  $^1$H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP)
have been studied in amorphous samples of trehalose sugar doped with TEMPO
radicals by means of mechanical milling, in the 1.6 K $\div$ 4.2 K temperature
range. The radical concentration was varied between 0.34 and 0.81 $\%$. The
highest polarization of 15 \% at 1.6 K, observed in the sample with
concentration $0.50 \%$, is of the same order of magnitude of that reported in
standard frozen solutions with TEMPO. The temperature and concentration
dependence of the spin-lattice relaxation rate $1/T_{\text{1}}$, dominated by
the coupling with the electron spins, were found to follow power laws with an
exponent close to $3$ in all samples. The observed proportionality between
$1/T_{\text{1}}$ and the polarization rate $1/T_{\text{pol}}$, with a
coefficient related to the electron polarization, is consistent with the
presence of Thermal Mixing (TM) and a good contact between the nuclear and the
electron spins. At high electron concentration additional relaxation channels
causing a decrease in the nuclear polarization must be considered. These
results provide further support for a more extensive use of amorphous DNP-ready
samples, obtained by means of comilling, in dissolution DNP experiments and
possibly for $\textit{in vivo}$ metabolic imaging.
