D3O is a material in a wide range of protective applications from phone cases to F1 cars. It's a popular option in MTB and Moto protection as it's a soft material that hardens upon impact and so doesn't restrict movement like hard shell protection and needs less bulk than many other methods.
The downsides are that it's a pretty dense material so is relatively heavy. D3O is not cheap and it's less malleable in the cold.