How can I permanently save value in string variable of master page using vb.net?

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VB

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Hello,

I am working on ASP.Net website with Visual Basic as backend language. I am working on Master-Child webpages. I have created a Public string property on Master page and want to permanently change its value. When I change value of property it is updated properly, but when I move to some other content page and try to access that value, than Master page string returns nothing.
Here is code of property.

Public _patient_id As String
   Public Property patient_id() As String
       Get
           Return _patient_id
       End Get
       Set(ByVal value As String)
           _patient_id = value
       End Set
   End Property

I will be really thankful if you suggest some solution to permanently update value of string.
Thank you