A catheter apparatus comprising: a tubular body having a distal end, a proximal end, and a lumen defined therein, the tubular body including at least two fluid impermeable layers defining a pressure-responsive chamber and at least one interstitial structure provided within the pressure-responsive chamber, the at least one interstitial structure includes a layered structure positioned between said at least two fluid impermeable layers, and activation means for selectably causing the tubular body to enter a first state and a second state; wherein, in the first state, the tubular body has a first value of a stiffness metric that is equal to or less than a predetermined navigatibility threshold; wherein, in the second state, the tubular body has a second value of the stiffness metric that is greater than the first value and that is greater than or equal to a predetermined rigidity threshold value; and wherein the activation means includes a controller communicatively coupled to the tubular body and adapted to place the tubular body in the second state by subjecting at least a portion of the tubular body to an increase in radial compression by causing a reduction in internal pressure within the pressure-responsive chamber thereby causing collapse of the pressure-responsive chamber, the at least one interstitial structure is adapted to exhibit radial compression in response to the reduction in internal pressure within the pressure responsive chamber caused by the controller, thereby increasing layer-to-layer friction of the layered structure; wherein the controller is configured to cause the change of state by pneumatic activation; wherein the pressure-responsive chamber is at atmospheric pressure in the first state; and wherein the pressure-responsive chamber is placed under negative pressure in the second state, characterized in that the at least one interstitial structure comprises a substantially cylindrical braided structure. a tubular body having a distal end, a proximal end, and a lumen defined therein, the tubular body including at least two fluid impermeable layers defining a pressure-responsive chamber and at least one interstitial structure provided within the pressure-responsive chamber, the at least one interstitial structure includes a layered structure positioned between said at least two fluid impermeable layers, and activation means for selectably causing the tubular body to enter a first state and a second state; wherein, in the first state, the tubular body has a first value of a stiffness metric that is equal to or less than a predetermined navigatibility threshold; wherein, in the second state, the tubular body has a second value of the stiffness metric that is greater than the first value and that is greater than or equal to a predetermined rigidity threshold value; and wherein the activation means includes a controller communicatively coupled to the tubular body and adapted to place the tubular body in the second state by subjecting at least a portion of the tubular body to an increase in radial compression by causing a reduction in internal pressure within the pressure-responsive chamber thereby causing collapse of the pressure-responsive chamber, the at least one interstitial structure is adapted to exhibit radial compression in response to the reduction in internal pressure within the pressure responsive chamber caused by the controller, thereby increasing layer-to-layer friction of the layered structure; wherein the controller is configured to cause the change of state by pneumatic activation; wherein the pressure-responsive chamber is at atmospheric pressure in the first state; and wherein the pressure-responsive chamber is placed under negative pressure in the second state, characterized in that the at least one interstitial structure comprises a substantially cylindrical braided structure. <EOS>