## 2 Materials and Methods
### Development of Smart Helmet Prototype  
A standard cyclist helmet (312 g) was utilised for the development of the smart helmet prototype (Lazer Bullet 1.0, Lazer Sport, Antwerp, Belgium). The helmet was equipped with a Lifebeam heart rate sensor (Lazer Sport, Antwerp, Belgium; Figure 3) and a \(3\times 3\) mm digital humidity and temperature sensor (CJMCU-1080 HTC1080, Texas Instruments, Dallas, Texas; accuracy: \(\pm 2\%\) for relative humidity and \(\pm 0.2~{}^{\circ}\)C for temperature) to measure the surrounding air humidity and temperature. Additionally, four temperature sensors (Negative-Temperature-Coefficient \"NTC\" temperature sensors, 100 k\(\Omega\) at 25 \({}^{\circ}\)C; Figure 4) were used at the front, back, right and left of the helmet inner body. The final weight of the equipped helmet was 358 g. All sensors were connected directly to a microcontroller (Adafruit Feather  
\begin{table}
\begin{tabular}{c l l l l l} \hline \hline \multicolumn{1}{c}{**Participant**} & \multicolumn{1}{c}{**Variables**} & \multicolumn{1}{c}{**Timeslot (1)**} & \multicolumn{1}{c}{**Timeslot (2)**} & \multicolumn{1}{c}{**Timeslot (3)**} & \multicolumn{1}{c}{**Timeslot (4)**} \\ \hline \multirow{4}{*}{_j_ = 1} & \(T_{a}\) (\({}^{\circ}\)C) & \(\downarrow\) & \(\downarrow\) & \(\downarrow\) & \(\downarrow\) \\  & \(v\) (m\(\cdot\)s\({}^{-1}\)) & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\downarrow\) \\  & \(p\) (\% PPER = 1) & \(\downarrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) \\  & \(R_{h}\) & – & \(\uparrow\) & – & \(\downarrow\) \\  & (m\({}^{2,\circ}\)C\(\cdot\)W\({}^{-1}\)) & – & \(\uparrow\) & – & \(\downarrow\) \\  & \(T_{a}\) (\({}^{\circ}\)C) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) \\ \multirow{4}{*}{_j_ = 8} & \(v\) (m\(\cdot\)s\({}^{-1}\)) & \(\uparrow\) & \(\uparrow\) & \(\downarrow\) & \(\downarrow\) \\  & \(P\) (\% PPER = 1) & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) \\  & \(R_{h}\) & – & – & \(\downarrow\) & \(\uparrow\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental design for test subjects 1 and 8, showing the four runs (combinations) of input variables with three different levels, namely, high (\(\uparrow\)), mid (\(-\)) and low (\(\downarrow\)).  
32u4 Bluefruit, Adafruit Industries, New York, NY, USA) that transmitted all data from the helmet to a smartphone via Bluetooth. The Adafruit Bluefruit was chosen as it is the smallest \"all-in-one\" Arduino-compatible and Bluetooth Low Energy microcontroller with built-in USB and battery charging. The developed system was compatible with a 3.7 V Li-polymer rechargeable battery (LP-523450-1S-3) with the ability to power the system for up to 10 h. A circuit diagram of the used electronics and sensors is shown in Figure 4. The impeded electronics and sensor technology in the smart helmet increased the final original weight of the helmet (312 g) by only 14.7% and did not alter the geometric and aerodynamic characteristics of the original standard helmet. As such, the developed smart helmet is comparable to the original standard helmet (Lazer Bullet 1.0, Lazer Sport, Antwerp, Belgium).  
Figure 4: Circuit diagram of the stand-alone sensor system impeded in the smart helmet prototype.  
Figure 3: The developed smart helmet prototype showing the microcontroller and the humidity and temperature sensor on the back side of the helmet (**left picture**) and the four NTC temperature sensors placed in the inner body of the helmet (**right picture**).  
An android-based application \"SmartHelmet App\" (Figure 5) was developed to simultaneously communicate with both the Adafruit Feather microcontroller and the Lifebeam heart rate monitor via a Bluetooth communication protocol. The SmartHelmet App was developed using the AppyBuilder online platform (App Inventor, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA). The application was designed to receive, display in real-time and store all the data from the SmartHelmet at a 0.2 Hz sampling rate.