## 3 Results
### Bioprinting Publications and Biotechnology Readiness Level (Figure 4)  
TRLs adaptations have been developed for all types of medical products, including pharmaceuticals (drugs, biologics, vaccines), medical devices, etc. The concept is that when a critical technology is  
Figure 3: Temporal evolution of keyword co-occurrence networks between 2006 and 2016 with a time interval of 5 years, (**a**): 2006–2010; (**b**): 2011–2015. The size of each node is proportionate to its degree and the thickness of the links represents the tie strength.  
incorporated into the fabrication of a product, the probability for a successful outcome is great when the level of readiness of the technologies is high. We analyzed the distribution of bioprinting original articles according to the bioprinting technology readiness level (BTRL), as described in the M&M section (Table 1 and Figure 4). The publication scale ranged from \"in vitro experiments\" to \"industrial commercialization\", mimicking the TRL. Over the decade, the groups were highly disproportioned. Group 1 (in vitro papers) accounted for 92% of the bioprinting original articles, when groups 2 and 3 (in vivo papers) represented respectively 7.3% and 0.6% (one article). There was not a single paper from group 4. Interestingly, the group 1 proportion increased with time, but not the in vivo groups. These results confirmed the actual lack of readiness of the bioprinting for medical applications and called for further analyses about the number of research actors.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Technology Readiness Level** & **Level** & **Biopernting Readiness Level** \\ \hline Basic principles observed and reported & 1 & Literature reviews and initial market surveys \\ \hline Technology concept and/or application formulated & 2 & Research plans and protocols are developed, peer reviewed, and approved. \\ \hline Analytical and experimental critical function & 3 & In vitro: Initial proof-of-concept is demonstrated in a limited number of laboratory models. \\ and/or characteristic proof of concept & 3 & In vitro: Proof-of-concept and safety of candidate systems demonstrated in defined laboratory models. \\ \hline Component and/or breadboard validation in & 4 & In vitro: Proof-of-concept and safety of candidate systems demonstrated in defined laboratory models. \\ \hline Component and/or breadboard validation in & 5 & In vivo animal: devices tested through simulation, in tissue or organ models, or animal models if required. Initial testing is complete. \\ \hline System/subsystem model or prototype & 6 & In vivo human: Clinical trials conducted to demonstrate safety of candidate medical device in a small number of humans. \\ \hline System prototype demonstration in an & 7 & In vivo human: Clinical safety and effectiveness trials conducted with a fully integrated medical device prototype in an operational environment. \\ \hline Actual system completed and qualified through & 8 & In vivo human: Implementation of clinical trials to gather information relative to the safety and effectiveness of the device. \\ \hline Actual system proven through successful & 9 & The medical device may be distributed/marketed. Follow-up after commercialization \\ \hline \hline \end{tabular}
\end{table}
Table 1: Bioprinting technology readiness level (BTRL) scale, adapted from the Medical Devices scale of the Medical Research and Materiel Command (MRMC) [18].  
Figure 4: Distribution of bioprinting original articles according to the bioprinting technology readiness level (BTRL). The 2006–2015 bioprinting articles were analyzed and distributed into 4 groups: 1 (in vitro, BTRL 1–4), 2 (animal-in vivo, BTRL 5), 3 (human-clinical grade, BTRL 6–8) and 4 (industrial, BTRL 9). There has never been any paper in group 4.