A device for injecting an agent, comprising: a syringe comprising a syringe cavity, a plunger element slidably received in the syringe cavity, and a hollow needle in fluid communication with the syringe cavity, wherein the plunger element is configured to move from a proximal position to a distal position, the plunger element having a hollow proximal portion; a power assembly configured to transmit force to the plunger element, the power assembly comprising a spring that is in contact with the plunger element configured to bias the plunger element toward the distal position, and a spring sleeve located between the spring and the plunger element; and a user-actuated brake assembly that is configured to reversibly resist movement of the plunger element in at least one intermediate position between the proximal position and the distal position, characterised in that the user-actuated brake assembly comprises at least one braking pad that is attached to the outer surface of a flexure of the hollow proximal portion of the plunger element, wherein the flexure is configured to be flexed radially outward such that the at least one braking pad forms a high-friction interface with the spring sleeve when a proximally biased wedge-shaped stopper located at a distal end of a rod fixedly attached to an end cap of a proximal housing and slidably positioned within the hollow proximal portion of the plunger element exerts a proximal force on a corresponding wedge-shaped interior of the plunger element, wherein the stopper is proximally biased by a biasing element configured to bias the end cap of the proximal housing and the plunger element away from each other, and wherein friction between the at least one braking pad and the spring sleeve is reduced or eliminated when the proximal force is removed by applying a distal force on the proximal housing that overcomes the biasing element. a syringe comprising a syringe cavity, a plunger element slidably received in the syringe cavity, and a hollow needle in fluid communication with the syringe cavity, wherein the plunger element is configured to move from a proximal position to a distal position, the plunger element having a hollow proximal portion; a power assembly configured to transmit force to the plunger element, the power assembly comprising a spring that is in contact with the plunger element configured to bias the plunger element toward the distal position, and a spring sleeve located between the spring and the plunger element; and a user-actuated brake assembly that is configured to reversibly resist movement of the plunger element in at least one intermediate position between the proximal position and the distal position, characterised in that the user-actuated brake assembly comprises at least one braking pad that is attached to the outer surface of a flexure of the hollow proximal portion of the plunger element, wherein the flexure is configured to be flexed radially outward such that the at least one braking pad forms a high-friction interface with the spring sleeve when a proximally biased wedge-shaped stopper located at a distal end of a rod fixedly attached to an end cap of a proximal housing and slidably positioned within the hollow proximal portion of the plunger element exerts a proximal force on a corresponding wedge-shaped interior of the plunger element, wherein the stopper is proximally biased by a biasing element configured to bias the end cap of the proximal housing and the plunger element away from each other, and wherein friction between the at least one braking pad and the spring sleeve is reduced or eliminated when the proximal force is removed by applying a distal force on the proximal housing that overcomes the biasing element.