CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated deficit	Convertible preferred stock Preferred Stock
Beginning balance (in shares) at Dec. 31, 2019		21,018,663			3,704,288
Beginning balance at Dec. 31, 2019	$ 20,015	$ 21	$ 431,305	$ (411,315)	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		21,166
Issuance of common stock upon exercise of options (in shares)		468
Issuance of common stock upon exercise of options	0
Issuance of common stock under Employee Stock Purchase Plan (in shares)		1,666
Issuance of common stock under Employee Stock Purchase Plan	1		1
Conversions of convertible preferred stock (in shares)		6,566,820			(656,682)
Conversions of convertible preferred stock	0	$ 7	(6)		$ (1)
Stock-based compensation expense	823		823
Net loss	(5,937)			(5,937)
Ending balance (in shares) at Mar. 31, 2020		27,608,783			3,047,606
Ending balance at Mar. 31, 2020	14,902	$ 28	432,123	(417,252)	$ 3
Beginning balance (in shares) at Dec. 31, 2019		21,018,663			3,704,288
Beginning balance at Dec. 31, 2019	20,015	$ 21	431,305	(411,315)	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(14,408)
Ending balance (in shares) at Sep. 30, 2020		39,163,096			1,896,215
Ending balance at Sep. 30, 2020	7,754	$ 39	433,436	(425,723)	$ 2
Beginning balance (in shares) at Mar. 31, 2020		27,608,783			3,047,606
Beginning balance at Mar. 31, 2020	14,902	$ 28	432,123	(417,252)	$ 3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		17,703
Conversions of convertible preferred stock (in shares)		8,296,430			(829,643)
Conversions of convertible preferred stock	0	$ 8	(7)		$ (1)
Stock-based compensation expense	629		629
Net loss	(6,947)			(6,947)
Ending balance (in shares) at Jun. 30, 2020		35,922,916			2,217,963
Ending balance at Jun. 30, 2020	8,584	$ 36	432,745	(424,199)	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		19,368
Issuance of common stock under Employee Stock Purchase Plan (in shares)		3,332
Issuance of common stock under Employee Stock Purchase Plan	2		2
Conversions of convertible preferred stock (in shares)		3,217,480			(321,748)
Conversions of convertible preferred stock	0	$ 3	(3)		$ 0
Stock-based compensation expense	692		692
Net loss	(1,524)			(1,524)
Ending balance (in shares) at Sep. 30, 2020		39,163,096			1,896,215
Ending balance at Sep. 30, 2020	7,754	$ 39	433,436	(425,723)	$ 2
Beginning balance (in shares) at Dec. 31, 2020		67,432,712			1,931,860
Beginning balance at Dec. 31, 2020	26,026	$ 67	453,002	(427,045)	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		16,902
Issuance of common stock upon exercise of options (in shares)		26,770
Issuance of common stock upon exercise of options	26		26
Issuance of common stock upon exercise of warrants (in shares)		2,418,681
Issuance of common stock upon exercise of warrants	666	$ 3	663
Issuance of common stock through ATM (in shares)		4,009,585
Issuance of common stock through ATM	5,713	$ 4	5,709
Issuance of common stock under Employee Stock Purchase Plan (in shares)		4,122
Issuance of common stock under Employee Stock Purchase Plan	2		2
Conversions of convertible preferred stock (in shares)		780,360			(78,036)
Conversions of convertible preferred stock	0	$ 1	(1)
Stock-based compensation expense	691		691
Net loss	(6,013)			(6,013)
Ending balance (in shares) at Mar. 31, 2021		74,689,132			1,853,824
Ending balance at Mar. 31, 2021	27,111	$ 75	460,092	(433,058)	$ 2
Beginning balance (in shares) at Dec. 31, 2020		67,432,712			1,931,860
Beginning balance at Dec. 31, 2020	26,026	$ 67	453,002	(427,045)	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(20,675)
Ending balance (in shares) at Sep. 30, 2021		87,047,832			1,846,239
Ending balance at Sep. 30, 2021	29,012	$ 87	476,643	(447,720)	$ 2
Beginning balance (in shares) at Mar. 31, 2021		74,689,132			1,853,824
Beginning balance at Mar. 31, 2021	27,111	$ 75	460,092	(433,058)	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		16,849
Issuance of common stock upon exercise of options (in shares)		958
Issuance of common stock upon exercise of options	0
Issuance of common stock upon exercise of warrants (in shares)		250,000
Issuance of common stock upon exercise of warrants	166		166
Issuance of common stock through ATM (in shares)		12,007,546
Issuance of common stock through ATM	14,831	$ 12	14,819
Conversions of convertible preferred stock (in shares)		75,850			(7,585)
Conversions of convertible preferred stock	0
Stock-based compensation expense	754		754
Net loss	(6,034)			(6,034)
Ending balance (in shares) at Jun. 30, 2021		87,040,335			1,846,239
Ending balance at Jun. 30, 2021	36,828	$ 87	475,831	(439,092)	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under Employee Stock Purchase Plan (in shares)		7,497
Issuance of common stock under Employee Stock Purchase Plan	5		5
Stock-based compensation expense	807		807
Net loss	(8,628)			(8,628)
Ending balance (in shares) at Sep. 30, 2021		87,047,832			1,846,239
Ending balance at Sep. 30, 2021	$ 29,012	$ 87	$ 476,643	$ (447,720)	$ 2