Engineering Gradient Biomaterials for Controlled Signaling during Stem Cell Differentiation

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Engineering Gradient Biomaterials for Controlled
Signaling during Stem Cell Differentiation

• Miguel Benson
• Kristin Riching
• Kristen Sipsma
• Kelly Toy

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University of Wisconsin - MadisonBiomedical
Engineering Design Courses

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  Student Design Consortium (SDC).
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  and the Advisor.
• 3) Agrees to keep this information in confidence
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• 4) Continued attendance at this presentation
  constitutes compliance with this agreement.

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Client

• Brian Peret, Graduate Student, Dept. of
  Biomedical Engineering
• William Murphy, Assistant Professor, Dept. of
  Biomedical Engineering

Advisor

• Mitch Tyler, Dept. of Biomedical Engineering

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Problem Statement/Motivation

• Design a mechanism by which a protein gradient
  within a hydrogel can be established and
  controlled
• Control of gradients and concentration of growth
  factors will facilitate the understanding of stem
  cell differentiation

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Design Requirements/Assumptions

• Medium must allow diffusion of proteins
• Source must not deplete during desired time frame
• Release of proteins from source is linear
• Reasonable time frame of diffusion
• Changing parameters need to give results
  predictive of actual diffusion patterns
• Make computer simulation

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Biological Rationale

• SHH and BMP growth factors released by certain
  cells in developing embryo
• Creates concentration gradient for diffusion
• Signals other cells to differentiate into
  specific cell types
• Modeling this gradient can lead to the control of
  stem cell differentiation

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Hydrogel

• Polymer or copolymer infused with water
• Hydrophilic
• Swells when in contact with water
• Cross linking of polymer counteracts swelling
  force
• UV cross linking
• Diffusivity
• Cross linking
• Degree of swelling
• Molecular weight

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Microspheres

• Encapsulate protein or growth factor
• Allow timed release of contents
• Biodegradable
• Biocompatible
• Contents released through both diffusion and
  breakdown of microsphere
• Poly(lactide-co-glycolide)
• Most widely used for biological applications
• Composition affects rate of breakdown

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Diffusion through Permeable Barrier

• Non-encapsulated proteins placed into section of
  hydrogel
• Porous barrier allows for time controlled
  continuous diffusion

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Analysis of Design 1

• Advantage
• Barrier creates constant rate of diffusion
• Disadvantages
• Source of proteins will deplete
• Equilibrium will be reached

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Diffusion from Planar Source of Microspheres

• Even distribution of microspheres along one edge
  of rectangular hydrogel
• Constant protein release from microsphere
• Protein diffuses through hydrogel
• Assume diffusion is one direction

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Analysis of Design 2

• Advantages
• Simplified equations of diffusion
• Constant source concentration
• Allows greater control of release rate and
  initial concentration
• Disadvantages
• Possible conformational change of some proteins
  during microsphere preparation

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Diffusion from Point Source of Microspheres

• Microspheres located at a single point in center
  of hydrogel
• Causes radial diffusion

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Analysis of Design 3

• Advantages
• Proteins move out into a wider area
• Disadvantages
• Two dimensional movement of proteins
• Microspheres must be extremely small
• Difficult to measure gradient

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Design Matrix
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Future Work

• Determine relationships between
• Diffusivity and molecular weight of hydrogel
  polymer
• Diffusivity and cross linking density
• Concentration and distance from source
• Create computer simulation model
• Test using appropriate hydrogel and microspheres
  with fluorescent tags

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• Questions?