A check valve comprising: a housing; and a spring contained within the housing, wherein the spring is formed from a single piece of a polymeric material and wherein the spring comprises: a proximal end; a distal end positioned opposite to the proximal end; a body positioned between the proximal end and the distal end, wherein the body comprises a wall, wherein the body comprises a proximal opening on a portion of the wall adjacent to the proximal end; wherein the spring further includes a sealing lip configured to engage and disengage with the housing of the check valve, characterized in that the spring comprises a distal opening on a portion of the wall adjacent to the distal end; the body further comprises a bellows between the proximal opening and the distal opening; in an initial position, in which the bellows are expanded and the sealing lip contacts a proximal interior sealing surface of the housing, fluid flow in either direction through the check valve is prevented, and if the pressure is equal to or greater than the cracking pressure of the check valve, the bellows compress, disengaging the sealing lip from the proximal interior sealing surface and moving the proximal end toward the distal end of the spring to allow a fluid to flow through the check valve and through the proximal opening and the distal opening. a housing; and a spring contained within the housing, wherein the spring is formed from a single piece of a polymeric material and wherein the spring comprises: a proximal end; a distal end positioned opposite to the proximal end; a body positioned between the proximal end and the distal end, wherein the body comprises a wall, wherein the body comprises a proximal opening on a portion of the wall adjacent to the proximal end; wherein the spring further includes a sealing lip configured to engage and disengage with the housing of the check valve, characterized in that the spring comprises a distal opening on a portion of the wall adjacent to the distal end; the body further comprises a bellows between the proximal opening and the distal opening; in an initial position, in which the bellows are expanded and the sealing lip contacts a proximal interior sealing surface of the housing, fluid flow in either direction through the check valve is prevented, and if the pressure is equal to or greater than the cracking pressure of the check valve, the bellows compress, disengaging the sealing lip from the proximal interior sealing surface and moving the proximal end toward the distal end of the spring to allow a fluid to flow through the check valve and through the proximal opening and the distal opening. a proximal end; a distal end positioned opposite to the proximal end; a body positioned between the proximal end and the distal end, wherein the body comprises a wall, wherein the body comprises a proximal opening on a portion of the wall adjacent to the proximal end; wherein the spring further includes a sealing lip configured to engage and disengage with the housing of the check valve, characterized in that the spring comprises a distal opening on a portion of the wall adjacent to the distal end; the body further comprises a bellows between the proximal opening and the distal opening; in an initial position, in which the bellows are expanded and the sealing lip contacts a proximal interior sealing surface of the housing, fluid flow in either direction through the check valve is prevented, and if the pressure is equal to or greater than the cracking pressure of the check valve, the bellows compress, disengaging the sealing lip from the proximal interior sealing surface and moving the proximal end toward the distal end of the spring to allow a fluid to flow through the check valve and through the proximal opening and the distal opening.