Load-dependent collagen fiber architecture data of representative bovine tendon and mitral valve anterior leaflet tissues as quantified by an integrated opto-mechanical system

The data presented in this article provide load-dependent collagen fiber architecture (CFA) of one representative bovine tendon tissue sample and two representative porcine mitral valve anterior leaflet tissues, and they are stored in a MATLAB MAT-file format. Each dataset contains: (i) the number of pixel points, (ii) the array of pixel's x- and y-coordinates, (iii) the three acquired pixel intensity arrays, and (iv) the Delaunay triangulation for visualization purpose. This dataset is associated with a companion journal article, which can be consulted for further information about the methodology, results, and discussion of the opto-mechanical characterization of the tissue's CFA's (Jett etal. [1]).


a b s t r a c t
The data presented in this article provide load-dependent collagen fiber architecture (CFA) of one representative bovine tendon tissue sample and two representative porcine mitral valve anterior leaflet tissues, and they are stored in a MATLAB MAT-file format. Each dataset contains: (i) the number of pixel points, (ii) the array of pixel's x-and y-coordinates, (iii) the three acquired pixel intensity arrays, and (iv) the Delaunay triangulation for visualization purpose. This dataset is associated with a companion journal article, which can be consulted for further information about the Degree of optical anisotropy Heart valve Load-dependence methodology, results, and discussion of the opto-mechanical characterization of the tissue's CFA's (Jett et al. [1] [1]. (This Data-in-Brief submission is a co-submission of the research article.)

Value of the Data
Describing the load-dependence of local, pixel-wise collagen fiber architectures (CFAs) in uniaxially-loaded tendon and biaxially-loaded mitral valve tissues Providing novel information about the tissue microstructures by examining the differences between unloaded and mechanically-loaded tissues Permitting researchers to build predictive models relating bulk mechanical loading to local microstructural changes in soft collagenous tissues Facilitating ongoing/future investigations of the spatial heterogeneity of mitral valve leaflet tissues microstructural responses to loads, improving understanding of the tissue's physiological behaviors Providing new research opportunities to the development tissue-engineered protheses, such as for heart valve surgical replacement, by mimicking the tissue mechanics and microstructure

Tissue retrieval and storage
Bovine tendon and porcine hearts were acquired from a local USDA-approved slaughterhouse (Country Home Meats Co., Edmond, OK) and frozen in a standard freezer at À20 C for storage purpose. Previous studies have shown effectiveness of this tissue storage protocol for maintaining the tissue integrity, microstructures and mechanics [2e7].

Tissue dissection and preparation
For tendon tissue preparation, the central region of bovine tendon was excised into thin tissue sample (width¼15 mm, length¼40 mm, thickness¼1.25 mm), with care taken to exclude the synovial sheath membrane enclosing the tendon and align the strip length direction with the native tendon axis (Fig.1a). For leaflet acquisition, porcine hearts were slowly thawed in a saline bath at room temperature and were then dissected to obtain the mitral valve anterior leaflet (MVAL) tissue specimens with an effective testing size of 10Â10 mm (Fig. 2a). The dissected tissue samples were placed in a labelled container of phosphate-buffered saline (PBS), and stored in a refrigerator at 4 C until testing (within two days).

