A non-contact printing system for printing biological materials, comprising: a printing liquid reservoir containing a printing liquid, the printing liquid having a volume of about 0.5 to 1.0 millilitres and comprising a reagent including cells or cell fragments or other biological particles, the printing liquid defining a first printing liquid surface; an expansion chamber in fluid communication with the printing liquid, the expansion chamber including a bore; and an aspirator element in fluid communication with the expansion chamber, the aspirator element including a piston arranged to reciprocate in the bore, wherein in use: the piston is movable from a first position to a second position in order to increase the volume in the expansion chamber so as to reduce a pressure of a gas in the expansion chamber which pressure acts on the first printing liquid surface, such that a pressure head of the printing liquid moves the printing liquid from the printing liquid reservoir to the expansion chamber to cause a second printing liquid surface defined by the expansion chamber to rise in the expansion chamber from a first level to a second level; and the piston is movable from the second position to the first position in order to reduce the volume in the expansion chamber so as to restore the pressure of the gas in the expansion chamber, such that the pressure head of the printing liquid moves the printing liquid from the expansion chamber to the printing liquid reservoir to return the second printing liquid surface from the second level to the first level, thereby to cause mixing of the printing liquid in the printing liquid reservoir by means of aspiration and subsequent dispense of a volume of the printing liquid in the printing liquid reservoir, so as to provide a stable concentration of the cells or cell fragments or other biological particles for printing over time without degradation of the cells or cell fragments or other biological particles. a printing liquid reservoir containing a printing liquid, the printing liquid having a volume of about 0.5 to 1.0 millilitres and comprising a reagent including cells or cell fragments or other biological particles, the printing liquid defining a first printing liquid surface; an expansion chamber in fluid communication with the printing liquid, the expansion chamber including a bore; and an aspirator element in fluid communication with the expansion chamber, the aspirator element including a piston arranged to reciprocate in the bore, wherein in use: the piston is movable from a first position to a second position in order to increase the volume in the expansion chamber so as to reduce a pressure of a gas in the expansion chamber which pressure acts on the first printing liquid surface, such that a pressure head of the printing liquid moves the printing liquid from the printing liquid reservoir to the expansion chamber to cause a second printing liquid surface defined by the expansion chamber to rise in the expansion chamber from a first level to a second level; and the piston is movable from the second position to the first position in order to reduce the volume in the expansion chamber so as to restore the pressure of the gas in the expansion chamber, such that the pressure head of the printing liquid moves the printing liquid from the expansion chamber to the printing liquid reservoir to return the second printing liquid surface from the second level to the first level, thereby to cause mixing of the printing liquid in the printing liquid reservoir by means of aspiration and subsequent dispense of a volume of the printing liquid in the printing liquid reservoir, so as to provide a stable concentration of the cells or cell fragments or other biological particles for printing over time without degradation of the cells or cell fragments or other biological particles. the piston is movable from a first position to a second position in order to increase the volume in the expansion chamber so as to reduce a pressure of a gas in the expansion chamber which pressure acts on the first printing liquid surface, such that a pressure head of the printing liquid moves the printing liquid from the printing liquid reservoir to the expansion chamber to cause a second printing liquid surface defined by the expansion chamber to rise in the expansion chamber from a first level to a second level; and the piston is movable from the second position to the first position in order to reduce the volume in the expansion chamber so as to restore the pressure of the gas in the expansion chamber, such that the pressure head of the printing liquid moves the printing liquid from the expansion chamber to the printing liquid reservoir to return the second printing liquid surface from the second level to the first level, thereby to cause mixing of the printing liquid in the printing liquid reservoir by means of aspiration and subsequent dispense of a volume of the printing liquid in the printing liquid reservoir, so as to provide a stable concentration of the cells or cell fragments or other biological particles for printing over time without degradation of the cells or cell fragments or other biological particles.