public static HealthItem[] itemsInInventory;

void Start()
{
    itemsInInventory = new HealthItem[5];
}

void OnTriggerEnter2D(Collider2D other)
    {
        if (other.gameObject.tag == "Collectable")
        {
            if (itemsInInventory.Length >= 9)
            {
                for (int i = 0; i < itemsInInventory.Length; i++)
                {
                    if (itemsInInventory[i] == null)
                    {
                        HealthItem tempItem = other.GetComponent<HealthItem> ();
                        itemsInInventory[i] = Instantiate(tempItem) as HealthItem;
                        Destroy(other.gameObject);
                        return;
                    }
                }
            }
        }