# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 10 Conclusions  
In this review, special attention was given to unobtrusive and no-contact sensors for vital sign monitoring that can be placed inside the car, i.e., in the seat, the safety belt, the steering wheel or the cockpit. Based on different physical principles, many technologies exist and were introduced. Table 5 compares different technical aspects of these sensors including the question of active energy injection, costs, allowable distance between subject and device. According to the number of car-related publications per technology, the former and present efforts to incorporate these technologies into the car is not equally distributed among them. While the seat-integrated techniques ECG, cECG and BCG have been applied in the automotive environment quite extensively, camera-based techniques, although already at a high readiness level, have not been investigated to the same extent. This might be due to their comparably high costs and the potential conflict with drivers feeling observed and violated in their privacy. Figure 16: Early concept for sensor fusion inside the _U-car_[34]. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Sensor Technique** & **Type of Contact** & **Measured Quantity** & **Energy Injection** & **Distance** & **Sensitivity to Positioning** & **Costs** & **RL** & **In-Car Use** \\ \hline ECG (steering wheel) & galvanic & electric biopotential & no & 0 & + & o & + & [20,21,24,25,27,29,31,32,33] \\ cECG & capacitive & electric biopotential & no & mm & + & o & o & [42,44,48,49,52,58,61,61] \\ BCG & mechanical & displacement, force & no & 0 & – & o & o & [42,74,76,78] \\ Video motion & optical & displacement & no & m & – & + & + & not yet \\ PPG (steering wheel) & optical & photon absorption & yes & mm & o & – & + & [23,25,26] \\ PPG & optical & photon absorption & yes & m & – & + & o & [115,121] \\ Thermography & optical & radiation, temperature & no & m & – & ++ & + & [124,126] \\ MI & electromagnetic & electric biimpedance & yes & cm & o & – & [74,164,165] \\ Radar & electromagnetic & displacement, velocity & yes & m & – & o & o & [177, 180] \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of different ambient unobtrusive cardiorespiratory monitoring techniques by their technical aspects. Part of the information in columns 5 to 7 have been taken and adapted from [3]. RL stands for technology _randuses level_ estimated by the authors. In this review, pilot applications and demonstrators as well as commercial products (where existing) were presented and discussed. However, measurement scenarios using any of the upcoming wearables, alone or in combination with fixed sensors, were not covered. In the long run, sensor fusion with such sensors is considered beneficial and should be further investigated. Today, available no-contact sensors share the problem of motion artefacts. For some of the technologies, additional challenges exist (like triboelectricity for cECG and varying light conditions in the visible and NIR frequency range). To increase robustness for such delicate measurement scenarios, sensor/signal fusion is one of the options, next to motion artifact cancellation. This requires multi-sensor and multi-channel measurement setups with redundant as well as complementary sensor arrangements. Conceptualization: all authors participated in defining the overall structure of the review article, composing the tables and images and selecting the literature to be included. Writing: [PERSON] mainly wrote Sections 3, 8 and 9, [PERSON] mainly wrote Sections 4, 5 and 7, [PERSON] mainly wrote Sections 2, 6 and 10. Review and Editing: All authors carefully reviewed and edited the complete text. There are no funding sponsors for this manuscript. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: