maybe something like
julia> dft = expand_derivatives(current)
20exp(-2t)
julia> dft_function = eval(build_function(dft, t))
#5 (generic function with 1 method)
julia> result = dft_function(1.0) # t = 1.0
2.706705664732254
julia> println("The result is $(result) mA")
The result is 2.706705664732254 mA
julia>