Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total revenue	$ 10,877	$ 10,591	$ 8,643	$ 7,521	$ 6,589	$ 5,734	$ 5,213	$ 5,338	$ 37,632	$ 22,874	$ 17,585
Operating Expense:
Total costs of goods sold and services									19,684	12,887	9,837
Research and development (including stock compensation of $513, $180, and $59 for the years ended December 31, 2018, and 2017, and 2016, respectively)	4,045	4,411	3,705	3,644	3,927	4,224	3,903	4,250	15,805	16,304	16,993
Selling, general and administrative (including stock compensation of $4,143, $1,912, and $851 for the years ended December 31, 2018, 2017, and 2016, respectively)	10,577	8,846	7,579	6,691	6,047	4,728	4,747	4,166	33,693	19,688	12,466
Total operating expenses	20,291	18,253	15,954	14,684	13,682	12,121	11,682	11,394	69,182	48,879
Loss from operations	(9,414)	(7,662)	(7,311)	(7,163)	(7,093)	(6,387)	(6,469)	(6,056)	(31,550)	(26,005)	(21,711)
Interest income (expense), net	24	30	16	(24)	(216)	(240)	(240)	(255)	46	(951)	(1,298)
Other (expense) income, net	81	(25)	(48)	(15)	(73)	13	77	(80)	(7)	(63)	(164)
Tax expense	(25)								(25)
Net loss	(9,334)	(7,657)	(7,343)	(7,202)	(7,382)	(6,614)	(6,632)	(6,391)	(31,536)	(27,019)	(23,173)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(9,334)	(7,657)	(7,343)	(7,202)	(7,382)	(6,614)	(6,632)	(6,391)	(31,536)	(27,019)	(23,173)
Accretion of preferred stock to redemption value					(809)	(1,112)	(1,099)	(1,090)		(4,110)	(4,437)
Accrued dividends on preferred stock					(11)	(16)	(16)	(16)		(59)	(8)
Net loss attributable to common stockholders	$ (9,334)	$ (7,657)	$ (7,343)	$ (7,202)	$ (8,202)	$ (7,742)	$ (7,747)	$ (7,497)	$ (31,536)	$ (31,188)	$ (27,618)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (0.35)	$ (0.34)	$ (0.33)	$ (1.06)	$ (3.13)	$ (3.21)	$ (3.18)	$ (1.43)	$ (8.30)	$ (12.89)
Weighted-average common shares outstanding, basic and diluted	22,221,305	22,670,786	21,890,978	21,788,605	7,731,514	2,475,166	2,416,984	2,357,503	21,994,317	3,756,954	2,142,840
Product revenue
Total revenue	$ 7,458	$ 5,962	$ 5,200	$ 4,745	$ 4,069	$ 3,293	$ 3,337	$ 3,425	$ 23,365	$ 14,124	$ 10,601
Operating Expense:
Total costs of goods sold and services	3,734	3,277	2,945	2,773	2,169	1,905	1,834	1,834	12,729	7,742	6,299
Service and other revenue
Total revenue	3,419	3,017	3,174	2,507	2,252	2,172	1,608	1,644	12,117	7,676	5,012
Operating Expense:
Total costs of goods sold and services	$ 1,935	1,719	1,725	1,576	1,539	1,264	1,198	1,144	6,955	5,145	3,163
Collaboration and license revenue
Total revenue		$ 1,612	$ 269	$ 269	$ 268	$ 269	$ 268	$ 269	$ 2,150	$ 1,074	1,972
Operating Expense:
Total costs of goods sold and services											$ 375