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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2020-08-12T15:07:03.2317866+00:00 -->
<!-- Version            : 5.0.1.476 -->
<!-- Package ID         : 01b9b824997547258c6dd212399014ed -->
<!-- Copyright (c) 2020 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="hsdt_ClassOfWarrantOrRightExpired" name="ClassOfWarrantOrRightExpired" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_ClassOfWarrantOrRightExercisePriceGrantedOfWarrantsOrRights" name="ClassOfWarrantOrRightExercisePriceGrantedOfWarrantsOrRights" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_ClassOfWarrantOrRightExercisePriceExpiredOfWarrantsOrRights" name="ClassOfWarrantOrRightExercisePriceExpiredOfWarrantsOrRights" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_ClassOfWarrantOrRightExercisePriceExercisedOfWarrantsOrRights" name="ClassOfWarrantOrRightExercisePriceExercisedOfWarrantsOrRights" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_WarrantsOneMember" name="WarrantsOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_WarrantsTwoMember" name="WarrantsTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_WarrantsThreeMember" name="WarrantsThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_WarrantsFourMember" name="WarrantsFourMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_WarrantsFiveMember" name="WarrantsFiveMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_WarrantsSixMember" name="WarrantsSixMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_CommonStockCapitalSharesReservedForFutureIssuanceNewShares" name="CommonStockCapitalSharesReservedForFutureIssuanceNewShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="hsdt_TwoThousandEighteenOmnibusIncentivePlanMember" name="TwoThousandEighteenOmnibusIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_TwoThousandSixteenOmnibusIncentivePlanMember" name="TwoThousandSixteenOmnibusIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_NonEmployeeStockOptionMember" name="NonEmployeeStockOptionMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_ConsultantsStockOptionsMember" name="ConsultantsStockOptionsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="hsdt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="hsdt_AccruedFranchiseFeeCurrent" name="AccruedFranchiseFeeCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="hsdt_PercentageOfRoyaltyOnNetRevenue" name="PercentageOfRoyaltyOnNetRevenue" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_CommitmentsAndContingenciesLineItems" name="CommitmentsAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_CommitmentsAndContingenciesTable" name="CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_AdvancedNeuroRehabilitationLimitedLiabilityCompanyMember" name="AdvancedNeuroRehabilitationLimitedLiabilityCompanyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_LicenseAgreementForExclusiveRightsOnPatentPendingTechnologyMember" name="LicenseAgreementForExclusiveRightsOnPatentPendingTechnologyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_AAndBCompanyLimitedMember" name="AAndBCompanyLimitedMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_AssetPurchaseAgreementMember" name="AssetPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_ManufacturingAndSupplyAgreementMember" name="ManufacturingAndSupplyAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_KeyTronicCorporationMember" name="KeyTronicCorporationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_PaycheckProtectionProgramMember" name="PaycheckProtectionProgramMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="hsdt_PenaltyPayable" name="PenaltyPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="hsdt_OperatingLeaseCommencementDate" name="OperatingLeaseCommencementDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_OperatingLeaseOptionToExtendLeaseTermYear" name="OperatingLeaseOptionToExtendLeaseTermYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_OperatingLeaseOptionToExtendLeaseTermDate" name="OperatingLeaseOptionToExtendLeaseTermDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_OperatingLeaseRentPlusUtilitiesBeginningDate" name="OperatingLeaseRentPlusUtilitiesBeginningDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_OperatingLeaseAnnualIncreaseInRentalPercentage" name="OperatingLeaseAnnualIncreaseInRentalPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="hsdt_OperatingLeaseOptionToExtendLeaseTerm" name="OperatingLeaseOptionToExtendLeaseTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_DecreaseInOperatingLeaseRightOfUseAssetPartialTermination" name="DecreaseInOperatingLeaseRightOfUseAssetPartialTermination" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="hsdt_DecreaseInOperatingLeaseLiability" name="DecreaseInOperatingLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="hsdt_LongTermPurchaseCommitmentAutoRenewalPeriod" name="LongTermPurchaseCommitmentAutoRenewalPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_LongTermPurchaseCommitmentNoticePeriodToCancelRenewalPeriod" name="LongTermPurchaseCommitmentNoticePeriodToCancelRenewalPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="hsdt_LongTermPurchaseCommitmentExtendedPeriod" name="LongTermPurchaseCommitmentExtendedPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
</xsd:schema>
