## 2 Materials and Methods
The stage was then placed into a Cressington sputter coater (Watford WD England, UK), where the samples were placed under a vacuum using argon gas. Samples were then sputter-coated in a mixture of gold and palladium. The sampleswere then imaged on a Hitachi SU8000 field emission scanning electron microscope under a vacuum. **Cell Culture:** The endothelial cells were pooled donor human umbilical vein endothelial cells (HUVECs, Lonza) that were pre-screened for angiogenesis. Endothelial basal medium-2 (EBM-2) was also obtained from Lonza, USA, along with the accompanying BulletKit. Cells were thawed and diluted at a ratio of 1:10 with fresh EGM-2 medium. This cell suspension was then transferred to a T75 flask and placed in a 37 \({}^{\circ}\)C, 5% CO\({}_{2}\) incubator. The medium was refreshed after the first 24 h and then every 48 h. HUVECs typically reached confluency in 3-5 days. All cellular experiments were conducted after 3 days of growth. **Proliferation and Viability Assay:** In a 24-well plate, 15 collagen scaffolds were created with concentrations of 0, 1, 10, 13, and 22 mM genipin. Cells were seeded onto these scaffolds at a density of 60,000 cells per scaffold. Pictures of the cell cultures were taken daily on an EVOS FL digital inverted microscope (Thermo Fisher, Waltham, MA, USA). The number of cells per picture was then quantified using ImageJ (Mad City Labs, Madison, WI, USA) (1.53). The pictures were edited through the subtraction of the image background and thresholding. After analyzing and counting the cells in each picture, the quantitative data were saved into a spreadsheet. This data were then imported into GraphPad Prism software to see the rate of cell proliferation and total cell counts among substrates. The LIVE/DEAD Viability/Cytotoxicity kit (Thermo Fisher, USA) was used to visualize cell viability across the collagen scaffolds. After conducting the proliferation assay, cells were washed of medium with D-PBS. After washing the cells, the assay solution, consisting of 4 uM ethidium homodimer 1 (EthD-1) and 2 uM calcein-am in D-PBS, was dispensed on top of each scaffold, ensuring that all cells were covered. The cells were incubated at room temperature for 45 min and then imaged under the microscope. ImageJ was used to quantify the fluorescence of living cells after using the cytotoxicity kit. Using pictures taken on the microscope, analysis was run--specifically measuring the area and integrated intensity--on both the cells and the background of the image. This was repeated three times per sample, and data were compiled into a spreadsheet. The corrected total cell fluorescence (_CTCF_) was calculated using (1)  
\[CTCF=D_{i}-\left( \begin{array}{cc}A_{cell}&*&F_{bg}\end{array} \right) \tag{1}\]  
where \(D_{i}\) is the integrated density, \(A_{cell}\) is the area of the selected cell(s), and \(F_{bg}\) is the mean background fluorescence. All fluorescence measurements, including _CTCF_, were calculated in relative fluorescence units (RFUs). **Transendothelial electrical resistance (TEER):** Transendothelial electrical resistance (TEER) was used as a nondestructive way to measure barrier integrity. In a 12 Transwell plate, 10-well inserts were coated in control, 1, 10, 13, and 22 mM gel solutions, and polymerized, resulting in two gels per treatment. After washing, a baseline resistance value was taken using an EVOM 2 Voltohnmeter. Cells were seeded at a density of 56,000 cells per scaffold and cultured for 3 days, with their resistance being measured daily. TEER was calculated using (2) and (3) to correct for any resistance due to the substrate. \[R_{tissue}=\left( \begin{array}{cc}R_{total}&-R_{blank}\end{array} \right) \tag{2}\]  
\[TEER_{reported}=R_{tissue}(\Omega)* \begin{array}{cc}M_{tissue}\left(cm^{2} \right)\end{array}  \tag{3}\]  
**Cell morphology Measurement**: Collagen scaffolds were created in the wells of a 96-well plate at concentrations of 0, 1, 10, 13, and 22 mM genipin.