A modular infusion pump for dispensing a fluid, comprising: a curved reservoir module, comprising: a tubular, curved reservoir containing the fluid, the curved reservoir having a proximal end and a distal end; a flexible drive train configured to slidably fit within the curved reservoir and to expel the fluid through the proximal end of the reservoir, the flexible drive train including: a plunger at a proximal end of the drive train, a cursor at a distal end of the drive train, and a plurality of balls positioned between the plunger and the cursor; and a control module comprising an actuation mechanism comprising a lead screw configured to mechanically engage the plurality of the balls of the drive train when the reservoir module is connected to the control module and to drive_the drive train through the reservoir, characterized in that the flexible drive train includes a flexible and elastic filament extending between the cursor and the plunger, wherein the plurality of balls is positioned along and connected by the filament, wherein the filament is configured to maintain the plurality of balls under axial compression. a curved reservoir module, comprising: a tubular, curved reservoir containing the fluid, the curved reservoir having a proximal end and a distal end; a flexible drive train configured to slidably fit within the curved reservoir and to expel the fluid through the proximal end of the reservoir, the flexible drive train including: a plunger at a proximal end of the drive train, a cursor at a distal end of the drive train, and a plurality of balls positioned between the plunger and the cursor; and a control module comprising an actuation mechanism comprising a lead screw configured to mechanically engage the plurality of the balls of the drive train when the reservoir module is connected to the control module and to drive_the drive train through the reservoir, a tubular, curved reservoir containing the fluid, the curved reservoir having a proximal end and a distal end; a flexible drive train configured to slidably fit within the curved reservoir and to expel the fluid through the proximal end of the reservoir, the flexible drive train including: a plunger at a proximal end of the drive train, a cursor at a distal end of the drive train, and a plurality of balls positioned between the plunger and the cursor; and a control module comprising an actuation mechanism comprising a lead screw configured to mechanically engage the plurality of the balls of the drive train when the reservoir module is connected to the control module and to drive_the drive train through the reservoir, a plunger at a proximal end of the drive train, a cursor at a distal end of the drive train, and a plurality of balls positioned between the plunger and the cursor; and a control module comprising an actuation mechanism comprising a lead screw configured to mechanically engage the plurality of the balls of the drive train when the reservoir module is connected to the control module and to drive_the drive train through the reservoir, characterized in that the flexible drive train includes a flexible and elastic filament extending between the cursor and the plunger, wherein the plurality of balls is positioned along and connected by the filament, wherein the filament is configured to maintain the plurality of balls under axial compression.