Project Objectives

What are Hydrogels and Why are they Important?

  • Three-dimensional, hydrophilic, polymeric networks capable of absorbing large amounts of water or biological fluids
  • Due to their high water content, porosity and soft consistency, they closely simulate natural living tissue
  • Hydrogels are often reversible and can be dissolved by changing environmental conditions, such a pH, and the ionic strength of solution or temperature
  • This project used a pH sensitive hydrogel, with the main component being acrylic acid. The poly acrylic acid (PAA) makes the hydrogel extremely acidic, so any slight increase in the pH of the gel will cause it to swell

The porous qualities of a superabsorbent hydrogel allows for it to expand and swell, which is conducive to being used in a drug delivery system

What are WGM Resonators?

  • Micro resonators are a fundamental structure in optical biosensing
  • They are micrometer-scale structures for confining light that reflect light internally at the edges of the resonator, which creates a series of standing-wave optical modes, or resonances
  • Whispering gallery mode resonators are toroidal shaped and function like the famous whispering gallery in St. Paul’s Cathedral
  • For this experiment the resonator acts as the mechanism to transport the drug by containing it in the hydrogel. As the hydrogel swells the resonator utilizes the wavelength network to control the release of the drug
  • The resonators used in this experiment had a diameter of 80𝜇m and the pinched “waist” of the toroid had a circumference of 40𝜇m

This flow chart breaks down the basic chain of events in optical biosensing.

Understanding the Quality Factor

  • One of the key characteristics that a WGM resonator must have in order to be considered a viable material for biomedical chips is it must have a high quality factor
  • The quality factor, or Q factor, is a dimensionless parameter that describes how under-damped an oscillator or resonator is, and characterizes a resonator’s bandwidth relative to its center frequency
    • Higher Q values indicates a lower rate of energy loss relative to the stored energy of the resonator
    • A high Q factor for a resonator in air is to the magnitude of 108-109, ideally when in the hydrogel a Q factor to the magnitude of 106 was desired

The graph depicts the bandwidth of a dampened oscillator. As the bandwidth decreases, the quality (Q) factor increases, which indicates more energy being stored in the resonator