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Prepaid Expenses and Other Current Assets (Tables)
 9 Months Ended
Sep. 30, 2021
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
   September 30,   December 31, 
(in thousands)  2021   2020 
Australia research and development credit receivable  $1,212   $2,148 
CRO prepayments   693    1,211 
Metals to be used in research and development   1,691    31 
Directors & Officers Insurance   934    
-
 
Other   202    112 
   $4,732   $3,502