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Stock Purchase Warrants	12 Months Ended
Stock Purchase Warrants	Dec. 31, 2019
Warrants and Rights Note Disclosure [Abstract]
Stock Purchase Warrants	Stock Purchase Warrants The Company has historically issued warrants to purchase shares of the Company’s common stock in connection with certain of its common stock offerings. The fair value of the warrants described in the table above is measured using the Black-Scholes valuation model. Inherent in the Black-Scholes valuation model are assumptions related to expected stock-price volatility, expected life, risk-free interest rate and dividend yield. The Company estimates the volatility of its common stock based on historical volatility that matches the expected remaining life of the warrants. The risk-free interest rate is based on the U.S. Treasury zero-coupon yield curve on the grant date for a maturity similar to the expected remaining life of the warrants. The expected life of the warrants is assumed to be equivalent to their remaining contractual term. The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. During the year ended December 31, 2019, the Company issued 19,808 shares of common stock upon the exercise warrants with an exercise price of 4.27. There are no warrants outstanding as of December 31, 2019. ICT Warrants On December 21, 2017, the Company received $5.2 million (gross of withholding tax) from Innovative Cellular Therapeutics CO., LTD. (ICT), of which $4.0 million was allocated to the purchase of a warrant for 818,424 shares of the Company's common stock based on the fair value on the date of grant and the remaining $1.2 million was allocated as consideration for the license agreement described in note 4. The fair value of the warrant was based on the closing price as of December 6, 2017 of $4.90 at an exercise price of $0.01 per share. On December 27, 2017, ICT exercised the warrant via a cashless exercise in exchange for 816,850 shares of the Company’s common stock. There were no warrants issued to ICT outstanding as of December 31, 2019.