies providing the  convenience  and  reduction  in  large  lab  set-ups.  But  many  low-resource  setting  do  not  have  the means or infrastructure to perform these tests. Low-cost is one of the main driving components when it comes to point-of-care diagnostics. Paper-based microfluidics has generated a great amount of   interest for  the  development  of  diagnostic  and  self-contained  analytical  devices. Satisfying  the  World  Health Organization’s   (WHO)   recommended   ASSURED   criteria;   Affordable,   Sensitive,   Specific, User-friendly,  Rapid  and  robust, Equipment  free,  Deliverable),  paper-based  microfluidics  have  made point-of-care testing more accessible. Applications range from healthcare, food safety, environmental monitoring,  among  others.  What  has  in  part  attracted  attention  is  the  low-cost,  ease-of-use,  and adaptability of these paper devices. Compared to conventional microfluidic devices, the   paper-based counterparts  are  able  to  utilize  paper’s  inherent  wicking  property  to  eliminate  the  external  pumping needed  to  drive  the  fluid.  Channels  are  easily  formed  by  either  selectively  removing  sections  of  the paper substrate or by pattering channel boundaries with a hydrophobic material.

In spite of the benefits and advantages described above, paper-based microfluidic technologies often  lack  the  necessary  sensitivity  and  sophistication  available  in  conventional  microfluidic  devices. In  order  to  be  a  competitive  alternative,  paper-based  microfluidics  require  improvement  and  novel development  of  feasible  detection  methods.  These  methods  will  likely  require  increasingly  complex chemistry  and  control  of  reagents.  Thus,  understanding  imbibition  as  well  as  obtaining  precise, accurate, and consistent fluid handling within the paper device will be crucial.

Although  considerable  knowledge  exists  on  techniques  to  manipulate  fluid  within  the  paper channel, what is lacking are studies on how non-laboratory conditions (e.g. relative humidity) influence fluid  flow.  This  presentation  aims  to  address  this  gap  with  particular  focus  on  the  effects  of  relative humidity and channel width. A series of controlled imbibition experiments is reported using cellulose papers  commonly  used  in  the  field  of  paper-based  microfluidics.  We  show  that  both  the  imposed relative  humidity  and  the  channel  width  have  critical  design  considerations  in  paper-based  devices. Additionally,  we  compare  three  models,  the  L-W  model,  the  Fries  et  al.  (2008)  model  which incorporates evaporation, and a newly developed water saturation model. We assess their accuracy in representing the experimental data and systematically evaluate the importance of evaporation and water saturation under a wide range of relative humidity conditions. The current study has created a library of paper-specific, imbibition-related properties for commonly used  filter and chromatography papers  for the first time.

Collectively, the  success  of  this  research  will  improve  the  development  of  future  diagnostic and analytical paper devices producing a user-friendly and cost effective point-of-care alternative.