Creating a "wall" node in SpriteKit - swift

Question

I'm working with SpriteKit, Xcode 6 and Swift
I have a spritenode declared like this:
let firstCircle = SKSpriteNode(imageNamed: "Circle")
firstCircle.physicsBody = SKPhysicsBody(circleOfRadius: 7)
firstCircle.physicsBody?.affectedByGravity = false
and another spritenode which is bigger circle declared like this :
let wall = SKSpriteNode(imageNamed: "Circle")
wall.physicsBody = SKPhysicsBody(circleOfRadius: 50)
wall.physicsBody?.affectedByGravity = false
The node firstcircle follows my finger and the node wall stay at the middle of the screen, and I want that the two nodes cannot enter in collision together, but with my actual code, firstcircle just cross the wall, what have I to do in order to fix that problem ?
Here is how I move the node :
let scale:CGFloat = 10.0
let damping:CGFloat = 0.94
var departCircleLocation = CGPoint()
var departTouchLocation = CGPoint()
var currentTouchLocation = CGPoint()
var isTouchActive = false
override func touchesBegan(touches: NSSet, withEvent event: UIEvent)
{
isTouchActive = true
for touch: AnyObject in touches
{
departTouchLocation = touch.locationInNode(self)
}
departCircleLocation = firstCircle.position
currentTouchLocation = departTouchLocation
}
override func touchesMoved(touches: NSSet, withEvent event: UIEvent)
{
for touch: AnyObject in touches
{
currentTouchLocation = touch.locationInNode(self)
}
moveCircles()
}
override func touchesEnded(touches: NSSet, withEvent event: UIEvent)
{
isTouchActive = false
}
override func update(currentTime: NSTimeInterval)
{
moveCircle()
}
func moveCircle()
{
if isTouchActive
{
var dx = CGFloat(departCircleLocation.x - departTouchLocation.x + currentTouchLocation.x - firstCircle.position.x) * scale
var dy = CGFloat(departCircleLocation.y - departTouchLocation.y + currentTouchLocation.y - firstCircle.position.y) * scale
firstCircle.physicsBody?.velocity = CGVectorMake(dx, dy)
}
else
{
let dx = firstCircle.physicsBody!.velocity.dx * damping
let dy = firstCircle.physicsBody!.velocity.dy * damping
firstCircle.physicsBody!.velocity = CGVectorMake(dx, dy)
}
}