## 2 Materials and Methods
### Thermal Manikin  
The convective and evaporative performance of the 5 bicycle helmets (Table 1) were measured using a sweating thermal manikin (Figure 1d) that was electrically heated such that the manikin surface-temperature remained constant in an environmental chamber. The thermal manikin used for this purpose was developed in-house based on an average head shape (Figure 1b), obtained by averaging the 3D point coordinates of the head over a population sample. The 3D point coordinates were acquired from 85 magnetic resonance imaging (MRI) T1-weighted fast field echo (FFE) scans (male and female (Figure 1a) from the International Consortium for Brain Mapping (ICBM) database) [40]. The population average was obtained in two steps. In the first step, a correspondence was built between the surfaces in the population by registering a template surface via elastic-surface deformation to each individual head, thereby representing it with a fixed number of (semi)landmarks that were at the corresponding locations. In the second step, a population average was derived from the corresponded surfaces by averaging the location of each landmark over the set of surfaces [40]. The ear shape obtained from the scans was distorted due to fixtures used while scanning, so the ears on the final head shape were reconstructed using standard ear shapes available in meshmixer. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**No.** & & **A1-Z1** & **A2-Armor** & **A3-Blade** & **A4-Century** & **H1** \\ \hline  & **Type/Size** & **Open/M** & **Closed/M** & **Open/M** & **Open/M** & **Closed** \\ \hline
1 & Weight (g) & 190 & 250 & 230 & 277 & 150 \\ \hline
2 & Material & \multicolumn{4}{c}{Polycarbonate outer shell with expanded polystyrene (EPS) foam inner layer and padding} & \multicolumn{2}{c}{Plaster shell with padding} \\ \hline
4 &
\begin{tabular}{c} Surface area \\ (m\({}^{2}\)) \\ \end{tabular} & 0.270 & 0.263 & 0.255 & 0.240 & 0.196 \\ \hline  & & & & Multidirectional & Twist cap & \\
5 & Remarks & \begin{tabular}{c} Padding: 5 x \\ 3 mm. \\ \end{tabular} & \begin{tabular}{c} Padding: 8 \\ mm thick on \\ the sides \\ \end{tabular} & \
\begin{tabular}{c} Padding: 8 \\ mm thick on \\ layer below \\ inner layer \\ \end{tabular} &
\begin{tabular}{c} Multidirectional \\ technology for \\ better \\ aerodynamics \\ and \\ ventilation \\ \end{tabular} & \multicolumn{2}{c}{Prototype design, 3 mm thick padding} \\ \hline
6 & Homologation & \multicolumn{4}{c}{EN 1078} & \multicolumn{2}{c}{Not available} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Helmets tested in the study and their properties. Figure 1: **(a)** Slice of head scan [32]; **(b)** average head shape obtained from magnetic resonance imaging (MRI) scans using shape-modeling approach; **(c)** manikin inner layer with marked thermal-sensor locations and sweat openings; **(d)** thermal manikin; **(e)** manikin measurement zones. The average head-form shape was 3D-printed in a Visjlet PXL core on a ProJet CJP 660 Pro and strengthened with Colorbond infiltrate. The manikin was constructed with two layers (Figure 1c), a 2 mm thick outer layer for zone visualization and sweat-duct placement, and a 2.5 mm thick inner layer for heating elements and the installation of 0.5 mm thick thermal sensors. The inner layer was lined with heating elements (Figure 1c) (Cu-Ni alloy) connected to a power source to simulate different metabolic heat-output rates. In addition to the heating elements, 40 NTC 10 k coated temperature-measurement sensors were installed in every measurement-zone location, as marked in Figure 1c. The temperature sensors provide zonal-temperature feedback that allows to control power input to maintain a constant zone temperature. The heating and feedback system were controlled using a dedicated proportional integral derivative (PID) controller for each zone. The outer layer of the thermal manikin head was provided with 16 openings (Figure 1c) on the surface to simulate sweat.