12. Concentrations

Credit—Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents and accounts receivable. The Company’s cash equivalents consist of cash and money market accounts with a financial institution that management believes to be of high-credit quality; however, at times, balances exceed federally insured limits. Amounts held on deposit at financial institutions in excess of Federal Deposit Insurance Corporation-insured amounts were $44,000 and $0.9 million as of September 30, 2018 and December 31, 2017, respectively.

Significant Customers—Credit risk with respect to accounts receivable is concentrated with two large customers that contribute a majority of the Company’s business and is mitigated by a relatively short collection period. Collateral is not required for accounts receivable. The fair value of accounts receivable approximates their carrying value. Revenue and accounts receivable concentrated with significant customers and their manufacturing subcontractors as a percentage of accounts receivable and total revenue were as follows:

	As of			As of
	September 30,			December 31,
	2018			2017
Accounts Receivable:
Customer A		44	%		49	%
Customer B		35			34

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2018			2017			2018			2017
Revenue:
Customer A		50	%		54	%		50	%		61	%
Customer B		30			34			29			29