Opto-mechanical testing e polarized spatial frequency domain imaging of the tissue samples
For the quantification of the load-dependent collagen fiber architecture (CFA) of both the bovine tendon and MVAL tissue specimens, an integrated instrument (Fig. 4a), which combines a commercial   biaxial mechanical testing system (BioTester, CellScale, Canada) and an in-house polarized spatial frequency domain imaging (pSFDI) device, was used.
In brief, the bovine tendon sample was then mounted to the BioTester via the CellScale clamp mounting fixture and subjected to various longitudinal strains (0%, 1%, 2% and 3%) along the tendon tissue's length direction (Fig. 1a). At each strain state, the collagen fiber orientation and the DOA within the sample were quantified using the integrated instrument with a spatial frequency of f x ¼0.20 mm À1 . Sample hydration was maintained by soaking the sample in PBS solution during the imaging tests.
For mitral valve leaflet testing, the entire anterior leaflet tissue samples were excised from the porcine mitral heart valve were mounted to the BioTester using the CellScale BioRakes fixture to create an effective testing region of 10Â10 mm (Fig. 2a). The MVAL tissue sample's circumferential and radial directions were aligned with the x-and y-axes of the tester, respectively, during mounting. The tissues were then immersed in a PBS solution at 37 C for the duration of mechanical testing to emulate the valve's physiological conditions. Prior to applying the mechanical testing protocols, the tissue samples  were preconditioned to restore their in vivo functional state using a standard force-controlled preconditioning protocol with a targeted maximum force of 1 N applied in both the circumferential and radial directions associated with the tissue's collagen fiber networks [8,9]. The targeted loading of 1 N was determined based on an assumed physiological membrane tension of 100 N/m [10,11] and a 10 mm effective edge length. The MVAL tissue sample was subjected to various biaxial loads: T circ :T rad ¼1:1 (equibiaxial loading), T circ :T rad ¼1:0.25, and T circ :T rad ¼0.25:1, where T circ and T rad are the membrane tensions applied in the MVAL tissue's circumferential and radial directions, respectively. During pSFDI imaging tests, a spatial frequency of f x ¼0.27 mm À1 was adopted.

pSFDI imaging procedure
The pSFDI imaging technique combines the ability of co-polarized imaging to quantify the birefringent fiber structures with the depth-discrimination capabilities of SFDI. Interested readers can refer to more details in Refs. [1,12e14]. The pSFDI system (Fig. 4b) utilized an LED-driven, micromirror-based pattern projection system (Texas Instruments, Dallas, TX) with a projection wavelength of 490 nm (cyan) and a 5-Megapixel CCD camera (Basler, Germany) with lens of f/1.9 and an exposure time of 50 ms. For controlled rotational polarization, our pSFDI system employed a nanoparticle linear polarizer with a diameter of 25 mm mounted into a rotational servo motor with a 0.1 resolution (Thorlabs, Newton, NJ). During pSFDI imaging, three phase-shifted images were projected sequentially, through a polarizer at angle q polarizer , and onto the tissue sample. The reflected light from the sample passed back through the same polarizer and was captured by the CCD camera. This projection-capture sequence was repeated at each of the 37 discrete polarization increments (5 increments from 0 to 180 ) using an in-house LabView controlling program (National Instruments, Austin, TX).
2.5. pSFDI image data analysis e quantification of fiber orientation angle q fiber and degree of optical anisotropy (DOA) After pSFDI imaging, the 37 phase-shifted images were first smoothed via convolution with a normalized 5 Â 5 uniform kernel and were then combined at each pixel and polarization state to obtain the resultant DC and AC intensities: the DC intensity I DC which provides equal weighting for each reflected photon by representing the conventional diffuse reflectance image, and the AC intensity I AC , which signifies the differences between the spatially-modulated intensity patterns.
; and I AC ¼ ffiffiffi 2 p 3 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ðI 0 À I 120 Þ 2 þ ðI 120 À I 240 Þ 2 þ ðI 240 À I 0 Þ 2 q : Herein, I 0 , I 120 , and I 240 are the pixel-wise intensity corresponding to the three phase shifts, respectively. The global maxima of these intensity functions occurs when the polarizer transmission axis q polarizer is parallel to and perpendicular to the fiber orientation angle q fiber , respectively (Fig. 1b).
Quantitatively, the birefringent reflected intensity I out of a group of collagen fibers (Fig. 4b) can be described by the following 3-term Fourier cosine series: where t sys is a bulk systemic coefficient encompassing non-birefringent intensity modifiers, such as the aperture of the camera, and a 0 , a 2 , and a 4 are the three Fourier coefficients. The magnitudes of the optical anisotropies provide a means of quantitatively examining the local dispersion of the collagen fibers, which is expressed in the degree of optical anisotropy (DOA), i.e., DOA ¼ a 2 þ a 4 a 0 þ a 2 þ a 4 : Please refer to more details about the step-by-step algorithmic procedures in Section 2.3 of [1].