Direct writing of calcium phosphate/graphite nanocomposite film using laser-induced graphitization of polyimides

Abstract

We report the direct writing of calcium phosphate/graphite nanocomposite using laser-induced graphitization of suspension-covered polyimide (PI) films. Calcium phosphate/graphite nanocomposites are desirable for the osteoconductive, osteoconductive, and resorptive properties of calcium phosphate and the electrical conductivity of graphitic carbon. Two forms of calcium phosphate, namely, EggCaP and CaOP, were synthesized by chemomechanical alloying of diammonium hydrogen phosphate (DHP) with eggshells – a waste material – and reagent-grade calcium oxide (CaO) powders, respectively. Fourier transform infrared (FTIR) spectroscopy of the synthesized powders revealed the phase change of starting materials to calcium phosphates (CaP), and powders were suspended in methanol to form printing suspension. Laser irradiation of synthesized CaP powder suspension-covered polyimide (PI) surfaces resulted in the formation of electrically conductive films due to laser-induced graphitization of PI. Laser-induced graphited films based on EggCaP suspension (ELIG) and CaOP (CLIG) had a sheet resistance of 25 Ω/∎ and 30 Ω/∎, respectively, and had lower resistance than the LIG films formed from bare PI (60 Ω/∎). Raman spectroscopy revealed that repeated laser irradiation of ELIG and CLIG film led to decreased defects and higher graphitization. ELIG had an ID/IG ratio of 0.65 after the first irradiation, but repeated irradiation reduced the ratio to 0.38. Similarly, CLIG had a higher ID/IG ratio of 0.55 during the first irradiation, and repeated irradiation reduced it to 0.42. Furthermore, CO, CO stretch modes, and various PO4 groups were also found in the Raman spectrum, indicating the formation of CaP/graphite nanocomposites in the graphitized films. SEM results showed that both graphitized films had closed foam-like microstructure with CaP particles deposited on the pore walls. Films formed after single irradiation had discrete CaP particle deposits on the surface, but repeated laser irradiation led to the integration of CaP particles into the pore walls. The ELIG films showed higher incorporation of the CaP particles and the highest electrical conductivity. The laser-induced graphitic transformation leads to incorporating the CaP particles into a conductive substrate, resulting in a CaP/graphite nanocomposite. The high electrical conductivity, closed-foam-like microstructure, porosity, biocompatibility, and hierarchical surface roughness of the ELIG and CLIG films make them attractive for biosensors, bioelectronics, and implant applications.