Tri-nitro benzene sulfonic acid (TNBS) assay. The degree of modification of collagen furan was quantified using TNBS colorimetric assay as described earlier with minor adjustments. Briefly, 2 mg of dry sample was dissolved in 200 µl of deionized water, followed by mixing with 1 ml of NaHCO3 (4% w/w) and 1 ml of TNBS solution (0.5% w/w in water) at 40 °C under mild shaking and dark condition for 4h. Afterward, 3 mL of HCl (6 м) solution was added to quench the reaction, followed by three times extraction of unreacted TNBS from the sample with diethyl ether. UV absorbance of samples was recorded at 346 nm using a Lambda 35 UV/Vis spectrophotometer (PerkinElmer, Sweden) against a blank prepared by the above procedure, except that the HCl solution was added before the addition of TNBS solution. The same procedure was used for pristine collagen samples which were used as controls. The degree of functionalization was calculated as follows: