## 4 Discussion
Hardness testing revealed strong integration of the sulfonated layer of PEEK with the natural PEEK structure, as no delamination or cracking was seen in Figure 8. Expulsion testing, Figure 9, revealed the strong interlocking ability of the roughened porous surface on the sfPEEK and sfPEEK-HT surfaces, as both required significantly more force for expulsion than PEEK. [PERSON] et al. tested expulsion on porous PEEK with ridges and flat porous PEEK and determined that the flat porous specimen type created the maximum contact area between the material surface and bone, thus increasing the required expulsion force [44]. The authors reported that the porosity on the PEEK material had an average strut spacing of 169-248 \(\upmu\)m and strut thickness of 73-119 \(\upmu\)m, with an average expulsion force of 466 N compared to 271 N for normal PEEK [44]. Another expulsion study with PEEK showed that smooth surface PEEK had an average expulsion force of 362 N and macro-patterned PEEK had an average expulsion force of 474 N [28]. There are few published studies on the expulsion of titanium interbody implants. One study showed that the cage insertion technique significantly affects the expulsion force [53]. This study demonstrated that pre-assembled cages had a substantially higher expulsion force of 392 \(\pm\) 19 N compared to cages assembled before insertion, with an expulsion force of 287 \(\pm\) 16 N [53]. Another study compared an enhanced expulsion-proof intervertebral fusion device (EEIFD) to traditional transforminal lumbar interbody fusion device (TTLID) [54]. Their analysis showed that the EEIFD with a blade rotated to hold the device in position had an expulsion force of 534 \(\pm\) 21 N, the EEIFD with the blade not rotated had an expulsion force of 476 \(\pm\) 24 N, and the TTLID had an expulsion force of 444 \(\pm\) 12 N [54]. Our study shows that both sulfonated PEEK surfaces had higher expulsion forces (605 N for sfPEEK-HT and 495 N for sfPEEK) compared to the studies with porous PEEK cages, macro-patterned PEEK, the pre-assembled cages, and the EEIFD, showing that the surfaces created through the sulfonation process in this study result in superior surfaces to resist expulsion. Furthermore, the smooth PEEK expulsion forces in this study (290 N) are very similar to those of the smooth PEEK specimens in comparative research. Live/dead imaging, shown in Figure 10, of the cells on day 7 revealed successful attachment and growth of cells on all surfaces, as indicated by the green staining and spreading of the cells on day 21. No cell death was seen from the live/dead imaging. The same can be seen on the surfaces at day 21, but more cell coverage and confluency are present for the sfPEEK and sfPEEK-HT specimens compared to PEEK. This confluency indicates the porous surface favors cell attachment and proliferation. This data aligns well with our previous research on sulfonated and hydrothermally treated PEEK surfaces, demonstrating higher cell viability, proliferation, differentiation, and mineralization of MCT3-E1 mouse pre-osteoblastic cells [49]. [PERSON] et al. studied the osteogenic capabilities of PEEK (PK), sulfonated hydrothermally treated PEEK (SPK), and modified sulfonated PEEK, finding results similar to those of this current study [60]. The authors discovered that SPK exhibited significantly higher cell proliferation rates after 1 and 7 days of culture than normal PEEK and demonstrated greater mineralization after 21 days, as determined by Alizarin staining [60]. This study indicated that the porous structure created by the sulfonation treatment promoted the MC3T3-E1 cells to higher adhesion and proliferation, as was shown in the current study and our past research [49; 60]. However, the researchers noted that SPK displayed higher levels of ALP activity on days 7 and 14 compared to PK, which contradicted previous research findings [49; 60]. [PERSON] et al. examined PEEK, sulfonated PEEK (SP), and sulfonated PEEK that had been hydrothermally treated (SPW25 and SPW120) [32].