title: Effects of mechanical indentation on diffuse reflectance spectra, light transmission, and intrinsic optical properties in ex vivo porcine skin.
abstract: OBJECTIVE
  Mechanical indentation has been shown to increase light transmission through turbid tissue. In this study, we investigated the effects of localized indentation on the optical properties of ex vivo porcine skin specimens by dynamically monitoring diffuse reflectance spectra, light transmission, and applied load while controlling tissue thickness.
  
  
  METHODS
  A custom-built diffuse reflectance spectroscopy (DRS) system was used to capture diffuse reflectance spectra from tissue specimens undergoing indentation. The DRS probe was designed to perform both optical sensing and tissue indentation. A mechanical load frame was used to dynamically control probe displacement and resultant specimen thickness change while recording applied load. Diffuse reflectance spectra, as well as light transmission at 630 nm, were recorded during stress relaxation tests where tissue specimens were displaced to and held at a final thickness. Tissue optical properties were extracted from reflectance spectra using a previously established look-up table (LUT) approach.
  
  
  RESULTS
  Indentation increased light transmission through tissue during linear displacement, and continued to increase transmission during subsequent stress relaxation at constant tissue thickness. The magnitude of relative transmission increases was shown to be a function of bulk tissue compressive strain (relative thickness change). Reduced scattering coefficients calculated from the LUT at 630 nm decreased during stress relaxation, with the relative decrease in scattering also depending strongly on tissue compressive strain. Reduced scattering coefficients decreased by 12.0 ± 4.7% at 0.44 ± 0.022 compressive strain, and reduced by 35.6 ± 1.3% at 0.71 ± 0.01 compressive strain.
  
  
  CONCLUSIONS
  DRS can be used to capture transient changes in intrinsic tissue optical properties during mechanical loading. Mechanical indentation modifies tissue optical properties and may be harnessed as a minimally-invasive optical clearing technique to improve optical diagnostics and therapeutics.
authors.affiliation: School of Biomedical Engineering and Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA. wcvogt@vt.edu
authors.forename: William C
authors.initials: WC
authors.lastname: Vogt
authors.forename: Alondra
authors.initials: A
authors.lastname: Izquierdo-Román
authors.forename: Brandon
authors.initials: B
authors.lastname: Nichols
authors.forename: Liang
authors.initials: L
authors.lastname: Lim
authors.forename: James W
authors.initials: JW
authors.lastname: Tunnell
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Rylander
mesh_terms: D000818:Animals; D005336:Fiber Optic Technology; D055620:Optical Phenomena; D011312:Pressure; D012879:Skin Physiological Phenomena; D013057:Spectrum Analysis / Q000295:instrumentation / Q000379:methods; D013314:Stress, Mechanical; D013552:Swine