Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (19,450)	$ (27,558)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	890	1,281
Amortization of debt issuance costs and debt discount	171	117
Stock-based compensation	2,091	3,080
Noncash interest expense and other charges	1,575	5,634
Common stock issued for services		106
Provision for excess and obsolete inventories	272	914
Other non-cash charges	(51)	126
Changes in operating assets and liabilities:
Accounts receivable	(248)	(160)
Inventories	(1,169)	(92)
Prepaid expenses and other current assets	128	5
Other assets	658	340
Accounts payable	(485)	(125)
Accrued compensation	585	334
Leasehold liability	(1,402)
Accrued expenses and other current liabilities	(795)	(2,208)
Other long-term liabilities	(34)	(260)
Net cash used in operating activities	(17,264)	(18,466)
Cash flows from investing activities
Purchase of property and equipment	(88)	(32)
Proceeds from sale of property and equipment	18	28
Net cash used in investing activities	(70)	(4)
Cash flows from financing activities
Proceeds from the issuance of convertible preferred stock, net of issuance costs		15,525
Proceeds from issuance of common stock and convertible preferred stock, net of issuance costs		10,179
Proceeds from issuance of common stock under officers’ and directors’ purchase plan	63	21
Proceeds from public offerings, net of issuance costs		3,026
Proceeds from exercise of common stock warrants	7,993	581
Debt discount in connection with loan amendment		(155)
Proceeds from the issuance of common stock	3,811	314
Net cash provided by financing activities	11,867	29,491
Net change in cash and cash equivalents	(5,467)	11,021
Cash and cash equivalents, beginning of period	16,410	5,389
Cash and cash equivalents, end of period	10,943	16,410
Supplemental disclosure of cash flow information
Cash paid for interest		60
Noncash investing and financing activities:
Conversion of CRG loan principal and accrued interest into Series A Convertible preferred stock		42,794
Issuance of Series A preferred stock as dividend payment	6,498
Disposal of fully depreciated property and equipment	54	2,849
Accretion of Series A Convertible preferred stock dividends	3,580	2,918
Reclassification of right of use asset to prepaid rent	462
Increase to right of use asset and leasehold liability arising from lease amendment	4,680
Transfer between inventory and property and equipment	$ 408	$ 51