A device for indicating violation of a temperature limit of an item stored in a temperature-controlled environment, wherein the device is positionable near the item and includes a microfluidic sensor configured to change its state permanently when the temperature limit is violated, wherein the change in sensor state is machine-readable to provide a permanent indication of the temperature limit violation, the microfluidic sensor including: a reservoir for storing a fluid that changes phase from a solid to a liquid when the temperature limit is violated, wherein the reservoir initially stores the fluid in a liquid state and upon placement of the device in the temperature-controlled environment, the fluid changes phase from a liquid to a solid and the microfluidic sensor adopts a resting state; a microfluidic channel in fluid communication with the reservoir for receiving flow of the fluid in liquid phase from the reservoir when the temperature limit is violated, the microfluidic channel being sized and shaped such that capillary forces drive the fluid flow from the reservoir into the microfluidic channel; and a barrier positioned between the reservoir and the microfluidic channel for preventing the fluid flow from permanently changing the sensor state until activation of the microfluidic sensor from the resting state, wherein the barrier is removable to activate the microfluidic sensor from the resting state to an activated state in which the microfluidic channel receives the fluid flow from the reservoir when the temperature limit is violated; wherein the flow of the fluid into the microfluidic channel causes a permanent change in an electrical property of the microfluidic sensor such that the state of the microfluidic sensor is permanently changed from the activated state to a triggered state, and wherein the microfluidic sensor further includes an electrical circuit configured to provide a machine-readable indicator of the permanent change in sensor state to the triggered state. a reservoir for storing a fluid that changes phase from a solid to a liquid when the temperature limit is violated, wherein the reservoir initially stores the fluid in a liquid state and upon placement of the device in the temperature-controlled environment, the fluid changes phase from a liquid to a solid and the microfluidic sensor adopts a resting state; a microfluidic channel in fluid communication with the reservoir for receiving flow of the fluid in liquid phase from the reservoir when the temperature limit is violated, the microfluidic channel being sized and shaped such that capillary forces drive the fluid flow from the reservoir into the microfluidic channel; and a barrier positioned between the reservoir and the microfluidic channel for preventing the fluid flow from permanently changing the sensor state until activation of the microfluidic sensor from the resting state, wherein the barrier is removable to activate the microfluidic sensor from the resting state to an activated state in which the microfluidic channel receives the fluid flow from the reservoir when the temperature limit is violated; wherein the flow of the fluid into the microfluidic channel causes a permanent change in an electrical property of the microfluidic sensor such that the state of the microfluidic sensor is permanently changed from the activated state to a triggered state, and wherein the microfluidic sensor further includes an electrical circuit configured to provide a machine-readable indicator of the permanent change in sensor state to the triggered state.