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				6 Months Ended
	Jun. 29, 2019	Mar. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 29, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Antidilutive securities excluded from computation (in shares)					6,559,565	5,296,339
Numerator:
Net loss	$ (9,441)	$ (6,649)	$ (7,396)	$ (5,696)	$ (16,090)	$ (13,092)
Denominator:
Weighted average common shares outstanding—basic (in shares)	39,081,359		6,072,319		23,206,203	5,933,806
Dilutive effect of stock equivalents resulting from stock options, RSUs, common stock warrants, preferred stock warrants and convertible preferred stock (as converted) (in shares)	0		0		0	0
Weighted average common shares outstanding-diluted (in shares)	39,081,359		6,072,319		23,206,203	5,933,806
Net loss per common share—basic and diluted (in dollars per share)	$ (0.24)		$ (1.22)		$ (0.69)	$ (2.21)