Net Loss Per Share Attributable to Common Stockholders - Schedule of basic and diluted net loss per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (6,649)	$ (5,696)
Denominator:
Weighted average common shares outstanding—basic and diluted (in shares)	6,974,301	5,793,801
Dilutive effect of stock equivalents resulting from stock options, common stock warrants, preferred stock warrants and convertible preferred stock (as converted) (in shares)	0	0
Weighted average common shares outstanding-diluted (in shares)	6,974,301	5,793,801
Net loss per common share—basic and diluted (in dollars per share)	$ (0.95)	$ (0.98)