## 3 Results and Discussion
### X-ray Photoelectron Spectroscopy (XPS)  
The presence of a Ca2p photoelectron peak in the XPS spectra of as-received PLA spool material, Figure 9, indicates the presence of calcium (at least in the surface region). The high-resolution scan of the Ca2p photoelectron region is consistent with CaCO\({}_{3}\) (Ca 2p\({}_{3/2}\)/Ca 2p\({}_{1/2}\) binding energy of 346.9 eV/350.8 eV). The C 1 s peak deconvolution shows the presence of C-C/C-H at 285.3 eV, C-OH at 286.3 eV, and O-C=O at 289.3 eV. These peaks are consistent with PLA, as well as carbonate, though the high intensity of the peak at 285.3 eV most likely represents additional hydrocarbons present on the surface of the spool material. These hydrocarbons can be associated with a coating layer used in storage, winding, or handling during the manufacturing process. It is believed that calcium carbonate in the PLA spool material is also due to the manufacturing process as a component to aid in the extrusion and overall stability of the PLA filament. Figure 8: Differential scanning calorimetry (DSC) comparison of PLA spool material and printed PLA. XPS analysis of printed PLA, Figure 10, indicates that calcium carbonate remains present on the surface of the despite extrusion conditions (melting at 230 \({}^{\circ}\)C and cooling rapidly to room temperature following printing). The C 1 s spectrum deconvolution shows a ratio of C-C/C-H (284.6 eV), C-OH (286.6 eV), and O-C=O (288.8 eV) closer to what one would expect from PLA with some additional enrichment of the higher energy peak due to the remaining presence of CO\({}_{3}\). However, the C 1 s spectrum from printed PLA now also shows a peak at 291.7 eV. We attributed this new peak to CO\({}_{2}\), which was seen to have a slightly elevated presence in the FTIR spectrum from printed PLA. While some CO\({}_{2}\) is noted by FTIR due to normal atmospheric conditions (as in the case of most materials) the presence under UHV (ultra-high vacuum) conditions (10\({}^{-9}\)-10\({}^{-1}\) torr) during XPS indicates some additional incorporation and sorption in the surface layer of the printed PLA. This is likely due to the decomposition of CaCO\({}_{3}\):  
\[\text{CaCO}_{3}\leftrightarrow\text{CaO}+\text{CO}_{2} \tag{1}\]  
Therefore, the increase in CO\({}_{2}\) seen in both the FTIR and this XPS spectra indicates that calcium carbonate is breaking down slightly and, due to the porous nature of printed PLA, it is likely becoming trapped (and subsequently chemisorbed) in the pores and voids of the material. This is consistent with results observed from laser confocal microscopy of the printed materials. In general, our results have shown that heating, melting, and recrystallization of the PLA as a function of the printing process has drastically changed the nature of the PLA, making it stronger (via tensile testing measurements) and more chemically reactive (indicated by the increase in abundance of the C=O ligand). However, while this more chemically reactive nature is of value for in situ synthesis and stabilization of metal nanoparticles [28], or for use as support structure for growth and calcification of cells, it also results in some disadvantages which can impact its use in medical and other settings, such as increased susceptibility to water, alcohol, acetone, and heat. Water will decompose the material over time and alcohols, acetone, and heating will degrade the printed material as well [29; 30; 31; 32; 33]. This means that sterilization with alcohol, typical in medical use, may alter the chemistry of FDM-printed  
Figure 9: XPS spectra of PLA spool material as received, showing the wide-scan spectra and analysis of the C 1s and Ca 2p photoelectron peaks. PLA samples. Additionally, PLA cannot be sterilized in an autoclave as both chemical alteration and degradation of the PLA will result. However, future testing of the PLA materials will look into using an FDA (Food and Drug Administration)-approved gas (ethylene oxide) used for sterilization of medial materials that cannot be subjected to alcohols or heat.